    const std = @import("std");
    const common = @import("common.zig");
    const FloatStream = @import("FloatStream.zig");
    const isEightDigits = common.isEightDigits;
    const Number = common.Number;

    /// Parse 8 digits, loaded as bytes in little-endian order.
    ///
    /// This uses the trick where every digit is in [0x030, 0x39],
    /// and therefore can be parsed in 3 multiplications, much
    /// faster than the normal 8.
    ///
    /// This is based off the algorithm described in "Fast numeric string to
    /// int", available here: <https://johnnylee-sde.github.io/Fast-numeric-string-to-int/>.
    fn parse8Digits(v_: u64) u64 {
        var v = v_;
        const mask = 0x0000_00ff_0000_00ff;
        const mul1 = 0x000f_4240_0000_0064;
        const mul2 = 0x0000_2710_0000_0001;
        v -= 0x3030_3030_3030_3030;
        v = (v * 10) + (v >> 8); // will not overflow, fits in 63 bits

        const v1 = (v & mask) *% mul1;
        const v2 = ((v >> 16) & mask) *% mul2;
        return @as(u64, @truncate(u32, (v1 +% v2) >> 32));
    }

    /// Parse digits until a non-digit character is found.
    fn tryParseDigits(comptime T: type, stream: *FloatStream, x: *T, comptime base: u8) void {
        // Try to parse 8 digits at a time, using an optimized algorithm.

        // This only supports decimal digits.

        if (base == 10) {
            while (stream.hasLen(8)) {
                const v = stream.readU64Unchecked();
                if (!isEightDigits(v)) {
                    break;
                }

                x.* = x.* *% 1_0000_0000 +% parse8Digits(v);
                stream.advance(8);
            }
        }

        while (stream.scanDigit(base)) |digit| {
            x.* *%= base;
            x.* +%= digit;
        }
    }

    fn min_n_digit_int(comptime T: type, digit_count: usize) T {
        var n: T = 1;
        var i: usize = 1;
        while (i < digit_count) : (i += 1) n *= 10;
        return n;
    }

    /// Parse up to N digits
    fn tryParseNDigits(comptime T: type, stream: *FloatStream, x: *T, comptime base: u8, comptime n: usize) void {
        while (x.* < min_n_digit_int(T, n)) {
            if (stream.scanDigit(base)) |digit| {
                x.* *%= base;
                x.* +%= digit;
            } else {
                break;
            }
        }
    }

    /// Parse the scientific notation component of a float.
    fn parseScientific(stream: *FloatStream) ?i64 {
        var exponent: i64 = 0;
        var negative = false;

        if (stream.first()) |c| {
            negative = c == '-';
            if (c == '-' or c == '+') {
                stream.advance(1);
            }
        }
        if (stream.firstIsDigit(10)) {
            while (stream.scanDigit(10)) |digit| {
                // no overflows here, saturate well before overflow

                if (exponent < 0x1000_0000) {
                    exponent = 10 * exponent + digit;
                }
            }

            return if (negative) -exponent else exponent;
        }

        return null;
    }

    const ParseInfo = struct {
        // 10 or 16

        base: u8,
        // 10^19 fits in u64, 16^16 fits in u64

        max_mantissa_digits: usize,
        // e.g. e or p (E and P also checked)

        exp_char_lower: u8,
    };

    fn parsePartialNumberBase(comptime T: type, stream: *FloatStream, negative: bool, n: *usize, comptime info: ParseInfo) ?Number(T) {
        const MantissaT = common.mantissaType(T);

        // parse initial digits before dot

        var mantissa: MantissaT = 0;
        tryParseDigits(MantissaT, stream, &mantissa, info.base);
        var int_end = stream.offsetTrue();
        var n_digits = @intCast(isize, stream.offsetTrue());

        // handle dot with the following digits

        var exponent: i64 = 0;
        if (stream.firstIs('.')) {
            stream.advance(1);
            const marker = stream.offsetTrue();
            tryParseDigits(MantissaT, stream, &mantissa, info.base);
            const n_after_dot = stream.offsetTrue() - marker;
            exponent = -@intCast(i64, n_after_dot);
            n_digits += @intCast(isize, n_after_dot);
        }

        // adjust required shift to offset mantissa for base-16 (2^4)

        if (info.base == 16) {
            exponent *= 4;
        }

        if (n_digits == 0) {
            return null;
        }

        // handle scientific format

        var exp_number: i64 = 0;
        if (stream.firstIsLower(info.exp_char_lower)) {
            stream.advance(1);
            exp_number = parseScientific(stream) orelse return null;
            exponent += exp_number;
        }

        const len = stream.offset; // length must be complete parsed length

        n.* = len;

        if (stream.underscore_count > 0 and !validUnderscores(stream.slice, info.base)) {
            return null;
        }

        // common case with not many digits

        if (n_digits <= info.max_mantissa_digits) {
            return Number(T){
                .exponent = exponent,
                .mantissa = mantissa,
                .negative = negative,
                .many_digits = false,
                .hex = info.base == 16,
            };
        }

        n_digits -= info.max_mantissa_digits;
        var many_digits = false;
        stream.reset(); // re-parse from beginning

        while (stream.firstIs3('0', '.', '_')) {
            // '0' = '.' + 2

            const next = stream.firstUnchecked();
            if (next != '_') {
                n_digits -= @intCast(isize, next -| ('0' - 1));
            } else {
                stream.underscore_count += 1;
            }
            stream.advance(1);
        }
        if (n_digits > 0) {
            // at this point we have more than max_mantissa_digits significant digits, let's try again

            many_digits = true;
            mantissa = 0;
            stream.reset();
            tryParseNDigits(MantissaT, stream, &mantissa, info.base, info.max_mantissa_digits);

            exponent = blk: {
                if (mantissa >= min_n_digit_int(MantissaT, info.max_mantissa_digits)) {
                    // big int

                    break :blk @intCast(i64, int_end) - @intCast(i64, stream.offsetTrue());
                } else {
                    // the next byte must be present and be '.'

                    // We know this is true because we had more than 19

                    // digits previously, so we overflowed a 64-bit integer,

                    // but parsing only the integral digits produced less

                    // than 19 digits. That means we must have a decimal

                    // point, and at least 1 fractional digit.

                    stream.advance(1);
                    var marker = stream.offsetTrue();
                    tryParseNDigits(MantissaT, stream, &mantissa, info.base, info.max_mantissa_digits);
                    break :blk @intCast(i64, marker) - @intCast(i64, stream.offsetTrue());
                }
            };
            // add back the explicit part

            exponent += exp_number;
        }

        return Number(T){
            .exponent = exponent,
            .mantissa = mantissa,
            .negative = negative,
            .many_digits = many_digits,
            .hex = info.base == 16,
        };
    }

    /// Parse a partial, non-special floating point number.
    ///
    /// This creates a representation of the float as the
    /// significant digits and the decimal exponent.
    fn parsePartialNumber(comptime T: type, s: []const u8, negative: bool, n: *usize) ?Number(T) {
        std.debug.assert(s.len != 0);
        var stream = FloatStream.init(s);
        const MantissaT = common.mantissaType(T);

        if (stream.hasLen(2) and stream.atUnchecked(0) == '0' and std.ascii.toLower(stream.atUnchecked(1)) == 'x') {
            stream.advance(2);
            return parsePartialNumberBase(T, &stream, negative, n, .{
                .base = 16,
                .max_mantissa_digits = if (MantissaT == u64) 16 else 32,
                .exp_char_lower = 'p',
            });
        } else {
            return parsePartialNumberBase(T, &stream, negative, n, .{
                .base = 10,
                .max_mantissa_digits = if (MantissaT == u64) 19 else 38,
                .exp_char_lower = 'e',
            });
        }
    }

    pub fn parseNumber(comptime T: type, s: []const u8, negative: bool) ?Number(T) {
        var consumed: usize = 0;
        if (parsePartialNumber(T, s, negative, &consumed)) |number| {
            // must consume entire float (no trailing data)

            if (s.len == consumed) {
                return number;
            }
        }
        return null;
    }

    fn parsePartialInfOrNan(comptime T: type, s: []const u8, negative: bool, n: *usize) ?T {
        // inf/infinity; infxxx should only consume inf.

        if (std.ascii.startsWithIgnoreCase(s, "inf")) {
            n.* = 3;
            if (std.ascii.startsWithIgnoreCase(s[3..], "inity")) {
                n.* = 8;
            }

            return if (!negative) std.math.inf(T) else -std.math.inf(T);
        }

        if (std.ascii.startsWithIgnoreCase(s, "nan")) {
            n.* = 3;
            return std.math.nan(T);
        }

        return null;
    }

    pub fn parseInfOrNan(comptime T: type, s: []const u8, negative: bool) ?T {
        var consumed: usize = 0;
        if (parsePartialInfOrNan(T, s, negative, &consumed)) |special| {
            if (s.len == consumed) {
                return special;
            }
        }
        return null;
    }

    pub fn validUnderscores(s: []const u8, comptime base: u8) bool {
        var i: usize = 0;
        while (i < s.len) : (i += 1) {
            if (s[i] == '_') {
                // underscore at start of end

                if (i == 0 or i + 1 == s.len) {
                    return false;
                }
                // consecutive underscores

                if (!common.isDigit(s[i - 1], base) or !common.isDigit(s[i + 1], base)) {
                    return false;
                }

                // next is guaranteed a digit, skip an extra

                i += 1;
            }
        }

        return true;
    }