    const std = @import("std.zig");
    const io = std.io;
    const math = std.math;
    const assert = std.debug.assert;
    const mem = std.mem;
    const unicode = std.unicode;
    const meta = std.meta;
    const builtin = @import("builtin");
    const errol = @import("fmt/errol.zig");
    const lossyCast = std.math.lossyCast;
    const expectFmt = std.testing.expectFmt;

    pub const default_max_depth = 3;

    pub const Alignment = enum {
        Left,
        Center,
        Right,
    };

    pub const FormatOptions = struct {
        precision: ?usize = null,
        width: ?usize = null,
        alignment: Alignment = .Right,
        fill: u8 = ' ',
    };

    /// Renders fmt string with args, calling `writer` with slices of bytes.
    /// If `writer` returns an error, the error is returned from `format` and
    /// `writer` is not called again.
    ///
    /// The format string must be comptime-known and may contain placeholders following
    /// this format:
    /// `{[argument][specifier]:[fill][alignment][width].[precision]}`
    ///
    /// Above, each word including its surrounding [ and ] is a parameter which you have to replace with something:
    ///
    /// - *argument* is either the numeric index or the field name of the argument that should be inserted
    ///   - when using a field name, you are required to enclose the field name (an identifier) in square
    ///     brackets, e.g. {[score]...} as opposed to the numeric index form which can be written e.g. {2...}
    /// - *specifier* is a type-dependent formatting option that determines how a type should formatted (see below)
    /// - *fill* is a single character which is used to pad the formatted text
    /// - *alignment* is one of the three characters `<`, `^` or `>`. they define if the text is *left*, *center*, or *right* aligned
    /// - *width* is the total width of the field in characters
    /// - *precision* specifies how many decimals a formatted number should have
    ///
    /// Note that most of the parameters are optional and may be omitted. Also you can leave out separators like `:` and `.` when
    /// all parameters after the separator are omitted.
    /// Only exception is the *fill* parameter. If *fill* is required, one has to specify *alignment* as well, as otherwise
    /// the digits after `:` is interpreted as *width*, not *fill*.
    ///
    /// The *specifier* has several options for types:
    /// - `x` and `X`: output numeric value in hexadecimal notation
    /// - `s`:
    ///   - for pointer-to-many and C pointers of u8, print as a C-string using zero-termination
    ///   - for slices of u8, print the entire slice as a string without zero-termination
    /// - `e`: output floating point value in scientific notation
    /// - `d`: output numeric value in decimal notation
    /// - `b`: output integer value in binary notation
    /// - `o`: output integer value in octal notation
    /// - `c`: output integer as an ASCII character. Integer type must have 8 bits at max.
    /// - `u`: output integer as an UTF-8 sequence. Integer type must have 21 bits at max.
    /// - `?`: output optional value as either the unwrapped value, or `null`; may be followed by a format specifier for the underlying value.
    /// - `!`: output error union value as either the unwrapped value, or the formatted error value; may be followed by a format specifier for the underlying value.
    /// - `*`: output the address of the value instead of the value itself.
    /// - `any`: output a value of any type using its default format.
    ///
    /// If a formatted user type contains a function of the type
    /// ```
    /// pub fn format(value: ?, comptime fmt: []const u8, options: std.fmt.FormatOptions, writer: anytype) !void
    /// ```
    /// with `?` being the type formatted, this function will be called instead of the default implementation.
    /// This allows user types to be formatted in a logical manner instead of dumping all fields of the type.
    ///
    /// A user type may be a `struct`, `vector`, `union` or `enum` type.
    ///
    /// To print literal curly braces, escape them by writing them twice, e.g. `{{` or `}}`.
    pub fn format(
        writer: anytype,
        comptime fmt: []const u8,
        args: anytype,
    ) !void {
        const ArgsType = @TypeOf(args);
        const args_type_info = @typeInfo(ArgsType);
        if (args_type_info != .Struct) {
            @compileError("expected tuple or struct argument, found " ++ @typeName(ArgsType));
        }

        const fields_info = args_type_info.Struct.fields;
        if (fields_info.len > max_format_args) {
            @compileError("32 arguments max are supported per format call");
        }

        @setEvalBranchQuota(2000000);
        comptime var arg_state: ArgState = .{ .args_len = fields_info.len };
        comptime var i = 0;
        inline while (i < fmt.len) {
            const start_index = i;

            inline while (i < fmt.len) : (i += 1) {
                switch (fmt[i]) {
                    '{', '}' => break,
                    else => {},
                }
            }

            comptime var end_index = i;
            comptime var unescape_brace = false;

            // Handle {{ and }}, those are un-escaped as single braces

            if (i + 1 < fmt.len and fmt[i + 1] == fmt[i]) {
                unescape_brace = true;
                // Make the first brace part of the literal...

                end_index += 1;
                // ...and skip both

                i += 2;
            }

            // Write out the literal

            if (start_index != end_index) {
                try writer.writeAll(fmt[start_index..end_index]);
            }

            // We've already skipped the other brace, restart the loop

            if (unescape_brace) continue;

            if (i >= fmt.len) break;

            if (fmt[i] == '}') {
                @compileError("missing opening {");
            }

            // Get past the {

            comptime assert(fmt[i] == '{');
            i += 1;

            const fmt_begin = i;
            // Find the closing brace

            inline while (i < fmt.len and fmt[i] != '}') : (i += 1) {}
            const fmt_end = i;

            if (i >= fmt.len) {
                @compileError("missing closing }");
            }

            // Get past the }

            comptime assert(fmt[i] == '}');
            i += 1;

            const placeholder = comptime parsePlaceholder(fmt[fmt_begin..fmt_end].*);
            const arg_pos = comptime switch (placeholder.arg) {
                .none => null,
                .number => |pos| pos,
                .named => |arg_name| meta.fieldIndex(ArgsType, arg_name) orelse
                    @compileError("no argument with name '" ++ arg_name ++ "'"),
            };

            const width = switch (placeholder.width) {
                .none => null,
                .number => |v| v,
                .named => |arg_name| blk: {
                    const arg_i = comptime meta.fieldIndex(ArgsType, arg_name) orelse
                        @compileError("no argument with name '" ++ arg_name ++ "'");
                    _ = comptime arg_state.nextArg(arg_i) orelse @compileError("too few arguments");
                    break :blk @field(args, arg_name);
                },
            };

            const precision = switch (placeholder.precision) {
                .none => null,
                .number => |v| v,
                .named => |arg_name| blk: {
                    const arg_i = comptime meta.fieldIndex(ArgsType, arg_name) orelse
                        @compileError("no argument with name '" ++ arg_name ++ "'");
                    _ = comptime arg_state.nextArg(arg_i) orelse @compileError("too few arguments");
                    break :blk @field(args, arg_name);
                },
            };

            const arg_to_print = comptime arg_state.nextArg(arg_pos) orelse
                @compileError("too few arguments");

            try formatType(
                @field(args, fields_info[arg_to_print].name),
                placeholder.specifier_arg,
                FormatOptions{
                    .fill = placeholder.fill,
                    .alignment = placeholder.alignment,
                    .width = width,
                    .precision = precision,
                },
                writer,
                default_max_depth,
            );
        }

        if (comptime arg_state.hasUnusedArgs()) {
            const missing_count = arg_state.args_len - @popCount(arg_state.used_args);
            switch (missing_count) {
                0 => unreachable,
                1 => @compileError("unused argument in '" ++ fmt ++ "'"),
                else => @compileError((comptime comptimePrint("{d}", .{missing_count})) ++ " unused arguments in '" ++ fmt ++ "'"),
            }
        }
    }

    fn parsePlaceholder(comptime str: anytype) Placeholder {
        comptime var parser = Parser{ .buf = &str };

        // Parse the positional argument number

        const arg = comptime parser.specifier() catch |err|
            @compileError(@errorName(err));

        // Parse the format specifier

        const specifier_arg = comptime parser.until(':');

        // Skip the colon, if present

        if (comptime parser.char()) |ch| {
            if (ch != ':') {
                @compileError("expected : or }, found '" ++ [1]u8{ch} ++ "'");
            }
        }

        // Parse the fill character

        // The fill parameter requires the alignment parameter to be specified

        // too

        const fill = comptime if (parser.peek(1)) |ch|
            switch (ch) {
                '<', '^', '>' => parser.char().?,
                else => ' ',
            }
        else
            ' ';

        // Parse the alignment parameter

        const alignment: Alignment = comptime if (parser.peek(0)) |ch| init: {
            switch (ch) {
                '<', '^', '>' => _ = parser.char(),
                else => {},
            }
            break :init switch (ch) {
                '<' => .Left,
                '^' => .Center,
                else => .Right,
            };
        } else .Right;

        // Parse the width parameter

        const width = comptime parser.specifier() catch |err|
            @compileError(@errorName(err));

        // Skip the dot, if present

        if (comptime parser.char()) |ch| {
            if (ch != '.') {
                @compileError("expected . or }, found '" ++ [1]u8{ch} ++ "'");
            }
        }

        // Parse the precision parameter

        const precision = comptime parser.specifier() catch |err|
            @compileError(@errorName(err));

        if (comptime parser.char()) |ch| {
            @compileError("extraneous trailing character '" ++ [1]u8{ch} ++ "'");
        }

        return Placeholder{
            .specifier_arg = cacheString(specifier_arg[0..specifier_arg.len].*),
            .fill = fill,
            .alignment = alignment,
            .arg = arg,
            .width = width,
            .precision = precision,
        };
    }

    fn cacheString(str: anytype) []const u8 {
        return &str;
    }

    const Placeholder = struct {
        specifier_arg: []const u8,
        fill: u8,
        alignment: Alignment,
        arg: Specifier,
        width: Specifier,
        precision: Specifier,
    };

    const Specifier = union(enum) {
        none,
        number: usize,
        named: []const u8,
    };

    const Parser = struct {
        buf: []const u8,
        pos: usize = 0,

        // Returns a decimal number or null if the current character is not a

        // digit

        fn number(self: *@This()) ?usize {
            var r: ?usize = null;

            while (self.pos < self.buf.len) : (self.pos += 1) {
                switch (self.buf[self.pos]) {
                    '0'...'9' => {
                        if (r == null) r = 0;
                        r.? *= 10;
                        r.? += self.buf[self.pos] - '0';
                    },
                    else => break,
                }
            }

            return r;
        }

        // Returns a substring of the input starting from the current position

        // and ending where `ch` is found or until the end if not found

        fn until(self: *@This(), ch: u8) []const u8 {
            const start = self.pos;

            if (start >= self.buf.len)
                return &[_]u8{};

            while (self.pos < self.buf.len) : (self.pos += 1) {
                if (self.buf[self.pos] == ch) break;
            }
            return self.buf[start..self.pos];
        }

        // Returns one character, if available

        fn char(self: *@This()) ?u8 {
            if (self.pos < self.buf.len) {
                const ch = self.buf[self.pos];
                self.pos += 1;
                return ch;
            }
            return null;
        }

        fn maybe(self: *@This(), val: u8) bool {
            if (self.pos < self.buf.len and self.buf[self.pos] == val) {
                self.pos += 1;
                return true;
            }
            return false;
        }

        // Returns a decimal number or null if the current character is not a

        // digit

        fn specifier(self: *@This()) !Specifier {
            if (self.maybe('[')) {
                const arg_name = self.until(']');

                if (!self.maybe(']'))
                    return @field(anyerror, "Expected closing ]");

                return Specifier{ .named = arg_name };
            }
            if (self.number()) |i|
                return Specifier{ .number = i };

            return Specifier{ .none = {} };
        }

        // Returns the n-th next character or null if that's past the end

        fn peek(self: *@This(), n: usize) ?u8 {
            return if (self.pos + n < self.buf.len) self.buf[self.pos + n] else null;
        }
    };

    const ArgSetType = u32;
    const max_format_args = @typeInfo(ArgSetType).Int.bits;

    const ArgState = struct {
        next_arg: usize = 0,
        used_args: ArgSetType = 0,
        args_len: usize,

        fn hasUnusedArgs(self: *@This()) bool {
            return @popCount(self.used_args) != self.args_len;
        }

        fn nextArg(self: *@This(), arg_index: ?usize) ?usize {
            const next_index = arg_index orelse init: {
                const arg = self.next_arg;
                self.next_arg += 1;
                break :init arg;
            };

            if (next_index >= self.args_len) {
                return null;
            }

            // Mark this argument as used

            self.used_args |= @as(ArgSetType, 1) << @intCast(u5, next_index);
            return next_index;
        }
    };

    pub fn formatAddress(value: anytype, options: FormatOptions, writer: anytype) @TypeOf(writer).Error!void {
        _ = options;
        const T = @TypeOf(value);

        switch (@typeInfo(T)) {
            .Pointer => |info| {
                try writer.writeAll(@typeName(info.child) ++ "@");
                if (info.size == .Slice)
                    try formatInt(@ptrToInt(value.ptr), 16, .lower, FormatOptions{}, writer)
                else
                    try formatInt(@ptrToInt(value), 16, .lower, FormatOptions{}, writer);
                return;
            },
            .Optional => |info| {
                if (@typeInfo(info.child) == .Pointer) {
                    try writer.writeAll(@typeName(info.child) ++ "@");
                    try formatInt(@ptrToInt(value), 16, .lower, FormatOptions{}, writer);
                    return;
                }
            },
            else => {},
        }

        @compileError("cannot format non-pointer type " ++ @typeName(T) ++ " with * specifier");
    }

    // This ANY const is a workaround for: https://github.com/ziglang/zig/issues/7948

    const ANY = "any";

    fn defaultSpec(comptime T: type) [:0]const u8 {
        switch (@typeInfo(T)) {
            .Array => |_| return ANY,
            .Pointer => |ptr_info| switch (ptr_info.size) {
                .One => switch (@typeInfo(ptr_info.child)) {
                    .Array => |_| return "*",
                    else => {},
                },
                .Many, .C => return "*",
                .Slice => return ANY,
            },
            .Optional => |info| return "?" ++ defaultSpec(info.child),
            .ErrorUnion => |info| return "!" ++ defaultSpec(info.payload),
            else => {},
        }
        return "";
    }

    fn stripOptionalOrErrorUnionSpec(comptime fmt: []const u8) []const u8 {
        return if (std.mem.eql(u8, fmt[1..], ANY))
            ANY
        else
            fmt[1..];
    }

    fn invalidFmtErr(comptime fmt: []const u8, value: anytype) void {
        @compileError("invalid format string '" ++ fmt ++ "' for type '" ++ @typeName(@TypeOf(value)) ++ "'");
    }

    pub fn formatType(
        value: anytype,
        comptime fmt: []const u8,
        options: FormatOptions,
        writer: anytype,
        max_depth: usize,
    ) @TypeOf(writer).Error!void {
        const T = @TypeOf(value);
        const actual_fmt = comptime if (std.mem.eql(u8, fmt, ANY))
            defaultSpec(@TypeOf(value))
        else if (fmt.len != 0 and (fmt[0] == '?' or fmt[0] == '!')) switch (@typeInfo(T)) {
            .Optional, .ErrorUnion => fmt,
            else => stripOptionalOrErrorUnionSpec(fmt),
        } else fmt;

        if (comptime std.mem.eql(u8, actual_fmt, "*")) {
            return formatAddress(value, options, writer);
        }

        if (comptime std.meta.trait.hasFn("format")(T)) {
            return try value.format(actual_fmt, options, writer);
        }

        switch (@typeInfo(T)) {
            .ComptimeInt, .Int, .ComptimeFloat, .Float => {
                return formatValue(value, actual_fmt, options, writer);
            },
            .Void => {
                if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
                return formatBuf("void", options, writer);
            },
            .Bool => {
                if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
                return formatBuf(if (value) "true" else "false", options, writer);
            },
            .Optional => {
                if (actual_fmt.len == 0 or actual_fmt[0] != '?')
                    @compileError("cannot format optional without a specifier (i.e. {?} or {any})");
                const remaining_fmt = comptime stripOptionalOrErrorUnionSpec(actual_fmt);
                if (value) |payload| {
                    return formatType(payload, remaining_fmt, options, writer, max_depth);
                } else {
                    return formatBuf("null", options, writer);
                }
            },
            .ErrorUnion => {
                if (actual_fmt.len == 0 or actual_fmt[0] != '!')
                    @compileError("cannot format error union without a specifier (i.e. {!} or {any})");
                const remaining_fmt = comptime stripOptionalOrErrorUnionSpec(actual_fmt);
                if (value) |payload| {
                    return formatType(payload, remaining_fmt, options, writer, max_depth);
                } else |err| {
                    return formatType(err, "", options, writer, max_depth);
                }
            },
            .ErrorSet => {
                if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
                try writer.writeAll("error.");
                return writer.writeAll(@errorName(value));
            },
            .Enum => |enumInfo| {
                try writer.writeAll(@typeName(T));
                if (enumInfo.is_exhaustive) {
                    if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
                    try writer.writeAll(".");
                    try writer.writeAll(@tagName(value));
                    return;
                }

                // Use @tagName only if value is one of known fields

                @setEvalBranchQuota(3 * enumInfo.fields.len);
                inline for (enumInfo.fields) |enumField| {
                    if (@enumToInt(value) == enumField.value) {
                        try writer.writeAll(".");
                        try writer.writeAll(@tagName(value));
                        return;
                    }
                }

                try writer.writeAll("(");
                try formatType(@enumToInt(value), actual_fmt, options, writer, max_depth);
                try writer.writeAll(")");
            },
            .Union => |info| {
                if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
                try writer.writeAll(@typeName(T));
                if (max_depth == 0) {
                    return writer.writeAll("{ ... }");
                }
                if (info.tag_type) |UnionTagType| {
                    try writer.writeAll("{ .");
                    try writer.writeAll(@tagName(@as(UnionTagType, value)));
                    try writer.writeAll(" = ");
                    inline for (info.fields) |u_field| {
                        if (value == @field(UnionTagType, u_field.name)) {
                            try formatType(@field(value, u_field.name), ANY, options, writer, max_depth - 1);
                        }
                    }
                    try writer.writeAll(" }");
                } else {
                    try format(writer, "@{x}", .{@ptrToInt(&value)});
                }
            },
            .Struct => |info| {
                if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
                if (info.is_tuple) {
                    // Skip the type and field names when formatting tuples.

                    if (max_depth == 0) {
                        return writer.writeAll("{ ... }");
                    }
                    try writer.writeAll("{");
                    inline for (info.fields) |f, i| {
                        if (i == 0) {
                            try writer.writeAll(" ");
                        } else {
                            try writer.writeAll(", ");
                        }
                        try formatType(@field(value, f.name), ANY, options, writer, max_depth - 1);
                    }
                    return writer.writeAll(" }");
                }
                try writer.writeAll(@typeName(T));
                if (max_depth == 0) {
                    return writer.writeAll("{ ... }");
                }
                try writer.writeAll("{");
                inline for (info.fields) |f, i| {
                    if (i == 0) {
                        try writer.writeAll(" .");
                    } else {
                        try writer.writeAll(", .");
                    }
                    try writer.writeAll(f.name);
                    try writer.writeAll(" = ");
                    try formatType(@field(value, f.name), ANY, options, writer, max_depth - 1);
                }
                try writer.writeAll(" }");
            },
            .Pointer => |ptr_info| switch (ptr_info.size) {
                .One => switch (@typeInfo(ptr_info.child)) {
                    .Array => |info| {
                        if (actual_fmt.len == 0)
                            @compileError("cannot format array ref without a specifier (i.e. {s} or {*})");
                        if (info.child == u8) {
                            switch (actual_fmt[0]) {
                                's', 'x', 'X', 'e', 'E' => {
                                    comptime checkTextFmt(actual_fmt);
                                    return formatBuf(value, options, writer);
                                },
                                else => {},
                            }
                        }
                        if (comptime std.meta.trait.isZigString(info.child)) {
                            for (value) |item, i| {
                                comptime checkTextFmt(actual_fmt);
                                if (i != 0) try formatBuf(", ", options, writer);
                                try formatBuf(item, options, writer);
                            }
                            return;
                        }
                        invalidFmtErr(fmt, value);
                    },
                    .Enum, .Union, .Struct => {
                        return formatType(value.*, actual_fmt, options, writer, max_depth);
                    },
                    else => return format(writer, "{s}@{x}", .{ @typeName(ptr_info.child), @ptrToInt(value) }),
                },
                .Many, .C => {
                    if (actual_fmt.len == 0)
                        @compileError("cannot format pointer without a specifier (i.e. {s} or {*})");
                    if (ptr_info.sentinel) |_| {
                        return formatType(mem.span(value), actual_fmt, options, writer, max_depth);
                    }
                    if (ptr_info.child == u8) {
                        switch (actual_fmt[0]) {
                            's', 'x', 'X', 'e', 'E' => {
                                comptime checkTextFmt(actual_fmt);
                                return formatBuf(mem.span(value), options, writer);
                            },
                            else => {},
                        }
                    }
                    invalidFmtErr(fmt, value);
                },
                .Slice => {
                    if (actual_fmt.len == 0)
                        @compileError("cannot format slice without a specifier (i.e. {s} or {any})");
                    if (max_depth == 0) {
                        return writer.writeAll("{ ... }");
                    }
                    if (ptr_info.child == u8) {
                        switch (actual_fmt[0]) {
                            's', 'x', 'X', 'e', 'E' => {
                                comptime checkTextFmt(actual_fmt);
                                return formatBuf(value, options, writer);
                            },
                            else => {},
                        }
                    }
                    try writer.writeAll("{ ");
                    for (value) |elem, i| {
                        try formatType(elem, actual_fmt, options, writer, max_depth - 1);
                        if (i != value.len - 1) {
                            try writer.writeAll(", ");
                        }
                    }
                    try writer.writeAll(" }");
                },
            },
            .Array => |info| {
                if (actual_fmt.len == 0)
                    @compileError("cannot format array without a specifier (i.e. {s} or {any})");
                if (max_depth == 0) {
                    return writer.writeAll("{ ... }");
                }
                if (info.child == u8) {
                    switch (actual_fmt[0]) {
                        's', 'x', 'X', 'e', 'E' => {
                            comptime checkTextFmt(actual_fmt);
                            return formatBuf(&value, options, writer);
                        },
                        else => {},
                    }
                }
                try writer.writeAll("{ ");
                for (value) |elem, i| {
                    try formatType(elem, actual_fmt, options, writer, max_depth - 1);
                    if (i < value.len - 1) {
                        try writer.writeAll(", ");
                    }
                }
                try writer.writeAll(" }");
            },
            .Vector => |info| {
                try writer.writeAll("{ ");
                var i: usize = 0;
                while (i < info.len) : (i += 1) {
                    try formatValue(value[i], actual_fmt, options, writer);
                    if (i < info.len - 1) {
                        try writer.writeAll(", ");
                    }
                }
                try writer.writeAll(" }");
            },
            .Fn => {
                if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
                return format(writer, "{s}@{x}", .{ @typeName(T), @ptrToInt(value) });
            },
            .Type => {
                if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
                return formatBuf(@typeName(value), options, writer);
            },
            .EnumLiteral => {
                if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
                const buffer = [_]u8{'.'} ++ @tagName(value);
                return formatBuf(buffer, options, writer);
            },
            .Null => {
                if (actual_fmt.len != 0) invalidFmtErr(fmt, value);
                return formatBuf("null", options, writer);
            },
            else => @compileError("unable to format type '" ++ @typeName(T) ++ "'"),
        }
    }

    fn formatValue(
        value: anytype,
        comptime fmt: []const u8,
        options: FormatOptions,
        writer: anytype,
    ) !void {
        if (comptime std.mem.eql(u8, fmt, "B")) {
            @compileError("specifier 'B' has been deprecated, wrap your argument in std.fmt.fmtIntSizeDec instead");
        } else if (comptime std.mem.eql(u8, fmt, "Bi")) {
            @compileError("specifier 'Bi' has been deprecated, wrap your argument in std.fmt.fmtIntSizeBin instead");
        }

        const T = @TypeOf(value);
        switch (@typeInfo(T)) {
            .Float, .ComptimeFloat => return formatFloatValue(value, fmt, options, writer),
            .Int, .ComptimeInt => return formatIntValue(value, fmt, options, writer),
            .Bool => return formatBuf(if (value) "true" else "false", options, writer),
            else => comptime unreachable,
        }
    }

    pub fn formatIntValue(
        value: anytype,
        comptime fmt: []const u8,
        options: FormatOptions,
        writer: anytype,
    ) !void {
        comptime var radix = 10;
        comptime var case: Case = .lower;

        const int_value = if (@TypeOf(value) == comptime_int) blk: {
            const Int = math.IntFittingRange(value, value);
            break :blk @as(Int, value);
        } else value;

        if (fmt.len == 0 or comptime std.mem.eql(u8, fmt, "d")) {
            radix = 10;
            case = .lower;
        } else if (comptime std.mem.eql(u8, fmt, "c")) {
            if (@typeInfo(@TypeOf(int_value)).Int.bits <= 8) {
                return formatAsciiChar(@as(u8, int_value), options, writer);
            } else {
                @compileError("cannot print integer that is larger than 8 bits as an ASCII character");
            }
        } else if (comptime std.mem.eql(u8, fmt, "u")) {
            if (@typeInfo(@TypeOf(int_value)).Int.bits <= 21) {
                return formatUnicodeCodepoint(@as(u21, int_value), options, writer);
            } else {
                @compileError("cannot print integer that is larger than 21 bits as an UTF-8 sequence");
            }
        } else if (comptime std.mem.eql(u8, fmt, "b")) {
            radix = 2;
            case = .lower;
        } else if (comptime std.mem.eql(u8, fmt, "x")) {
            radix = 16;
            case = .lower;
        } else if (comptime std.mem.eql(u8, fmt, "X")) {
            radix = 16;
            case = .upper;
        } else if (comptime std.mem.eql(u8, fmt, "o")) {
            radix = 8;
            case = .lower;
        } else {
            invalidFmtErr(fmt, value);
        }

        return formatInt(int_value, radix, case, options, writer);
    }

    fn formatFloatValue(
        value: anytype,
        comptime fmt: []const u8,
        options: FormatOptions,
        writer: anytype,
    ) !void {
        // this buffer should be enough to display all decimal places of a decimal f64 number.

        var buf: [512]u8 = undefined;
        var buf_stream = std.io.fixedBufferStream(&buf);

        if (fmt.len == 0 or comptime std.mem.eql(u8, fmt, "e")) {
            formatFloatScientific(value, options, buf_stream.writer()) catch |err| switch (err) {
                error.NoSpaceLeft => unreachable,
            };
        } else if (comptime std.mem.eql(u8, fmt, "d")) {
            formatFloatDecimal(value, options, buf_stream.writer()) catch |err| switch (err) {
                error.NoSpaceLeft => unreachable,
            };
        } else if (comptime std.mem.eql(u8, fmt, "x")) {
            formatFloatHexadecimal(value, options, buf_stream.writer()) catch |err| switch (err) {
                error.NoSpaceLeft => unreachable,
            };
        } else {
            invalidFmtErr(fmt, value);
        }

        return formatBuf(buf_stream.getWritten(), options, writer);
    }

    pub const Case = enum { lower, upper };

    fn formatSliceHexImpl(comptime case: Case) type {
        const charset = "0123456789" ++ if (case == .upper) "ABCDEF" else "abcdef";

        return struct {
            pub fn f(
                bytes: []const u8,
                comptime fmt: []const u8,
                options: std.fmt.FormatOptions,
                writer: anytype,
            ) !void {
                _ = fmt;
                _ = options;
                var buf: [2]u8 = undefined;

                for (bytes) |c| {
                    buf[0] = charset[c >> 4];
                    buf[1] = charset[c & 15];
                    try writer.writeAll(&buf);
                }
            }
        };
    }

    const formatSliceHexLower = formatSliceHexImpl(.lower).f;
    const formatSliceHexUpper = formatSliceHexImpl(.upper).f;

    /// Return a Formatter for a []const u8 where every byte is formatted as a pair
    /// of lowercase hexadecimal digits.
    pub fn fmtSliceHexLower(bytes: []const u8) std.fmt.Formatter(formatSliceHexLower) {
        return .{ .data = bytes };
    }

    /// Return a Formatter for a []const u8 where every byte is formatted as pair
    /// of uppercase hexadecimal digits.
    pub fn fmtSliceHexUpper(bytes: []const u8) std.fmt.Formatter(formatSliceHexUpper) {
        return .{ .data = bytes };
    }

    fn formatSliceEscapeImpl(comptime case: Case) type {
        const charset = "0123456789" ++ if (case == .upper) "ABCDEF" else "abcdef";

        return struct {
            pub fn f(
                bytes: []const u8,
                comptime fmt: []const u8,
                options: std.fmt.FormatOptions,
                writer: anytype,
            ) !void {
                _ = fmt;
                _ = options;
                var buf: [4]u8 = undefined;

                buf[0] = '\\';
                buf[1] = 'x';

                for (bytes) |c| {
                    if (std.ascii.isPrint(c)) {
                        try writer.writeByte(c);
                    } else {
                        buf[2] = charset[c >> 4];
                        buf[3] = charset[c & 15];
                        try writer.writeAll(&buf);
                    }
                }
            }
        };
    }

    const formatSliceEscapeLower = formatSliceEscapeImpl(.lower).f;
    const formatSliceEscapeUpper = formatSliceEscapeImpl(.upper).f;

    /// Return a Formatter for a []const u8 where every non-printable ASCII
    /// character is escaped as \xNN, where NN is the character in lowercase
    /// hexadecimal notation.
    pub fn fmtSliceEscapeLower(bytes: []const u8) std.fmt.Formatter(formatSliceEscapeLower) {
        return .{ .data = bytes };
    }

    /// Return a Formatter for a []const u8 where every non-printable ASCII
    /// character is escaped as \xNN, where NN is the character in uppercase
    /// hexadecimal notation.
    pub fn fmtSliceEscapeUpper(bytes: []const u8) std.fmt.Formatter(formatSliceEscapeUpper) {
        return .{ .data = bytes };
    }

    fn formatSizeImpl(comptime radix: comptime_int) type {
        return struct {
            fn f(
                value: u64,
                comptime fmt: []const u8,
                options: FormatOptions,
                writer: anytype,
            ) !void {
                _ = fmt;
                if (value == 0) {
                    return formatBuf("0B", options, writer);
                }
                // The worst case in terms of space needed is 32 bytes + 3 for the suffix.

                var buf: [35]u8 = undefined;
                var bufstream = io.fixedBufferStream(buf[0..]);

                const mags_si = " kMGTPEZY";
                const mags_iec = " KMGTPEZY";

                const log2 = math.log2(value);
                const magnitude = switch (radix) {
                    1000 => math.min(log2 / comptime math.log2(1000), mags_si.len - 1),
                    1024 => math.min(log2 / 10, mags_iec.len - 1),
                    else => unreachable,
                };
                const new_value = lossyCast(f64, value) / math.pow(f64, lossyCast(f64, radix), lossyCast(f64, magnitude));
                const suffix = switch (radix) {
                    1000 => mags_si[magnitude],
                    1024 => mags_iec[magnitude],
                    else => unreachable,
                };

                formatFloatDecimal(new_value, options, bufstream.writer()) catch |err| switch (err) {
                    error.NoSpaceLeft => unreachable, // 35 bytes should be enough

                };

                bufstream.writer().writeAll(if (suffix == ' ')
                    "B"
                else switch (radix) {
                    1000 => &[_]u8{ suffix, 'B' },
                    1024 => &[_]u8{ suffix, 'i', 'B' },
                    else => unreachable,
                }) catch |err| switch (err) {
                    error.NoSpaceLeft => unreachable,
                };
                return formatBuf(bufstream.getWritten(), options, writer);
            }
        };
    }

    const formatSizeDec = formatSizeImpl(1000).f;
    const formatSizeBin = formatSizeImpl(1024).f;

    /// Return a Formatter for a u64 value representing a file size.
    /// This formatter represents the number as multiple of 1000 and uses the SI
    /// measurement units (kB, MB, GB, ...).
    pub fn fmtIntSizeDec(value: u64) std.fmt.Formatter(formatSizeDec) {
        return .{ .data = value };
    }

    /// Return a Formatter for a u64 value representing a file size.
    /// This formatter represents the number as multiple of 1024 and uses the IEC
    /// measurement units (KiB, MiB, GiB, ...).
    pub fn fmtIntSizeBin(value: u64) std.fmt.Formatter(formatSizeBin) {
        return .{ .data = value };
    }

    fn checkTextFmt(comptime fmt: []const u8) void {
        if (fmt.len != 1)
            @compileError("unsupported format string '" ++ fmt ++ "' when formatting text");
        switch (fmt[0]) {
            'x' => @compileError("specifier 'x' has been deprecated, wrap your argument in std.fmt.fmtSliceHexLower instead"),
            'X' => @compileError("specifier 'X' has been deprecated, wrap your argument in std.fmt.fmtSliceHexUpper instead"),
            'e' => @compileError("specifier 'e' has been deprecated, wrap your argument in std.fmt.fmtSliceEscapeLower instead"),
            'E' => @compileError("specifier 'E' has been deprecated, wrap your argument in std.fmt.fmtSliceEscapeUpper instead"),
            'z' => @compileError("specifier 'z' has been deprecated, wrap your argument in std.zig.fmtId instead"),
            'Z' => @compileError("specifier 'Z' has been deprecated, wrap your argument in std.zig.fmtEscapes instead"),
            else => {},
        }
    }

    pub fn formatText(
        bytes: []const u8,
        comptime fmt: []const u8,
        options: FormatOptions,
        writer: anytype,
    ) !void {
        comptime checkTextFmt(fmt);
        return formatBuf(bytes, options, writer);
    }

    pub fn formatAsciiChar(
        c: u8,
        options: FormatOptions,
        writer: anytype,
    ) !void {
        return formatBuf(@as(*const [1]u8, &c), options, writer);
    }

    pub fn formatUnicodeCodepoint(
        c: u21,
        options: FormatOptions,
        writer: anytype,
    ) !void {
        var buf: [4]u8 = undefined;
        const len = unicode.utf8Encode(c, &buf) catch |err| switch (err) {
            error.Utf8CannotEncodeSurrogateHalf, error.CodepointTooLarge => {
                const len = unicode.utf8Encode(unicode.replacement_character, &buf) catch unreachable;
                return formatBuf(buf[0..len], options, writer);
            },
        };
        return formatBuf(buf[0..len], options, writer);
    }

    pub fn formatBuf(
        buf: []const u8,
        options: FormatOptions,
        writer: anytype,
    ) !void {
        if (options.width) |min_width| {
            // In case of error assume the buffer content is ASCII-encoded

            const width = unicode.utf8CountCodepoints(buf) catch buf.len;
            const padding = if (width < min_width) min_width - width else 0;

            if (padding == 0)
                return writer.writeAll(buf);

            switch (options.alignment) {
                .Left => {
                    try writer.writeAll(buf);
                    try writer.writeByteNTimes(options.fill, padding);
                },
                .Center => {
                    const left_padding = padding / 2;
                    const right_padding = (padding + 1) / 2;
                    try writer.writeByteNTimes(options.fill, left_padding);
                    try writer.writeAll(buf);
                    try writer.writeByteNTimes(options.fill, right_padding);
                },
                .Right => {
                    try writer.writeByteNTimes(options.fill, padding);
                    try writer.writeAll(buf);
                },
            }
        } else {
            // Fast path, avoid counting the number of codepoints

            try writer.writeAll(buf);
        }
    }

    /// Print a float in scientific notation to the specified precision. Null uses full precision.
    /// It should be the case that every full precision, printed value can be re-parsed back to the
    /// same type unambiguously.
    pub fn formatFloatScientific(
        value: anytype,
        options: FormatOptions,
        writer: anytype,
    ) !void {
        var x = @floatCast(f64, value);

        // Errol doesn't handle these special cases.

        if (math.signbit(x)) {
            try writer.writeAll("-");
            x = -x;
        }

        if (math.isNan(x)) {
            return writer.writeAll("nan");
        }
        if (math.isPositiveInf(x)) {
            return writer.writeAll("inf");
        }
        if (x == 0.0) {
            try writer.writeAll("0");

            if (options.precision) |precision| {
                if (precision != 0) {
                    try writer.writeAll(".");
                    var i: usize = 0;
                    while (i < precision) : (i += 1) {
                        try writer.writeAll("0");
                    }
                }
            } else {
                try writer.writeAll(".0");
            }

            try writer.writeAll("e+00");
            return;
        }

        var buffer: [32]u8 = undefined;
        var float_decimal = errol.errol3(x, buffer[0..]);

        if (options.precision) |precision| {
            errol.roundToPrecision(&float_decimal, precision, errol.RoundMode.Scientific);

            try writer.writeAll(float_decimal.digits[0..1]);

            // {e0} case prints no `.`

            if (precision != 0) {
                try writer.writeAll(".");

                var printed: usize = 0;
                if (float_decimal.digits.len > 1) {
                    const num_digits = math.min(float_decimal.digits.len, precision + 1);
                    try writer.writeAll(float_decimal.digits[1..num_digits]);
                    printed += num_digits - 1;
                }

                while (printed < precision) : (printed += 1) {
                    try writer.writeAll("0");
                }
            }
        } else {
            try writer.writeAll(float_decimal.digits[0..1]);
            try writer.writeAll(".");
            if (float_decimal.digits.len > 1) {
                const num_digits = if (@TypeOf(value) == f32) math.min(@as(usize, 9), float_decimal.digits.len) else float_decimal.digits.len;

                try writer.writeAll(float_decimal.digits[1..num_digits]);
            } else {
                try writer.writeAll("0");
            }
        }

        try writer.writeAll("e");
        const exp = float_decimal.exp - 1;

        if (exp >= 0) {
            try writer.writeAll("+");
            if (exp > -10 and exp < 10) {
                try writer.writeAll("0");
            }
            try formatInt(exp, 10, .lower, FormatOptions{ .width = 0 }, writer);
        } else {
            try writer.writeAll("-");
            if (exp > -10 and exp < 10) {
                try writer.writeAll("0");
            }
            try formatInt(-exp, 10, .lower, FormatOptions{ .width = 0 }, writer);
        }
    }

    pub fn formatFloatHexadecimal(
        value: anytype,
        options: FormatOptions,
        writer: anytype,
    ) !void {
        if (math.signbit(value)) {
            try writer.writeByte('-');
        }
        if (math.isNan(value)) {
            return writer.writeAll("nan");
        }
        if (math.isInf(value)) {
            return writer.writeAll("inf");
        }

        const T = @TypeOf(value);
        const TU = std.meta.Int(.unsigned, @bitSizeOf(T));

        const mantissa_bits = math.floatMantissaBits(T);
        const fractional_bits = math.floatFractionalBits(T);
        const exponent_bits = math.floatExponentBits(T);
        const mantissa_mask = (1 << mantissa_bits) - 1;
        const exponent_mask = (1 << exponent_bits) - 1;
        const exponent_bias = (1 << (exponent_bits - 1)) - 1;

        const as_bits = @bitCast(TU, value);
        var mantissa = as_bits & mantissa_mask;
        var exponent: i32 = @truncate(u16, (as_bits >> mantissa_bits) & exponent_mask);

        const is_denormal = exponent == 0 and mantissa != 0;
        const is_zero = exponent == 0 and mantissa == 0;

        if (is_zero) {
            // Handle this case here to simplify the logic below.

            try writer.writeAll("0x0");
            if (options.precision) |precision| {
                if (precision > 0) {
                    try writer.writeAll(".");
                    try writer.writeByteNTimes('0', precision);
                }
            } else {
                try writer.writeAll(".0");
            }
            try writer.writeAll("p0");
            return;
        }

        if (is_denormal) {
            // Adjust the exponent for printing.

            exponent += 1;
        } else {
            if (fractional_bits == mantissa_bits)
                mantissa |= 1 << fractional_bits; // Add the implicit integer bit.

        }

        const mantissa_digits = (fractional_bits + 3) / 4;
        // Fill in zeroes to round the fraction width to a multiple of 4.

        mantissa <<= mantissa_digits * 4 - fractional_bits;

        if (options.precision) |precision| {
            // Round if needed.

            if (precision < mantissa_digits) {
                // We always have at least 4 extra bits.

                var extra_bits = (mantissa_digits - precision) * 4;
                // The result LSB is the Guard bit, we need two more (Round and

                // Sticky) to round the value.

                while (extra_bits > 2) {
                    mantissa = (mantissa >> 1) | (mantissa & 1);
                    extra_bits -= 1;
                }
                // Round to nearest, tie to even.

                mantissa |= @boolToInt(mantissa & 0b100 != 0);
                mantissa += 1;
                // Drop the excess bits.

                mantissa >>= 2;
                // Restore the alignment.

                mantissa <<= @intCast(math.Log2Int(TU), (mantissa_digits - precision) * 4);

                const overflow = mantissa & (1 << 1 + mantissa_digits * 4) != 0;
                // Prefer a normalized result in case of overflow.

                if (overflow) {
                    mantissa >>= 1;
                    exponent += 1;
                }
            }
        }

        // +1 for the decimal part.

        var buf: [1 + mantissa_digits]u8 = undefined;
        _ = formatIntBuf(&buf, mantissa, 16, .lower, .{ .fill = '0', .width = 1 + mantissa_digits });

        try writer.writeAll("0x");
        try writer.writeByte(buf[0]);
        const trimmed = mem.trimRight(u8, buf[1..], "0");
        if (options.precision) |precision| {
            if (precision > 0) try writer.writeAll(".");
        } else if (trimmed.len > 0) {
            try writer.writeAll(".");
        }
        try writer.writeAll(trimmed);
        // Add trailing zeros if explicitly requested.

        if (options.precision) |precision| if (precision > 0) {
            if (precision > trimmed.len)
                try writer.writeByteNTimes('0', precision - trimmed.len);
        };
        try writer.writeAll("p");
        try formatInt(exponent - exponent_bias, 10, .lower, .{}, writer);
    }

    /// Print a float of the format x.yyyyy where the number of y is specified by the precision argument.
    /// By default floats are printed at full precision (no rounding).
    pub fn formatFloatDecimal(
        value: anytype,
        options: FormatOptions,
        writer: anytype,
    ) !void {
        var x = @as(f64, value);

        // Errol doesn't handle these special cases.

        if (math.signbit(x)) {
            try writer.writeAll("-");
            x = -x;
        }

        if (math.isNan(x)) {
            return writer.writeAll("nan");
        }
        if (math.isPositiveInf(x)) {
            return writer.writeAll("inf");
        }
        if (x == 0.0) {
            try writer.writeAll("0");

            if (options.precision) |precision| {
                if (precision != 0) {
                    try writer.writeAll(".");
                    var i: usize = 0;
                    while (i < precision) : (i += 1) {
                        try writer.writeAll("0");
                    }
                }
            }

            return;
        }

        // non-special case, use errol3

        var buffer: [32]u8 = undefined;
        var float_decimal = errol.errol3(x, buffer[0..]);

        if (options.precision) |precision| {
            errol.roundToPrecision(&float_decimal, precision, errol.RoundMode.Decimal);

            // exp < 0 means the leading is always 0 as errol result is normalized.

            var num_digits_whole = if (float_decimal.exp > 0) @intCast(usize, float_decimal.exp) else 0;

            // the actual slice into the buffer, we may need to zero-pad between num_digits_whole and this.

            var num_digits_whole_no_pad = math.min(num_digits_whole, float_decimal.digits.len);

            if (num_digits_whole > 0) {
                // We may have to zero pad, for instance 1e4 requires zero padding.

                try writer.writeAll(float_decimal.digits[0..num_digits_whole_no_pad]);

                var i = num_digits_whole_no_pad;
                while (i < num_digits_whole) : (i += 1) {
                    try writer.writeAll("0");
                }
            } else {
                try writer.writeAll("0");
            }

            // {.0} special case doesn't want a trailing '.'

            if (precision == 0) {
                return;
            }

            try writer.writeAll(".");

            // Keep track of fractional count printed for case where we pre-pad then post-pad with 0's.

            var printed: usize = 0;

            // Zero-fill until we reach significant digits or run out of precision.

            if (float_decimal.exp <= 0) {
                const zero_digit_count = @intCast(usize, -float_decimal.exp);
                const zeros_to_print = math.min(zero_digit_count, precision);

                var i: usize = 0;
                while (i < zeros_to_print) : (i += 1) {
                    try writer.writeAll("0");
                    printed += 1;
                }

                if (printed >= precision) {
                    return;
                }
            }

            // Remaining fractional portion, zero-padding if insufficient.

            assert(precision >= printed);
            if (num_digits_whole_no_pad + precision - printed < float_decimal.digits.len) {
                try writer.writeAll(float_decimal.digits[num_digits_whole_no_pad .. num_digits_whole_no_pad + precision - printed]);
                return;
            } else {
                try writer.writeAll(float_decimal.digits[num_digits_whole_no_pad..]);
                printed += float_decimal.digits.len - num_digits_whole_no_pad;

                while (printed < precision) : (printed += 1) {
                    try writer.writeAll("0");
                }
            }
        } else {
            // exp < 0 means the leading is always 0 as errol result is normalized.

            var num_digits_whole = if (float_decimal.exp > 0) @intCast(usize, float_decimal.exp) else 0;

            // the actual slice into the buffer, we may need to zero-pad between num_digits_whole and this.

            var num_digits_whole_no_pad = math.min(num_digits_whole, float_decimal.digits.len);

            if (num_digits_whole > 0) {
                // We may have to zero pad, for instance 1e4 requires zero padding.

                try writer.writeAll(float_decimal.digits[0..num_digits_whole_no_pad]);

                var i = num_digits_whole_no_pad;
                while (i < num_digits_whole) : (i += 1) {
                    try writer.writeAll("0");
                }
            } else {
                try writer.writeAll("0");
            }

            // Omit `.` if no fractional portion

            if (float_decimal.exp >= 0 and num_digits_whole_no_pad == float_decimal.digits.len) {
                return;
            }

            try writer.writeAll(".");

            // Zero-fill until we reach significant digits or run out of precision.

            if (float_decimal.exp < 0) {
                const zero_digit_count = @intCast(usize, -float_decimal.exp);

                var i: usize = 0;
                while (i < zero_digit_count) : (i += 1) {
                    try writer.writeAll("0");
                }
            }

            try writer.writeAll(float_decimal.digits[num_digits_whole_no_pad..]);
        }
    }

    pub fn formatInt(
        value: anytype,
        base: u8,
        case: Case,
        options: FormatOptions,
        writer: anytype,
    ) !void {
        assert(base >= 2);

        const int_value = if (@TypeOf(value) == comptime_int) blk: {
            const Int = math.IntFittingRange(value, value);
            break :blk @as(Int, value);
        } else value;

        const value_info = @typeInfo(@TypeOf(int_value)).Int;

        // The type must have the same size as `base` or be wider in order for the

        // division to work

        const min_int_bits = comptime math.max(value_info.bits, 8);
        const MinInt = std.meta.Int(.unsigned, min_int_bits);

        const abs_value = math.absCast(int_value);
        // The worst case in terms of space needed is base 2, plus 1 for the sign

        var buf: [1 + math.max(value_info.bits, 1)]u8 = undefined;

        var a: MinInt = abs_value;
        var index: usize = buf.len;
        while (true) {
            const digit = a % base;
            index -= 1;
            buf[index] = digitToChar(@intCast(u8, digit), case);
            a /= base;
            if (a == 0) break;
        }

        if (value_info.signedness == .signed) {
            if (value < 0) {
                // Negative integer

                index -= 1;
                buf[index] = '-';
            } else if (options.width == null or options.width.? == 0) {
                // Positive integer, omit the plus sign

            } else {
                // Positive integer

                index -= 1;
                buf[index] = '+';
            }
        }

        return formatBuf(buf[index..], options, writer);
    }

    pub fn formatIntBuf(out_buf: []u8, value: anytype, base: u8, case: Case, options: FormatOptions) usize {
        var fbs = std.io.fixedBufferStream(out_buf);
        formatInt(value, base, case, options, fbs.writer()) catch unreachable;
        return fbs.pos;
    }

    const FormatDurationData = struct {
        ns: u64,
        negative: bool = false,
    };

    fn formatDuration(data: FormatDurationData, comptime fmt: []const u8, options: std.fmt.FormatOptions, writer: anytype) !void {
        _ = fmt;

        // worst case: "-XXXyXXwXXdXXhXXmXX.XXXs".len = 24

        var buf: [24]u8 = undefined;
        var fbs = std.io.fixedBufferStream(&buf);
        var buf_writer = fbs.writer();
        if (data.negative) {
            try buf_writer.writeByte('-');
        }

        var ns_remaining = data.ns;
        inline for (.{
            .{ .ns = 365 * std.time.ns_per_day, .sep = 'y' },
            .{ .ns = std.time.ns_per_week, .sep = 'w' },
            .{ .ns = std.time.ns_per_day, .sep = 'd' },
            .{ .ns = std.time.ns_per_hour, .sep = 'h' },
            .{ .ns = std.time.ns_per_min, .sep = 'm' },
        }) |unit| {
            if (ns_remaining >= unit.ns) {
                const units = ns_remaining / unit.ns;
                try formatInt(units, 10, .lower, .{}, buf_writer);
                try buf_writer.writeByte(unit.sep);
                ns_remaining -= units * unit.ns;
                if (ns_remaining == 0)
                    return formatBuf(fbs.getWritten(), options, writer);
            }
        }

        inline for (.{
            .{ .ns = std.time.ns_per_s, .sep = "s" },
            .{ .ns = std.time.ns_per_ms, .sep = "ms" },
            .{ .ns = std.time.ns_per_us, .sep = "us" },
        }) |unit| {
            const kunits = ns_remaining * 1000 / unit.ns;
            if (kunits >= 1000) {
                try formatInt(kunits / 1000, 10, .lower, .{}, buf_writer);
                const frac = kunits % 1000;
                if (frac > 0) {
                    // Write up to 3 decimal places

                    var decimal_buf = [_]u8{ '.', 0, 0, 0 };
                    _ = formatIntBuf(decimal_buf[1..], frac, 10, .lower, .{ .fill = '0', .width = 3 });
                    var end: usize = 4;
                    while (end > 1) : (end -= 1) {
                        if (decimal_buf[end - 1] != '0') break;
                    }
                    try buf_writer.writeAll(decimal_buf[0..end]);
                }
                try buf_writer.writeAll(unit.sep);
                return formatBuf(fbs.getWritten(), options, writer);
            }
        }

        try formatInt(ns_remaining, 10, .lower, .{}, buf_writer);
        try buf_writer.writeAll("ns");
        return formatBuf(fbs.getWritten(), options, writer);
    }

    /// Return a Formatter for number of nanoseconds according to its magnitude:
    /// [#y][#w][#d][#h][#m]#[.###][n|u|m]s
    pub fn fmtDuration(ns: u64) Formatter(formatDuration) {
        const data = FormatDurationData{ .ns = ns };
        return .{ .data = data };
    }

    test "fmtDuration" {
        var buf: [24]u8 = undefined;
        inline for (.{
            .{ .s = "0ns", .d = 0 },
            .{ .s = "1ns", .d = 1 },
            .{ .s = "999ns", .d = std.time.ns_per_us - 1 },
            .{ .s = "1us", .d = std.time.ns_per_us },
            .{ .s = "1.45us", .d = 1450 },
            .{ .s = "1.5us", .d = 3 * std.time.ns_per_us / 2 },
            .{ .s = "14.5us", .d = 14500 },
            .{ .s = "145us", .d = 145000 },
            .{ .s = "999.999us", .d = std.time.ns_per_ms - 1 },
            .{ .s = "1ms", .d = std.time.ns_per_ms + 1 },
            .{ .s = "1.5ms", .d = 3 * std.time.ns_per_ms / 2 },
            .{ .s = "1.11ms", .d = 1110000 },
            .{ .s = "1.111ms", .d = 1111000 },
            .{ .s = "1.111ms", .d = 1111100 },
            .{ .s = "999.999ms", .d = std.time.ns_per_s - 1 },
            .{ .s = "1s", .d = std.time.ns_per_s },
            .{ .s = "59.999s", .d = std.time.ns_per_min - 1 },
            .{ .s = "1m", .d = std.time.ns_per_min },
            .{ .s = "1h", .d = std.time.ns_per_hour },
            .{ .s = "1d", .d = std.time.ns_per_day },
            .{ .s = "1w", .d = std.time.ns_per_week },
            .{ .s = "1y", .d = 365 * std.time.ns_per_day },
            .{ .s = "1y52w23h59m59.999s", .d = 730 * std.time.ns_per_day - 1 }, // 365d = 52w1d

            .{ .s = "1y1h1.001s", .d = 365 * std.time.ns_per_day + std.time.ns_per_hour + std.time.ns_per_s + std.time.ns_per_ms },
            .{ .s = "1y1h1s", .d = 365 * std.time.ns_per_day + std.time.ns_per_hour + std.time.ns_per_s + 999 * std.time.ns_per_us },
            .{ .s = "1y1h999.999us", .d = 365 * std.time.ns_per_day + std.time.ns_per_hour + std.time.ns_per_ms - 1 },
            .{ .s = "1y1h1ms", .d = 365 * std.time.ns_per_day + std.time.ns_per_hour + std.time.ns_per_ms },
            .{ .s = "1y1h1ms", .d = 365 * std.time.ns_per_day + std.time.ns_per_hour + std.time.ns_per_ms + 1 },
            .{ .s = "1y1m999ns", .d = 365 * std.time.ns_per_day + std.time.ns_per_min + 999 },
            .{ .s = "584y49w23h34m33.709s", .d = math.maxInt(u64) },
        }) |tc| {
            const slice = try bufPrint(&buf, "{}", .{fmtDuration(tc.d)});
            try std.testing.expectEqualStrings(tc.s, slice);
        }

        inline for (.{
            .{ .s = "=======0ns", .f = "{s:=>10}", .d = 0 },
            .{ .s = "1ns=======", .f = "{s:=<10}", .d = 1 },
            .{ .s = "  999ns   ", .f = "{s:^10}", .d = std.time.ns_per_us - 1 },
        }) |tc| {
            const slice = try bufPrint(&buf, tc.f, .{fmtDuration(tc.d)});
            try std.testing.expectEqualStrings(tc.s, slice);
        }
    }

    fn formatDurationSigned(ns: i64, comptime fmt: []const u8, options: std.fmt.FormatOptions, writer: anytype) !void {
        if (ns < 0) {
            const data = FormatDurationData{ .ns = @intCast(u64, -ns), .negative = true };
            try formatDuration(data, fmt, options, writer);
        } else {
            const data = FormatDurationData{ .ns = @intCast(u64, ns) };
            try formatDuration(data, fmt, options, writer);
        }
    }

    /// Return a Formatter for number of nanoseconds according to its signed magnitude:
    /// [#y][#w][#d][#h][#m]#[.###][n|u|m]s
    pub fn fmtDurationSigned(ns: i64) Formatter(formatDurationSigned) {
        return .{ .data = ns };
    }

    test "fmtDurationSigned" {
        var buf: [24]u8 = undefined;
        inline for (.{
            .{ .s = "0ns", .d = 0 },
            .{ .s = "1ns", .d = 1 },
            .{ .s = "-1ns", .d = -(1) },
            .{ .s = "999ns", .d = std.time.ns_per_us - 1 },
            .{ .s = "-999ns", .d = -(std.time.ns_per_us - 1) },
            .{ .s = "1us", .d = std.time.ns_per_us },
            .{ .s = "-1us", .d = -(std.time.ns_per_us) },
            .{ .s = "1.45us", .d = 1450 },
            .{ .s = "-1.45us", .d = -(1450) },
            .{ .s = "1.5us", .d = 3 * std.time.ns_per_us / 2 },
            .{ .s = "-1.5us", .d = -(3 * std.time.ns_per_us / 2) },
            .{ .s = "14.5us", .d = 14500 },
            .{ .s = "-14.5us", .d = -(14500) },
            .{ .s = "145us", .d = 145000 },
            .{ .s = "-145us", .d = -(145000) },
            .{ .s = "999.999us", .d = std.time.ns_per_ms - 1 },
            .{ .s = "-999.999us", .d = -(std.time.ns_per_ms - 1) },
            .{ .s = "1ms", .d = std.time.ns_per_ms + 1 },
            .{ .s = "-1ms", .d = -(std.time.ns_per_ms + 1) },
            .{ .s = "1.5ms", .d = 3 * std.time.ns_per_ms / 2 },
            .{ .s = "-1.5ms", .d = -(3 * std.time.ns_per_ms / 2) },
            .{ .s = "1.11ms", .d = 1110000 },
            .{ .s = "-1.11ms", .d = -(1110000) },
            .{ .s = "1.111ms", .d = 1111000 },
            .{ .s = "-1.111ms", .d = -(1111000) },
            .{ .s = "1.111ms", .d = 1111100 },
            .{ .s = "-1.111ms", .d = -(1111100) },
            .{ .s = "999.999ms", .d = std.time.ns_per_s - 1 },
            .{ .s = "-999.999ms", .d = -(std.time.ns_per_s - 1) },
            .{ .s = "1s", .d = std.time.ns_per_s },
            .{ .s = "-1s", .d = -(std.time.ns_per_s) },
            .{ .s = "59.999s", .d = std.time.ns_per_min - 1 },
            .{ .s = "-59.999s", .d = -(std.time.ns_per_min - 1) },
            .{ .s = "1m", .d = std.time.ns_per_min },
            .{ .s = "-1m", .d = -(std.time.ns_per_min) },
            .{ .s = "1h", .d = std.time.ns_per_hour },
            .{ .s = "-1h", .d = -(std.time.ns_per_hour) },
            .{ .s = "1d", .d = std.time.ns_per_day },
            .{ .s = "-1d", .d = -(std.time.ns_per_day) },
            .{ .s = "1w", .d = std.time.ns_per_week },
            .{ .s = "-1w", .d = -(std.time.ns_per_week) },
            .{ .s = "1y", .d = 365 * std.time.ns_per_day },
            .{ .s = "-1y", .d = -(365 * std.time.ns_per_day) },
            .{ .s = "1y52w23h59m59.999s", .d = 730 * std.time.ns_per_day - 1 }, // 365d = 52w1d

            .{ .s = "-1y52w23h59m59.999s", .d = -(730 * std.time.ns_per_day - 1) }, // 365d = 52w1d

            .{ .s = "1y1h1.001s", .d = 365 * std.time.ns_per_day + std.time.ns_per_hour + std.time.ns_per_s + std.time.ns_per_ms },
            .{ .s = "-1y1h1.001s", .d = -(365 * std.time.ns_per_day + std.time.ns_per_hour + std.time.ns_per_s + std.time.ns_per_ms) },
            .{ .s = "1y1h1s", .d = 365 * std.time.ns_per_day + std.time.ns_per_hour + std.time.ns_per_s + 999 * std.time.ns_per_us },
            .{ .s = "-1y1h1s", .d = -(365 * std.time.ns_per_day + std.time.ns_per_hour + std.time.ns_per_s + 999 * std.time.ns_per_us) },
            .{ .s = "1y1h999.999us", .d = 365 * std.time.ns_per_day + std.time.ns_per_hour + std.time.ns_per_ms - 1 },
            .{ .s = "-1y1h999.999us", .d = -(365 * std.time.ns_per_day + std.time.ns_per_hour + std.time.ns_per_ms - 1) },
            .{ .s = "1y1h1ms", .d = 365 * std.time.ns_per_day + std.time.ns_per_hour + std.time.ns_per_ms },
            .{ .s = "-1y1h1ms", .d = -(365 * std.time.ns_per_day + std.time.ns_per_hour + std.time.ns_per_ms) },
            .{ .s = "1y1h1ms", .d = 365 * std.time.ns_per_day + std.time.ns_per_hour + std.time.ns_per_ms + 1 },
            .{ .s = "-1y1h1ms", .d = -(365 * std.time.ns_per_day + std.time.ns_per_hour + std.time.ns_per_ms + 1) },
            .{ .s = "1y1m999ns", .d = 365 * std.time.ns_per_day + std.time.ns_per_min + 999 },
            .{ .s = "-1y1m999ns", .d = -(365 * std.time.ns_per_day + std.time.ns_per_min + 999) },
            .{ .s = "292y24w3d23h47m16.854s", .d = math.maxInt(i64) },
            .{ .s = "-292y24w3d23h47m16.854s", .d = math.minInt(i64) + 1 },
        }) |tc| {
            const slice = try bufPrint(&buf, "{}", .{fmtDurationSigned(tc.d)});
            try std.testing.expectEqualStrings(tc.s, slice);
        }

        inline for (.{
            .{ .s = "=======0ns", .f = "{s:=>10}", .d = 0 },
            .{ .s = "1ns=======", .f = "{s:=<10}", .d = 1 },
            .{ .s = "-1ns======", .f = "{s:=<10}", .d = -(1) },
            .{ .s = "  -999ns  ", .f = "{s:^10}", .d = -(std.time.ns_per_us - 1) },
        }) |tc| {
            const slice = try bufPrint(&buf, tc.f, .{fmtDurationSigned(tc.d)});
            try std.testing.expectEqualStrings(tc.s, slice);
        }
    }

    pub const ParseIntError = error{
        /// The result cannot fit in the type specified
        Overflow,

        /// The input was empty or had a byte that was not a digit
        InvalidCharacter,
    };

    /// Creates a Formatter type from a format function. Wrapping data in Formatter(func) causes
    /// the data to be formatted using the given function `func`.  `func` must be of the following
    /// form:
    ///
    ///     fn formatExample(
    ///         data: T,
    ///         comptime fmt: []const u8,
    ///         options: std.fmt.FormatOptions,
    ///         writer: anytype,
    ///     ) !void;
    ///
    pub fn Formatter(comptime format_fn: anytype) type {
        const Data = @typeInfo(@TypeOf(format_fn)).Fn.args[0].arg_type.?;
        return struct {
            data: Data,
            pub fn format(
                self: @This(),
                comptime fmt: []const u8,
                options: std.fmt.FormatOptions,
                writer: anytype,
            ) @TypeOf(writer).Error!void {
                try format_fn(self.data, fmt, options, writer);
            }
        };
    }

    /// Parses the string `buf` as signed or unsigned representation in the
    /// specified radix of an integral value of type `T`.
    ///
    /// When `radix` is zero the string prefix is examined to detect the true radix:
    ///  * A prefix of "0b" implies radix=2,
    ///  * A prefix of "0o" implies radix=8,
    ///  * A prefix of "0x" implies radix=16,
    ///  * Otherwise radix=10 is assumed.
    ///
    /// Ignores '_' character in `buf`.
    /// See also `parseUnsigned`.
    pub fn parseInt(comptime T: type, buf: []const u8, radix: u8) ParseIntError!T {
        if (buf.len == 0) return error.InvalidCharacter;
        if (buf[0] == '+') return parseWithSign(T, buf[1..], radix, .Pos);
        if (buf[0] == '-') return parseWithSign(T, buf[1..], radix, .Neg);
        return parseWithSign(T, buf, radix, .Pos);
    }

    test "parseInt" {
        try std.testing.expect((try parseInt(i32, "-10", 10)) == -10);
        try std.testing.expect((try parseInt(i32, "+10", 10)) == 10);
        try std.testing.expect((try parseInt(u32, "+10", 10)) == 10);
        try std.testing.expectError(error.Overflow, parseInt(u32, "-10", 10));
        try std.testing.expectError(error.InvalidCharacter, parseInt(u32, " 10", 10));
        try std.testing.expectError(error.InvalidCharacter, parseInt(u32, "10 ", 10));
        try std.testing.expectError(error.InvalidCharacter, parseInt(u32, "_10_", 10));
        try std.testing.expectError(error.InvalidCharacter, parseInt(u32, "0x_10_", 10));
        try std.testing.expectError(error.InvalidCharacter, parseInt(u32, "0x10_", 10));
        try std.testing.expectError(error.InvalidCharacter, parseInt(u32, "0x_10", 10));
        try std.testing.expect((try parseInt(u8, "255", 10)) == 255);
        try std.testing.expectError(error.Overflow, parseInt(u8, "256", 10));

        // +0 and -0 should work for unsigned

        try std.testing.expect((try parseInt(u8, "-0", 10)) == 0);
        try std.testing.expect((try parseInt(u8, "+0", 10)) == 0);

        // ensure minInt is parsed correctly

        try std.testing.expect((try parseInt(i8, "-128", 10)) == math.minInt(i8));
        try std.testing.expect((try parseInt(i43, "-4398046511104", 10)) == math.minInt(i43));

        // empty string or bare +- is invalid

        try std.testing.expectError(error.InvalidCharacter, parseInt(u32, "", 10));
        try std.testing.expectError(error.InvalidCharacter, parseInt(i32, "", 10));
        try std.testing.expectError(error.InvalidCharacter, parseInt(u32, "+", 10));
        try std.testing.expectError(error.InvalidCharacter, parseInt(i32, "+", 10));
        try std.testing.expectError(error.InvalidCharacter, parseInt(u32, "-", 10));
        try std.testing.expectError(error.InvalidCharacter, parseInt(i32, "-", 10));

        // autodectect the radix

        try std.testing.expect((try parseInt(i32, "111", 0)) == 111);
        try std.testing.expect((try parseInt(i32, "1_1_1", 0)) == 111);
        try std.testing.expect((try parseInt(i32, "1_1_1", 0)) == 111);
        try std.testing.expect((try parseInt(i32, "+0b111", 0)) == 7);
        try std.testing.expect((try parseInt(i32, "+0B111", 0)) == 7);
        try std.testing.expect((try parseInt(i32, "+0b1_11", 0)) == 7);
        try std.testing.expect((try parseInt(i32, "+0o111", 0)) == 73);
        try std.testing.expect((try parseInt(i32, "+0O111", 0)) == 73);
        try std.testing.expect((try parseInt(i32, "+0o11_1", 0)) == 73);
        try std.testing.expect((try parseInt(i32, "+0x111", 0)) == 273);
        try std.testing.expect((try parseInt(i32, "-0b111", 0)) == -7);
        try std.testing.expect((try parseInt(i32, "-0b11_1", 0)) == -7);
        try std.testing.expect((try parseInt(i32, "-0o111", 0)) == -73);
        try std.testing.expect((try parseInt(i32, "-0x111", 0)) == -273);
        try std.testing.expect((try parseInt(i32, "-0X111", 0)) == -273);
        try std.testing.expect((try parseInt(i32, "-0x1_11", 0)) == -273);

        // bare binary/octal/decimal prefix is invalid

        try std.testing.expectError(error.InvalidCharacter, parseInt(u32, "0b", 0));
        try std.testing.expectError(error.InvalidCharacter, parseInt(u32, "0o", 0));
        try std.testing.expectError(error.InvalidCharacter, parseInt(u32, "0x", 0));
    }

    fn parseWithSign(
        comptime T: type,
        buf: []const u8,
        radix: u8,
        comptime sign: enum { Pos, Neg },
    ) ParseIntError!T {
        if (buf.len == 0) return error.InvalidCharacter;

        var buf_radix = radix;
        var buf_start = buf;
        if (radix == 0) {
            // Treat is as a decimal number by default.

            buf_radix = 10;
            // Detect the radix by looking at buf prefix.

            if (buf.len > 2 and buf[0] == '0') {
                switch (std.ascii.toLower(buf[1])) {
                    'b' => {
                        buf_radix = 2;
                        buf_start = buf[2..];
                    },
                    'o' => {
                        buf_radix = 8;
                        buf_start = buf[2..];
                    },
                    'x' => {
                        buf_radix = 16;
                        buf_start = buf[2..];
                    },
                    else => {},
                }
            }
        }

        const add = switch (sign) {
            .Pos => math.add,
            .Neg => math.sub,
        };

        var x: T = 0;

        if (buf_start[0] == '_' or buf_start[buf_start.len - 1] == '_') return error.InvalidCharacter;

        for (buf_start) |c| {
            if (c == '_') continue;
            const digit = try charToDigit(c, buf_radix);

            if (x != 0) x = try math.mul(T, x, math.cast(T, buf_radix) orelse return error.Overflow);
            x = try add(T, x, math.cast(T, digit) orelse return error.Overflow);
        }

        return x;
    }

    /// Parses the string `buf` as  unsigned representation in the specified radix
    /// of an integral value of type `T`.
    ///
    /// When `radix` is zero the string prefix is examined to detect the true radix:
    ///  * A prefix of "0b" implies radix=2,
    ///  * A prefix of "0o" implies radix=8,
    ///  * A prefix of "0x" implies radix=16,
    ///  * Otherwise radix=10 is assumed.
    ///
    /// Ignores '_' character in `buf`.
    /// See also `parseInt`.
    pub fn parseUnsigned(comptime T: type, buf: []const u8, radix: u8) ParseIntError!T {
        return parseWithSign(T, buf, radix, .Pos);
    }

    test "parseUnsigned" {
        try std.testing.expect((try parseUnsigned(u16, "050124", 10)) == 50124);
        try std.testing.expect((try parseUnsigned(u16, "65535", 10)) == 65535);
        try std.testing.expect((try parseUnsigned(u16, "65_535", 10)) == 65535);
        try std.testing.expectError(error.Overflow, parseUnsigned(u16, "65536", 10));

        try std.testing.expect((try parseUnsigned(u64, "0ffffffffffffffff", 16)) == 0xffffffffffffffff);
        try std.testing.expect((try parseUnsigned(u64, "0f_fff_fff_fff_fff_fff", 16)) == 0xffffffffffffffff);
        try std.testing.expectError(error.Overflow, parseUnsigned(u64, "10000000000000000", 16));

        try std.testing.expect((try parseUnsigned(u32, "DeadBeef", 16)) == 0xDEADBEEF);

        try std.testing.expect((try parseUnsigned(u7, "1", 10)) == 1);
        try std.testing.expect((try parseUnsigned(u7, "1000", 2)) == 8);

        try std.testing.expectError(error.InvalidCharacter, parseUnsigned(u32, "f", 10));
        try std.testing.expectError(error.InvalidCharacter, parseUnsigned(u8, "109", 8));

        try std.testing.expect((try parseUnsigned(u32, "NUMBER", 36)) == 1442151747);

        // these numbers should fit even though the radix itself doesn't fit in the destination type

        try std.testing.expect((try parseUnsigned(u1, "0", 10)) == 0);
        try std.testing.expect((try parseUnsigned(u1, "1", 10)) == 1);
        try std.testing.expectError(error.Overflow, parseUnsigned(u1, "2", 10));
        try std.testing.expect((try parseUnsigned(u1, "001", 16)) == 1);
        try std.testing.expect((try parseUnsigned(u2, "3", 16)) == 3);
        try std.testing.expectError(error.Overflow, parseUnsigned(u2, "4", 16));

        // parseUnsigned does not expect a sign

        try std.testing.expectError(error.InvalidCharacter, parseUnsigned(u8, "+0", 10));
        try std.testing.expectError(error.InvalidCharacter, parseUnsigned(u8, "-0", 10));

        // test empty string error

        try std.testing.expectError(error.InvalidCharacter, parseUnsigned(u8, "", 10));
    }

    pub const parseFloat = @import("fmt/parse_float.zig").parseFloat;
    pub const ParseFloatError = @import("fmt/parse_float.zig").ParseFloatError;

    test {
        _ = parseFloat;
    }

    pub fn charToDigit(c: u8, radix: u8) (error{InvalidCharacter}!u8) {
        const value = switch (c) {
            '0'...'9' => c - '0',
            'A'...'Z' => c - 'A' + 10,
            'a'...'z' => c - 'a' + 10,
            else => return error.InvalidCharacter,
        };

        if (value >= radix) return error.InvalidCharacter;

        return value;
    }

    pub fn digitToChar(digit: u8, case: Case) u8 {
        return switch (digit) {
            0...9 => digit + '0',
            10...35 => digit + ((if (case == .upper) @as(u8, 'A') else @as(u8, 'a')) - 10),
            else => unreachable,
        };
    }

    pub const BufPrintError = error{
        /// As much as possible was written to the buffer, but it was too small to fit all the printed bytes.
        NoSpaceLeft,
    };

    /// print a Formatter string into `buf`. Actually just a thin wrapper around `format` and `fixedBufferStream`.
    /// returns a slice of the bytes printed to.
    pub fn bufPrint(buf: []u8, comptime fmt: []const u8, args: anytype) BufPrintError![]u8 {
        var fbs = std.io.fixedBufferStream(buf);
        try format(fbs.writer(), fmt, args);
        return fbs.getWritten();
    }

    pub fn bufPrintZ(buf: []u8, comptime fmt: []const u8, args: anytype) BufPrintError![:0]u8 {
        const result = try bufPrint(buf, fmt ++ "\x00", args);
        return result[0 .. result.len - 1 :0];
    }

    /// Count the characters needed for format. Useful for preallocating memory
    pub fn count(comptime fmt: []const u8, args: anytype) u64 {
        var counting_writer = std.io.countingWriter(std.io.null_writer);
        format(counting_writer.writer(), fmt, args) catch |err| switch (err) {};
        return counting_writer.bytes_written;
    }

    pub const AllocPrintError = error{OutOfMemory};

    pub fn allocPrint(allocator: mem.Allocator, comptime fmt: []const u8, args: anytype) AllocPrintError![]u8 {
        const size = math.cast(usize, count(fmt, args)) orelse return error.OutOfMemory;
        const buf = try allocator.alloc(u8, size);
        return bufPrint(buf, fmt, args) catch |err| switch (err) {
            error.NoSpaceLeft => unreachable, // we just counted the size above

        };
    }

    pub fn allocPrintZ(allocator: mem.Allocator, comptime fmt: []const u8, args: anytype) AllocPrintError![:0]u8 {
        const result = try allocPrint(allocator, fmt ++ "\x00", args);
        return result[0 .. result.len - 1 :0];
    }

    test "bufPrintInt" {
        var buffer: [100]u8 = undefined;
        const buf = buffer[0..];

        try std.testing.expectEqualSlices(u8, "-1", bufPrintIntToSlice(buf, @as(i1, -1), 10, .lower, FormatOptions{}));

        try std.testing.expectEqualSlices(u8, "-101111000110000101001110", bufPrintIntToSlice(buf, @as(i32, -12345678), 2, .lower, FormatOptions{}));
        try std.testing.expectEqualSlices(u8, "-12345678", bufPrintIntToSlice(buf, @as(i32, -12345678), 10, .lower, FormatOptions{}));
        try std.testing.expectEqualSlices(u8, "-bc614e", bufPrintIntToSlice(buf, @as(i32, -12345678), 16, .lower, FormatOptions{}));
        try std.testing.expectEqualSlices(u8, "-BC614E", bufPrintIntToSlice(buf, @as(i32, -12345678), 16, .upper, FormatOptions{}));

        try std.testing.expectEqualSlices(u8, "12345678", bufPrintIntToSlice(buf, @as(u32, 12345678), 10, .upper, FormatOptions{}));

        try std.testing.expectEqualSlices(u8, "   666", bufPrintIntToSlice(buf, @as(u32, 666), 10, .lower, FormatOptions{ .width = 6 }));
        try std.testing.expectEqualSlices(u8, "  1234", bufPrintIntToSlice(buf, @as(u32, 0x1234), 16, .lower, FormatOptions{ .width = 6 }));
        try std.testing.expectEqualSlices(u8, "1234", bufPrintIntToSlice(buf, @as(u32, 0x1234), 16, .lower, FormatOptions{ .width = 1 }));

        try std.testing.expectEqualSlices(u8, "+42", bufPrintIntToSlice(buf, @as(i32, 42), 10, .lower, FormatOptions{ .width = 3 }));
        try std.testing.expectEqualSlices(u8, "-42", bufPrintIntToSlice(buf, @as(i32, -42), 10, .lower, FormatOptions{ .width = 3 }));
    }

    pub fn bufPrintIntToSlice(buf: []u8, value: anytype, base: u8, case: Case, options: FormatOptions) []u8 {
        return buf[0..formatIntBuf(buf, value, base, case, options)];
    }

    pub fn comptimePrint(comptime fmt: []const u8, args: anytype) *const [count(fmt, args):0]u8 {
        comptime {
            var buf: [count(fmt, args):0]u8 = undefined;
            _ = bufPrint(&buf, fmt, args) catch unreachable;
            buf[buf.len] = 0;
            return &buf;
        }
    }

    test "comptimePrint" {
        @setEvalBranchQuota(2000);
        try std.testing.expectEqual(*const [3:0]u8, @TypeOf(comptime comptimePrint("{}", .{100})));
        try std.testing.expectEqualSlices(u8, "100", comptime comptimePrint("{}", .{100}));
    }

    test "parse u64 digit too big" {
        _ = parseUnsigned(u64, "123a", 10) catch |err| {
            if (err == error.InvalidCharacter) return;
            unreachable;
        };
        unreachable;
    }

    test "parse unsigned comptime" {
        comptime {
            try std.testing.expect((try parseUnsigned(usize, "2", 10)) == 2);
        }
    }

    test "escaped braces" {
        try expectFmt("escaped: {{foo}}\n", "escaped: {{{{foo}}}}\n", .{});
        try expectFmt("escaped: {foo}\n", "escaped: {{foo}}\n", .{});
    }

    test "optional" {
        {
            const value: ?i32 = 1234;
            try expectFmt("optional: 1234\n", "optional: {?}\n", .{value});
            try expectFmt("optional: 1234\n", "optional: {?d}\n", .{value});
            try expectFmt("optional: 4d2\n", "optional: {?x}\n", .{value});
        }
        {
            const value: ?[]const u8 = "string";
            try expectFmt("optional: string\n", "optional: {?s}\n", .{value});
        }
        {
            const value: ?i32 = null;
            try expectFmt("optional: null\n", "optional: {?}\n", .{value});
        }
        {
            const value = @intToPtr(?*i32, 0xf000d000);
            try expectFmt("optional: *i32@f000d000\n", "optional: {*}\n", .{value});
        }
    }

    test "error" {
        {
            const value: anyerror!i32 = 1234;
            try expectFmt("error union: 1234\n", "error union: {!}\n", .{value});
            try expectFmt("error union: 1234\n", "error union: {!d}\n", .{value});
            try expectFmt("error union: 4d2\n", "error union: {!x}\n", .{value});
        }
        {
            const value: anyerror![]const u8 = "string";
            try expectFmt("error union: string\n", "error union: {!s}\n", .{value});
        }
        {
            const value: anyerror!i32 = error.InvalidChar;
            try expectFmt("error union: error.InvalidChar\n", "error union: {!}\n", .{value});
        }
    }

    test "int.small" {
        {
            const value: u3 = 0b101;
            try expectFmt("u3: 5\n", "u3: {}\n", .{value});
        }
    }

    test "int.specifier" {
        {
            const value: u8 = 'a';
            try expectFmt("u8: a\n", "u8: {c}\n", .{value});
        }
        {
            const value: u8 = 0b1100;
            try expectFmt("u8: 0b1100\n", "u8: 0b{b}\n", .{value});
        }
        {
            const value: u16 = 0o1234;
            try expectFmt("u16: 0o1234\n", "u16: 0o{o}\n", .{value});
        }
        {
            const value: u8 = 'a';
            try expectFmt("UTF-8: a\n", "UTF-8: {u}\n", .{value});
        }
        {
            const value: u21 = 0x1F310;
            try expectFmt("UTF-8: 🌐\n", "UTF-8: {u}\n", .{value});
        }
        {
            const value: u21 = 0xD800;
            try expectFmt("UTF-8: �\n", "UTF-8: {u}\n", .{value});
        }
        {
            const value: u21 = 0x110001;
            try expectFmt("UTF-8: �\n", "UTF-8: {u}\n", .{value});
        }
    }

    test "int.padded" {
        try expectFmt("u8: '   1'", "u8: '{:4}'", .{@as(u8, 1)});
        try expectFmt("u8: '1000'", "u8: '{:0<4}'", .{@as(u8, 1)});
        try expectFmt("u8: '0001'", "u8: '{:0>4}'", .{@as(u8, 1)});
        try expectFmt("u8: '0100'", "u8: '{:0^4}'", .{@as(u8, 1)});
        try expectFmt("i8: '-1  '", "i8: '{:<4}'", .{@as(i8, -1)});
        try expectFmt("i8: '  -1'", "i8: '{:>4}'", .{@as(i8, -1)});
        try expectFmt("i8: ' -1 '", "i8: '{:^4}'", .{@as(i8, -1)});
        try expectFmt("i16: '-1234'", "i16: '{:4}'", .{@as(i16, -1234)});
        try expectFmt("i16: '+1234'", "i16: '{:4}'", .{@as(i16, 1234)});
        try expectFmt("i16: '-12345'", "i16: '{:4}'", .{@as(i16, -12345)});
        try expectFmt("i16: '+12345'", "i16: '{:4}'", .{@as(i16, 12345)});
        try expectFmt("u16: '12345'", "u16: '{:4}'", .{@as(u16, 12345)});

        try expectFmt("UTF-8: 'ü   '", "UTF-8: '{u:<4}'", .{'ü'});
        try expectFmt("UTF-8: '   ü'", "UTF-8: '{u:>4}'", .{'ü'});
        try expectFmt("UTF-8: ' ü  '", "UTF-8: '{u:^4}'", .{'ü'});
    }

    test "buffer" {
        {
            var buf1: [32]u8 = undefined;
            var fbs = std.io.fixedBufferStream(&buf1);
            try formatType(1234, "", FormatOptions{}, fbs.writer(), default_max_depth);
            try std.testing.expect(mem.eql(u8, fbs.getWritten(), "1234"));

            fbs.reset();
            try formatType('a', "c", FormatOptions{}, fbs.writer(), default_max_depth);
            try std.testing.expect(mem.eql(u8, fbs.getWritten(), "a"));

            fbs.reset();
            try formatType(0b1100, "b", FormatOptions{}, fbs.writer(), default_max_depth);
            try std.testing.expect(mem.eql(u8, fbs.getWritten(), "1100"));
        }
    }

    test "array" {
        {
            const value: [3]u8 = "abc".*;
            try expectFmt("array: abc\n", "array: {s}\n", .{value});
            try expectFmt("array: abc\n", "array: {s}\n", .{&value});
            try expectFmt("array: { 97, 98, 99 }\n", "array: {d}\n", .{value});

            var buf: [100]u8 = undefined;
            try expectFmt(
                try bufPrint(buf[0..], "array: [3]u8@{x}\n", .{@ptrToInt(&value)}),
                "array: {*}\n",
                .{&value},
            );
        }
    }

    test "slice" {
        {
            const value: []const u8 = "abc";
            try expectFmt("slice: abc\n", "slice: {s}\n", .{value});
        }
        {
            var runtime_zero: usize = 0;
            const value = @intToPtr([*]align(1) const []const u8, 0xdeadbeef)[runtime_zero..runtime_zero];
            try expectFmt("slice: []const u8@deadbeef\n", "slice: {*}\n", .{value});
        }
        {
            const null_term_slice: [:0]const u8 = "\x00hello\x00";
            try expectFmt("buf: \x00hello\x00\n", "buf: {s}\n", .{null_term_slice});
        }

        try expectFmt("buf:  Test\n", "buf: {s:5}\n", .{"Test"});
        try expectFmt("buf: Test\n Other text", "buf: {s}\n Other text", .{"Test"});

        {
            var int_slice = [_]u32{ 1, 4096, 391891, 1111111111 };
            var runtime_zero: usize = 0;
            try expectFmt("int: { 1, 4096, 391891, 1111111111 }", "int: {any}", .{int_slice[runtime_zero..]});
            try expectFmt("int: { 1, 4096, 391891, 1111111111 }", "int: {d}", .{int_slice[runtime_zero..]});
            try expectFmt("int: { 1, 1000, 5fad3, 423a35c7 }", "int: {x}", .{int_slice[runtime_zero..]});
            try expectFmt("int: { 00001, 01000, 5fad3, 423a35c7 }", "int: {x:0>5}", .{int_slice[runtime_zero..]});
        }
    }

    test "escape non-printable" {
        try expectFmt("abc", "{s}", .{fmtSliceEscapeLower("abc")});
        try expectFmt("ab\\xffc", "{s}", .{fmtSliceEscapeLower("ab\xffc")});
        try expectFmt("ab\\xFFc", "{s}", .{fmtSliceEscapeUpper("ab\xffc")});
    }

    test "pointer" {
        {
            const value = @intToPtr(*align(1) i32, 0xdeadbeef);
            try expectFmt("pointer: i32@deadbeef\n", "pointer: {}\n", .{value});
            try expectFmt("pointer: i32@deadbeef\n", "pointer: {*}\n", .{value});
        }
        const FnPtr = if (builtin.zig_backend == .stage1) fn () void else *align(1) const fn () void;
        {
            const value = @intToPtr(FnPtr, 0xdeadbeef);
            try expectFmt("pointer: fn() void@deadbeef\n", "pointer: {}\n", .{value});
        }
        {
            const value = @intToPtr(FnPtr, 0xdeadbeef);
            try expectFmt("pointer: fn() void@deadbeef\n", "pointer: {}\n", .{value});
        }
    }

    test "cstr" {
        try expectFmt(
            "cstr: Test C\n",
            "cstr: {s}\n",
            .{@ptrCast([*c]const u8, "Test C")},
        );
        try expectFmt(
            "cstr:     Test C\n",
            "cstr: {s:10}\n",
            .{@ptrCast([*c]const u8, "Test C")},
        );
    }

    test "filesize" {
        try expectFmt("file size: 42B\n", "file size: {}\n", .{fmtIntSizeDec(42)});
        try expectFmt("file size: 42B\n", "file size: {}\n", .{fmtIntSizeBin(42)});
        try expectFmt("file size: 63MB\n", "file size: {}\n", .{fmtIntSizeDec(63 * 1000 * 1000)});
        try expectFmt("file size: 63MiB\n", "file size: {}\n", .{fmtIntSizeBin(63 * 1024 * 1024)});
        try expectFmt("file size: 66.06MB\n", "file size: {:.2}\n", .{fmtIntSizeDec(63 * 1024 * 1024)});
        try expectFmt("file size: 60.08MiB\n", "file size: {:.2}\n", .{fmtIntSizeBin(63 * 1000 * 1000)});
        try expectFmt("file size: =66.06MB=\n", "file size: {:=^9.2}\n", .{fmtIntSizeDec(63 * 1024 * 1024)});
        try expectFmt("file size:   66.06MB\n", "file size: {: >9.2}\n", .{fmtIntSizeDec(63 * 1024 * 1024)});
        try expectFmt("file size: 66.06MB  \n", "file size: {: <9.2}\n", .{fmtIntSizeDec(63 * 1024 * 1024)});
        try expectFmt("file size: 0.01844674407370955ZB\n", "file size: {}\n", .{fmtIntSizeDec(math.maxInt(u64))});
    }

    test "struct" {
        {
            const Struct = struct {
                field: u8,
            };
            const value = Struct{ .field = 42 };
            try expectFmt("struct: Struct{ .field = 42 }\n", "struct: {}\n", .{value});
            try expectFmt("struct: Struct{ .field = 42 }\n", "struct: {}\n", .{&value});
        }
        {
            const Struct = struct {
                a: u0,
                b: u1,
            };
            const value = Struct{ .a = 0, .b = 1 };
            try expectFmt("struct: Struct{ .a = 0, .b = 1 }\n", "struct: {}\n", .{value});
        }
    }

    test "enum" {
        if (builtin.zig_backend == .stage1) {
            // stage1 starts the typename with 'std' which might also be desireable for stage2

            return error.SkipZigTest;
        }
        const Enum = enum {
            One,
            Two,
        };
        const value = Enum.Two;
        try expectFmt("enum: Enum.Two\n", "enum: {}\n", .{value});
        try expectFmt("enum: Enum.Two\n", "enum: {}\n", .{&value});
        try expectFmt("enum: Enum.One\n", "enum: {}\n", .{Enum.One});
        try expectFmt("enum: Enum.Two\n", "enum: {}\n", .{Enum.Two});

        // test very large enum to verify ct branch quota is large enough

        try expectFmt("enum: os.windows.win32error.Win32Error.INVALID_FUNCTION\n", "enum: {}\n", .{std.os.windows.Win32Error.INVALID_FUNCTION});
    }

    test "non-exhaustive enum" {
        if (builtin.zig_backend == .stage1) {
            // stage1 fails to return fully qualified namespaces.

            return error.SkipZigTest;
        }
        const Enum = enum(u16) {
            One = 0x000f,
            Two = 0xbeef,
            _,
        };
        try expectFmt("enum: fmt.test.non-exhaustive enum.Enum.One\n", "enum: {}\n", .{Enum.One});
        try expectFmt("enum: fmt.test.non-exhaustive enum.Enum.Two\n", "enum: {}\n", .{Enum.Two});
        try expectFmt("enum: fmt.test.non-exhaustive enum.Enum(4660)\n", "enum: {}\n", .{@intToEnum(Enum, 0x1234)});
        try expectFmt("enum: fmt.test.non-exhaustive enum.Enum.One\n", "enum: {x}\n", .{Enum.One});
        try expectFmt("enum: fmt.test.non-exhaustive enum.Enum.Two\n", "enum: {x}\n", .{Enum.Two});
        try expectFmt("enum: fmt.test.non-exhaustive enum.Enum.Two\n", "enum: {X}\n", .{Enum.Two});
        try expectFmt("enum: fmt.test.non-exhaustive enum.Enum(1234)\n", "enum: {x}\n", .{@intToEnum(Enum, 0x1234)});
    }

    test "float.scientific" {
        try expectFmt("f32: 1.34000003e+00", "f32: {e}", .{@as(f32, 1.34)});
        try expectFmt("f32: 1.23400001e+01", "f32: {e}", .{@as(f32, 12.34)});
        try expectFmt("f64: -1.234e+11", "f64: {e}", .{@as(f64, -12.34e10)});
        try expectFmt("f64: 9.99996e-40", "f64: {e}", .{@as(f64, 9.999960e-40)});
    }

    test "float.scientific.precision" {
        try expectFmt("f64: 1.40971e-42", "f64: {e:.5}", .{@as(f64, 1.409706e-42)});
        try expectFmt("f64: 1.00000e-09", "f64: {e:.5}", .{@as(f64, @bitCast(f32, @as(u32, 814313563)))});
        try expectFmt("f64: 7.81250e-03", "f64: {e:.5}", .{@as(f64, @bitCast(f32, @as(u32, 1006632960)))});
        // libc rounds 1.000005e+05 to 1.00000e+05 but zig does 1.00001e+05.

        // In fact, libc doesn't round a lot of 5 cases up when one past the precision point.

        try expectFmt("f64: 1.00001e+05", "f64: {e:.5}", .{@as(f64, @bitCast(f32, @as(u32, 1203982400)))});
    }

    test "float.special" {
        try expectFmt("f64: nan", "f64: {}", .{math.nan_f64});
        // negative nan is not defined by IEE 754,

        // and ARM thus normalizes it to positive nan

        if (builtin.target.cpu.arch != .arm) {
            try expectFmt("f64: -nan", "f64: {}", .{-math.nan_f64});
        }
        try expectFmt("f64: inf", "f64: {}", .{math.inf(f64)});
        try expectFmt("f64: -inf", "f64: {}", .{-math.inf(f64)});
    }

    test "float.hexadecimal.special" {
        try expectFmt("f64: nan", "f64: {x}", .{math.nan_f64});
        // negative nan is not defined by IEE 754,

        // and ARM thus normalizes it to positive nan

        if (builtin.target.cpu.arch != .arm) {
            try expectFmt("f64: -nan", "f64: {x}", .{-math.nan_f64});
        }
        try expectFmt("f64: inf", "f64: {x}", .{math.inf(f64)});
        try expectFmt("f64: -inf", "f64: {x}", .{-math.inf(f64)});

        try expectFmt("f64: 0x0.0p0", "f64: {x}", .{@as(f64, 0)});
        try expectFmt("f64: -0x0.0p0", "f64: {x}", .{-@as(f64, 0)});
    }

    test "float.hexadecimal" {
        try expectFmt("f16: 0x1.554p-2", "f16: {x}", .{@as(f16, 1.0 / 3.0)});
        try expectFmt("f32: 0x1.555556p-2", "f32: {x}", .{@as(f32, 1.0 / 3.0)});
        try expectFmt("f64: 0x1.5555555555555p-2", "f64: {x}", .{@as(f64, 1.0 / 3.0)});
        try expectFmt("f80: 0x1.5555555555555556p-2", "f80: {x}", .{@as(f80, 1.0 / 3.0)});
        try expectFmt("f128: 0x1.5555555555555555555555555555p-2", "f128: {x}", .{@as(f128, 1.0 / 3.0)});

        try expectFmt("f16: 0x1p-14", "f16: {x}", .{math.floatMin(f16)});
        try expectFmt("f32: 0x1p-126", "f32: {x}", .{math.floatMin(f32)});
        try expectFmt("f64: 0x1p-1022", "f64: {x}", .{math.floatMin(f64)});
        try expectFmt("f80: 0x1p-16382", "f80: {x}", .{math.floatMin(f80)});
        try expectFmt("f128: 0x1p-16382", "f128: {x}", .{math.floatMin(f128)});

        try expectFmt("f16: 0x0.004p-14", "f16: {x}", .{math.floatTrueMin(f16)});
        try expectFmt("f32: 0x0.000002p-126", "f32: {x}", .{math.floatTrueMin(f32)});
        try expectFmt("f64: 0x0.0000000000001p-1022", "f64: {x}", .{math.floatTrueMin(f64)});
        try expectFmt("f80: 0x0.0000000000000002p-16382", "f80: {x}", .{math.floatTrueMin(f80)});
        try expectFmt("f128: 0x0.0000000000000000000000000001p-16382", "f128: {x}", .{math.floatTrueMin(f128)});

        try expectFmt("f16: 0x1.ffcp15", "f16: {x}", .{math.floatMax(f16)});
        try expectFmt("f32: 0x1.fffffep127", "f32: {x}", .{math.floatMax(f32)});
        try expectFmt("f64: 0x1.fffffffffffffp1023", "f64: {x}", .{math.floatMax(f64)});
        try expectFmt("f80: 0x1.fffffffffffffffep16383", "f80: {x}", .{math.floatMax(f80)});
        try expectFmt("f128: 0x1.ffffffffffffffffffffffffffffp16383", "f128: {x}", .{math.floatMax(f128)});
    }

    test "float.hexadecimal.precision" {
        try expectFmt("f16: 0x1.5p-2", "f16: {x:.1}", .{@as(f16, 1.0 / 3.0)});
        try expectFmt("f32: 0x1.555p-2", "f32: {x:.3}", .{@as(f32, 1.0 / 3.0)});
        try expectFmt("f64: 0x1.55555p-2", "f64: {x:.5}", .{@as(f64, 1.0 / 3.0)});
        try expectFmt("f80: 0x1.5555555p-2", "f80: {x:.7}", .{@as(f80, 1.0 / 3.0)});
        try expectFmt("f128: 0x1.555555555p-2", "f128: {x:.9}", .{@as(f128, 1.0 / 3.0)});

        try expectFmt("f16: 0x1.00000p0", "f16: {x:.5}", .{@as(f16, 1.0)});
        try expectFmt("f32: 0x1.00000p0", "f32: {x:.5}", .{@as(f32, 1.0)});
        try expectFmt("f64: 0x1.00000p0", "f64: {x:.5}", .{@as(f64, 1.0)});
        try expectFmt("f80: 0x1.00000p0", "f80: {x:.5}", .{@as(f80, 1.0)});
        try expectFmt("f128: 0x1.00000p0", "f128: {x:.5}", .{@as(f128, 1.0)});
    }

    test "float.decimal" {
        try expectFmt("f64: 152314000000000000000000000000", "f64: {d}", .{@as(f64, 1.52314e+29)});
        try expectFmt("f32: 0", "f32: {d}", .{@as(f32, 0.0)});
        try expectFmt("f32: 0", "f32: {d:.0}", .{@as(f32, 0.0)});
        try expectFmt("f32: 1.1", "f32: {d:.1}", .{@as(f32, 1.1234)});
        try expectFmt("f32: 1234.57", "f32: {d:.2}", .{@as(f32, 1234.567)});
        // -11.1234 is converted to f64 -11.12339... internally (errol3() function takes f64).

        // -11.12339... is rounded back up to -11.1234

        try expectFmt("f32: -11.1234", "f32: {d:.4}", .{@as(f32, -11.1234)});
        try expectFmt("f32: 91.12345", "f32: {d:.5}", .{@as(f32, 91.12345)});
        try expectFmt("f64: 91.1234567890", "f64: {d:.10}", .{@as(f64, 91.12345678901235)});
        try expectFmt("f64: 0.00000", "f64: {d:.5}", .{@as(f64, 0.0)});
        try expectFmt("f64: 6", "f64: {d:.0}", .{@as(f64, 5.700)});
        try expectFmt("f64: 10.0", "f64: {d:.1}", .{@as(f64, 9.999)});
        try expectFmt("f64: 1.000", "f64: {d:.3}", .{@as(f64, 1.0)});
        try expectFmt("f64: 0.00030000", "f64: {d:.8}", .{@as(f64, 0.0003)});
        try expectFmt("f64: 0.00000", "f64: {d:.5}", .{@as(f64, 1.40130e-45)});
        try expectFmt("f64: 0.00000", "f64: {d:.5}", .{@as(f64, 9.999960e-40)});
        try expectFmt("f64: 10000000000000.00", "f64: {d:.2}", .{@as(f64, 9999999999999.999)});
        try expectFmt("f64: 10000000000000000000000000000000000000", "f64: {d}", .{@as(f64, 1e37)});
        try expectFmt("f64: 100000000000000000000000000000000000000", "f64: {d}", .{@as(f64, 1e38)});
    }

    test "float.libc.sanity" {
        try expectFmt("f64: 0.00001", "f64: {d:.5}", .{@as(f64, @bitCast(f32, @as(u32, 916964781)))});
        try expectFmt("f64: 0.00001", "f64: {d:.5}", .{@as(f64, @bitCast(f32, @as(u32, 925353389)))});
        try expectFmt("f64: 0.10000", "f64: {d:.5}", .{@as(f64, @bitCast(f32, @as(u32, 1036831278)))});
        try expectFmt("f64: 1.00000", "f64: {d:.5}", .{@as(f64, @bitCast(f32, @as(u32, 1065353133)))});
        try expectFmt("f64: 10.00000", "f64: {d:.5}", .{@as(f64, @bitCast(f32, @as(u32, 1092616192)))});

        // libc differences

        //

        // This is 0.015625 exactly according to gdb. We thus round down,

        // however glibc rounds up for some reason. This occurs for all

        // floats of the form x.yyyy25 on a precision point.

        try expectFmt("f64: 0.01563", "f64: {d:.5}", .{@as(f64, @bitCast(f32, @as(u32, 1015021568)))});
        // errol3 rounds to ... 630 but libc rounds to ...632. Grisu3

        // also rounds to 630 so I'm inclined to believe libc is not

        // optimal here.

        try expectFmt("f64: 18014400656965630.00000", "f64: {d:.5}", .{@as(f64, @bitCast(f32, @as(u32, 1518338049)))});
    }

    test "custom" {
        const Vec2 = struct {
            const SelfType = @This();
            x: f32,
            y: f32,

            pub fn format(
                self: SelfType,
                comptime fmt: []const u8,
                options: FormatOptions,
                writer: anytype,
            ) !void {
                _ = options;
                if (fmt.len == 0 or comptime std.mem.eql(u8, fmt, "p")) {
                    return std.fmt.format(writer, "({d:.3},{d:.3})", .{ self.x, self.y });
                } else if (comptime std.mem.eql(u8, fmt, "d")) {
                    return std.fmt.format(writer, "{d:.3}x{d:.3}", .{ self.x, self.y });
                } else {
                    @compileError("unknown format character: '" ++ fmt ++ "'");
                }
            }
        };

        var value = Vec2{
            .x = 10.2,
            .y = 2.22,
        };
        try expectFmt("point: (10.200,2.220)\n", "point: {}\n", .{&value});
        try expectFmt("dim: 10.200x2.220\n", "dim: {d}\n", .{&value});

        // same thing but not passing a pointer

        try expectFmt("point: (10.200,2.220)\n", "point: {}\n", .{value});
        try expectFmt("dim: 10.200x2.220\n", "dim: {d}\n", .{value});
    }

    test "struct" {
        if (builtin.zig_backend == .stage1) {
            // stage1 fails to return fully qualified namespaces.

            return error.SkipZigTest;
        }
        const S = struct {
            a: u32,
            b: anyerror,
        };

        const inst = S{
            .a = 456,
            .b = error.Unused,
        };

        try expectFmt("fmt.test.struct.S{ .a = 456, .b = error.Unused }", "{}", .{inst});
        // Tuples

        try expectFmt("{ }", "{}", .{.{}});
        try expectFmt("{ -1 }", "{}", .{.{-1}});
        try expectFmt("{ -1, 42, 2.5e+04 }", "{}", .{.{ -1, 42, 0.25e5 }});
    }

    test "union" {
        if (builtin.zig_backend == .stage1) {
            // stage1 fails to return fully qualified namespaces.

            return error.SkipZigTest;
        }
        const TU = union(enum) {
            float: f32,
            int: u32,
        };

        const UU = union {
            float: f32,
            int: u32,
        };

        const EU = extern union {
            float: f32,
            int: u32,
        };

        const tu_inst = TU{ .int = 123 };
        const uu_inst = UU{ .int = 456 };
        const eu_inst = EU{ .float = 321.123 };

        try expectFmt("fmt.test.union.TU{ .int = 123 }", "{}", .{tu_inst});

        var buf: [100]u8 = undefined;
        const uu_result = try bufPrint(buf[0..], "{}", .{uu_inst});
        try std.testing.expect(mem.eql(u8, uu_result[0..18], "fmt.test.union.UU@"));

        const eu_result = try bufPrint(buf[0..], "{}", .{eu_inst});
        try std.testing.expect(mem.eql(u8, eu_result[0..18], "fmt.test.union.EU@"));
    }

    test "enum" {
        if (builtin.zig_backend == .stage1) {
            // stage1 fails to return fully qualified namespaces.

            return error.SkipZigTest;
        }
        const E = enum {
            One,
            Two,
            Three,
        };

        const inst = E.Two;

        try expectFmt("fmt.test.enum.E.Two", "{}", .{inst});
    }

    test "struct.self-referential" {
        if (builtin.zig_backend == .stage1) {
            // stage1 fails to return fully qualified namespaces.

            return error.SkipZigTest;
        }
        const S = struct {
            const SelfType = @This();
            a: ?*SelfType,
        };

        var inst = S{
            .a = null,
        };
        inst.a = &inst;

        try expectFmt("fmt.test.struct.self-referential.S{ .a = fmt.test.struct.self-referential.S{ .a = fmt.test.struct.self-referential.S{ .a = fmt.test.struct.self-referential.S{ ... } } } }", "{}", .{inst});
    }

    test "struct.zero-size" {
        if (builtin.zig_backend == .stage1) {
            // stage1 fails to return fully qualified namespaces.

            return error.SkipZigTest;
        }
        const A = struct {
            fn foo() void {}
        };
        const B = struct {
            a: A,
            c: i32,
        };

        const a = A{};
        const b = B{ .a = a, .c = 0 };

        try expectFmt("fmt.test.struct.zero-size.B{ .a = fmt.test.struct.zero-size.A{ }, .c = 0 }", "{}", .{b});
    }

    test "bytes.hex" {
        const some_bytes = "\xCA\xFE\xBA\xBE";
        try expectFmt("lowercase: cafebabe\n", "lowercase: {x}\n", .{fmtSliceHexLower(some_bytes)});
        try expectFmt("uppercase: CAFEBABE\n", "uppercase: {X}\n", .{fmtSliceHexUpper(some_bytes)});
        //Test Slices

        try expectFmt("uppercase: CAFE\n", "uppercase: {X}\n", .{fmtSliceHexUpper(some_bytes[0..2])});
        try expectFmt("lowercase: babe\n", "lowercase: {x}\n", .{fmtSliceHexLower(some_bytes[2..])});
        const bytes_with_zeros = "\x00\x0E\xBA\xBE";
        try expectFmt("lowercase: 000ebabe\n", "lowercase: {x}\n", .{fmtSliceHexLower(bytes_with_zeros)});
    }

    /// Decodes the sequence of bytes represented by the specified string of
    /// hexadecimal characters.
    /// Returns a slice of the output buffer containing the decoded bytes.
    pub fn hexToBytes(out: []u8, input: []const u8) ![]u8 {
        // Expect 0 or n pairs of hexadecimal digits.

        if (input.len & 1 != 0)
            return error.InvalidLength;
        if (out.len * 2 < input.len)
            return error.NoSpaceLeft;

        var in_i: usize = 0;
        while (in_i < input.len) : (in_i += 2) {
            const hi = try charToDigit(input[in_i], 16);
            const lo = try charToDigit(input[in_i + 1], 16);
            out[in_i / 2] = (hi << 4) | lo;
        }

        return out[0 .. in_i / 2];
    }

    test "hexToBytes" {
        var buf: [32]u8 = undefined;
        try expectFmt("90" ** 32, "{s}", .{fmtSliceHexUpper(try hexToBytes(&buf, "90" ** 32))});
        try expectFmt("ABCD", "{s}", .{fmtSliceHexUpper(try hexToBytes(&buf, "ABCD"))});
        try expectFmt("", "{s}", .{fmtSliceHexUpper(try hexToBytes(&buf, ""))});
        try std.testing.expectError(error.InvalidCharacter, hexToBytes(&buf, "012Z"));
        try std.testing.expectError(error.InvalidLength, hexToBytes(&buf, "AAA"));
        try std.testing.expectError(error.NoSpaceLeft, hexToBytes(buf[0..1], "ABAB"));
    }

    test "formatIntValue with comptime_int" {
        const value: comptime_int = 123456789123456789;

        var buf: [20]u8 = undefined;
        var fbs = std.io.fixedBufferStream(&buf);
        try formatIntValue(value, "", FormatOptions{}, fbs.writer());
        try std.testing.expect(mem.eql(u8, fbs.getWritten(), "123456789123456789"));
    }

    test "formatFloatValue with comptime_float" {
        const value: comptime_float = 1.0;

        var buf: [20]u8 = undefined;
        var fbs = std.io.fixedBufferStream(&buf);
        try formatFloatValue(value, "", FormatOptions{}, fbs.writer());
        try std.testing.expect(mem.eql(u8, fbs.getWritten(), "1.0e+00"));

        try expectFmt("1.0e+00", "{}", .{value});
        try expectFmt("1.0e+00", "{}", .{1.0});
    }

    test "formatType max_depth" {
        if (builtin.zig_backend == .stage1) {
            // stage1 fails to return fully qualified namespaces.

            return error.SkipZigTest;
        }
        const Vec2 = struct {
            const SelfType = @This();
            x: f32,
            y: f32,

            pub fn format(
                self: SelfType,
                comptime fmt: []const u8,
                options: FormatOptions,
                writer: anytype,
            ) !void {
                _ = options;
                if (fmt.len == 0) {
                    return std.fmt.format(writer, "({d:.3},{d:.3})", .{ self.x, self.y });
                } else {
                    @compileError("unknown format string: '" ++ fmt ++ "'");
                }
            }
        };
        const E = enum {
            One,
            Two,
            Three,
        };
        const TU = union(enum) {
            const SelfType = @This();
            float: f32,
            int: u32,
            ptr: ?*SelfType,
        };
        const S = struct {
            const SelfType = @This();
            a: ?*SelfType,
            tu: TU,
            e: E,
            vec: Vec2,
        };

        var inst = S{
            .a = null,
            .tu = TU{ .ptr = null },
            .e = E.Two,
            .vec = Vec2{ .x = 10.2, .y = 2.22 },
        };
        inst.a = &inst;
        inst.tu.ptr = &inst.tu;

        var buf: [1000]u8 = undefined;
        var fbs = std.io.fixedBufferStream(&buf);
        try formatType(inst, "", FormatOptions{}, fbs.writer(), 0);
        try std.testing.expect(mem.eql(u8, fbs.getWritten(), "fmt.test.formatType max_depth.S{ ... }"));

        fbs.reset();
        try formatType(inst, "", FormatOptions{}, fbs.writer(), 1);
        try std.testing.expect(mem.eql(u8, fbs.getWritten(), "fmt.test.formatType max_depth.S{ .a = fmt.test.formatType max_depth.S{ ... }, .tu = fmt.test.formatType max_depth.TU{ ... }, .e = fmt.test.formatType max_depth.E.Two, .vec = (10.200,2.220) }"));

        fbs.reset();
        try formatType(inst, "", FormatOptions{}, fbs.writer(), 2);
        try std.testing.expect(mem.eql(u8, fbs.getWritten(), "fmt.test.formatType max_depth.S{ .a = fmt.test.formatType max_depth.S{ .a = fmt.test.formatType max_depth.S{ ... }, .tu = fmt.test.formatType max_depth.TU{ ... }, .e = fmt.test.formatType max_depth.E.Two, .vec = (10.200,2.220) }, .tu = fmt.test.formatType max_depth.TU{ .ptr = fmt.test.formatType max_depth.TU{ ... } }, .e = fmt.test.formatType max_depth.E.Two, .vec = (10.200,2.220) }"));

        fbs.reset();
        try formatType(inst, "", FormatOptions{}, fbs.writer(), 3);
        try std.testing.expect(mem.eql(u8, fbs.getWritten(), "fmt.test.formatType max_depth.S{ .a = fmt.test.formatType max_depth.S{ .a = fmt.test.formatType max_depth.S{ .a = fmt.test.formatType max_depth.S{ ... }, .tu = fmt.test.formatType max_depth.TU{ ... }, .e = fmt.test.formatType max_depth.E.Two, .vec = (10.200,2.220) }, .tu = fmt.test.formatType max_depth.TU{ .ptr = fmt.test.formatType max_depth.TU{ ... } }, .e = fmt.test.formatType max_depth.E.Two, .vec = (10.200,2.220) }, .tu = fmt.test.formatType max_depth.TU{ .ptr = fmt.test.formatType max_depth.TU{ .ptr = fmt.test.formatType max_depth.TU{ ... } } }, .e = fmt.test.formatType max_depth.E.Two, .vec = (10.200,2.220) }"));
    }

    test "positional" {
        try expectFmt("2 1 0", "{2} {1} {0}", .{ @as(usize, 0), @as(usize, 1), @as(usize, 2) });
        try expectFmt("2 1 0", "{2} {1} {}", .{ @as(usize, 0), @as(usize, 1), @as(usize, 2) });
        try expectFmt("0 0", "{0} {0}", .{@as(usize, 0)});
        try expectFmt("0 1", "{} {1}", .{ @as(usize, 0), @as(usize, 1) });
        try expectFmt("1 0 0 1", "{1} {} {0} {}", .{ @as(usize, 0), @as(usize, 1) });
    }

    test "positional with specifier" {
        try expectFmt("10.0", "{0d:.1}", .{@as(f64, 9.999)});
    }

    test "positional/alignment/width/precision" {
        try expectFmt("10.0", "{0d: >3.1}", .{@as(f64, 9.999)});
    }

    test "vector" {
        if (builtin.target.cpu.arch == .riscv64) {
            // https://github.com/ziglang/zig/issues/4486

            return error.SkipZigTest;
        }

        if (builtin.zig_backend == .stage1) {
            // Regressed in LLVM 14:

            // https://github.com/llvm/llvm-project/issues/55522

            return error.SkipZigTest;
        }

        const vbool: @Vector(4, bool) = [_]bool{ true, false, true, false };
        const vi64: @Vector(4, i64) = [_]i64{ -2, -1, 0, 1 };
        const vu64: @Vector(4, u64) = [_]u64{ 1000, 2000, 3000, 4000 };

        try expectFmt("{ true, false, true, false }", "{}", .{vbool});
        try expectFmt("{ -2, -1, 0, 1 }", "{}", .{vi64});
        try expectFmt("{    -2,    -1,    +0,    +1 }", "{d:5}", .{vi64});
        try expectFmt("{ 1000, 2000, 3000, 4000 }", "{}", .{vu64});
        try expectFmt("{ 3e8, 7d0, bb8, fa0 }", "{x}", .{vu64});
    }

    test "enum-literal" {
        try expectFmt(".hello_world", "{}", .{.hello_world});
    }

    test "padding" {
        try expectFmt("Simple", "{s}", .{"Simple"});
        try expectFmt("      true", "{:10}", .{true});
        try expectFmt("      true", "{:>10}", .{true});
        try expectFmt("======true", "{:=>10}", .{true});
        try expectFmt("true======", "{:=<10}", .{true});
        try expectFmt("   true   ", "{:^10}", .{true});
        try expectFmt("===true===", "{:=^10}", .{true});
        try expectFmt("           Minimum width", "{s:18} width", .{"Minimum"});
        try expectFmt("==================Filled", "{s:=>24}", .{"Filled"});
        try expectFmt("        Centered        ", "{s:^24}", .{"Centered"});
        try expectFmt("-", "{s:-^1}", .{""});
        try expectFmt("==crêpe===", "{s:=^10}", .{"crêpe"});
        try expectFmt("=====crêpe", "{s:=>10}", .{"crêpe"});
        try expectFmt("crêpe=====", "{s:=<10}", .{"crêpe"});
        try expectFmt("====a", "{c:=>5}", .{'a'});
        try expectFmt("==a==", "{c:=^5}", .{'a'});
        try expectFmt("a====", "{c:=<5}", .{'a'});
    }

    test "decimal float padding" {
        var number: f32 = 3.1415;
        try expectFmt("left-pad:   **3.141\n", "left-pad:   {d:*>7.3}\n", .{number});
        try expectFmt("center-pad: *3.141*\n", "center-pad: {d:*^7.3}\n", .{number});
        try expectFmt("right-pad:  3.141**\n", "right-pad:  {d:*<7.3}\n", .{number});
    }

    test "sci float padding" {
        var number: f32 = 3.1415;
        try expectFmt("left-pad:   **3.141e+00\n", "left-pad:   {e:*>11.3}\n", .{number});
        try expectFmt("center-pad: *3.141e+00*\n", "center-pad: {e:*^11.3}\n", .{number});
        try expectFmt("right-pad:  3.141e+00**\n", "right-pad:  {e:*<11.3}\n", .{number});
    }

    test "null" {
        const inst = null;
        try expectFmt("null", "{}", .{inst});
    }

    test "type" {
        try expectFmt("u8", "{}", .{u8});
        try expectFmt("?f32", "{}", .{?f32});
        try expectFmt("[]const u8", "{}", .{[]const u8});
    }

    test "named arguments" {
        try expectFmt("hello world!", "{s} world{c}", .{ "hello", '!' });
        try expectFmt("hello world!", "{[greeting]s} world{[punctuation]c}", .{ .punctuation = '!', .greeting = "hello" });
        try expectFmt("hello world!", "{[1]s} world{[0]c}", .{ '!', "hello" });
    }

    test "runtime width specifier" {
        var width: usize = 9;
        try expectFmt("~~hello~~", "{s:~^[1]}", .{ "hello", width });
        try expectFmt("~~hello~~", "{s:~^[width]}", .{ .string = "hello", .width = width });
        try expectFmt("    hello", "{s:[1]}", .{ "hello", width });
        try expectFmt("42     hello", "{d} {s:[2]}", .{ 42, "hello", width });
    }

    test "runtime precision specifier" {
        var number: f32 = 3.1415;
        var precision: usize = 2;
        try expectFmt("3.14e+00", "{:1.[1]}", .{ number, precision });
        try expectFmt("3.14e+00", "{:1.[precision]}", .{ .number = number, .precision = precision });
    }