    ///! Contains all constants and types representing the wasm
    ///! binary format, as specified by:
    ///! https://webassembly.github.io/spec/core/
    const std = @import("std.zig");
    const testing = std.testing;

    // TODO: Add support for multi-byte ops (e.g. table operations)

    /// Wasm instruction opcodes
    ///
    /// All instructions are defined as per spec:
    /// https://webassembly.github.io/spec/core/appendix/index-instructions.html
    pub const Opcode = enum(u8) {
        @"unreachable" = 0x00,
        nop = 0x01,
        block = 0x02,
        loop = 0x03,
        @"if" = 0x04,
        @"else" = 0x05,
        end = 0x0B,
        br = 0x0C,
        br_if = 0x0D,
        br_table = 0x0E,
        @"return" = 0x0F,
        call = 0x10,
        call_indirect = 0x11,
        drop = 0x1A,
        select = 0x1B,
        local_get = 0x20,
        local_set = 0x21,
        local_tee = 0x22,
        global_get = 0x23,
        global_set = 0x24,
        i32_load = 0x28,
        i64_load = 0x29,
        f32_load = 0x2A,
        f64_load = 0x2B,
        i32_load8_s = 0x2C,
        i32_load8_u = 0x2D,
        i32_load16_s = 0x2E,
        i32_load16_u = 0x2F,
        i64_load8_s = 0x30,
        i64_load8_u = 0x31,
        i64_load16_s = 0x32,
        i64_load16_u = 0x33,
        i64_load32_s = 0x34,
        i64_load32_u = 0x35,
        i32_store = 0x36,
        i64_store = 0x37,
        f32_store = 0x38,
        f64_store = 0x39,
        i32_store8 = 0x3A,
        i32_store16 = 0x3B,
        i64_store8 = 0x3C,
        i64_store16 = 0x3D,
        i64_store32 = 0x3E,
        memory_size = 0x3F,
        memory_grow = 0x40,
        i32_const = 0x41,
        i64_const = 0x42,
        f32_const = 0x43,
        f64_const = 0x44,
        i32_eqz = 0x45,
        i32_eq = 0x46,
        i32_ne = 0x47,
        i32_lt_s = 0x48,
        i32_lt_u = 0x49,
        i32_gt_s = 0x4A,
        i32_gt_u = 0x4B,
        i32_le_s = 0x4C,
        i32_le_u = 0x4D,
        i32_ge_s = 0x4E,
        i32_ge_u = 0x4F,
        i64_eqz = 0x50,
        i64_eq = 0x51,
        i64_ne = 0x52,
        i64_lt_s = 0x53,
        i64_lt_u = 0x54,
        i64_gt_s = 0x55,
        i64_gt_u = 0x56,
        i64_le_s = 0x57,
        i64_le_u = 0x58,
        i64_ge_s = 0x59,
        i64_ge_u = 0x5A,
        f32_eq = 0x5B,
        f32_ne = 0x5C,
        f32_lt = 0x5D,
        f32_gt = 0x5E,
        f32_le = 0x5F,
        f32_ge = 0x60,
        f64_eq = 0x61,
        f64_ne = 0x62,
        f64_lt = 0x63,
        f64_gt = 0x64,
        f64_le = 0x65,
        f64_ge = 0x66,
        i32_clz = 0x67,
        i32_ctz = 0x68,
        i32_popcnt = 0x69,
        i32_add = 0x6A,
        i32_sub = 0x6B,
        i32_mul = 0x6C,
        i32_div_s = 0x6D,
        i32_div_u = 0x6E,
        i32_rem_s = 0x6F,
        i32_rem_u = 0x70,
        i32_and = 0x71,
        i32_or = 0x72,
        i32_xor = 0x73,
        i32_shl = 0x74,
        i32_shr_s = 0x75,
        i32_shr_u = 0x76,
        i32_rotl = 0x77,
        i32_rotr = 0x78,
        i64_clz = 0x79,
        i64_ctz = 0x7A,
        i64_popcnt = 0x7B,
        i64_add = 0x7C,
        i64_sub = 0x7D,
        i64_mul = 0x7E,
        i64_div_s = 0x7F,
        i64_div_u = 0x80,
        i64_rem_s = 0x81,
        i64_rem_u = 0x82,
        i64_and = 0x83,
        i64_or = 0x84,
        i64_xor = 0x85,
        i64_shl = 0x86,
        i64_shr_s = 0x87,
        i64_shr_u = 0x88,
        i64_rotl = 0x89,
        i64_rotr = 0x8A,
        f32_abs = 0x8B,
        f32_neg = 0x8C,
        f32_ceil = 0x8D,
        f32_floor = 0x8E,
        f32_trunc = 0x8F,
        f32_nearest = 0x90,
        f32_sqrt = 0x91,
        f32_add = 0x92,
        f32_sub = 0x93,
        f32_mul = 0x94,
        f32_div = 0x95,
        f32_min = 0x96,
        f32_max = 0x97,
        f32_copysign = 0x98,
        f64_abs = 0x99,
        f64_neg = 0x9A,
        f64_ceil = 0x9B,
        f64_floor = 0x9C,
        f64_trunc = 0x9D,
        f64_nearest = 0x9E,
        f64_sqrt = 0x9F,
        f64_add = 0xA0,
        f64_sub = 0xA1,
        f64_mul = 0xA2,
        f64_div = 0xA3,
        f64_min = 0xA4,
        f64_max = 0xA5,
        f64_copysign = 0xA6,
        i32_wrap_i64 = 0xA7,
        i32_trunc_f32_s = 0xA8,
        i32_trunc_f32_u = 0xA9,
        i32_trunc_f64_s = 0xAA,
        i32_trunc_f64_u = 0xAB,
        i64_extend_i32_s = 0xAC,
        i64_extend_i32_u = 0xAD,
        i64_trunc_f32_s = 0xAE,
        i64_trunc_f32_u = 0xAF,
        i64_trunc_f64_s = 0xB0,
        i64_trunc_f64_u = 0xB1,
        f32_convert_i32_s = 0xB2,
        f32_convert_i32_u = 0xB3,
        f32_convert_i64_s = 0xB4,
        f32_convert_i64_u = 0xB5,
        f32_demote_f64 = 0xB6,
        f64_convert_i32_s = 0xB7,
        f64_convert_i32_u = 0xB8,
        f64_convert_i64_s = 0xB9,
        f64_convert_i64_u = 0xBA,
        f64_promote_f32 = 0xBB,
        i32_reinterpret_f32 = 0xBC,
        i64_reinterpret_f64 = 0xBD,
        f32_reinterpret_i32 = 0xBE,
        f64_reinterpret_i64 = 0xBF,
        i32_extend8_s = 0xC0,
        i32_extend16_s = 0xC1,
        i64_extend8_s = 0xC2,
        i64_extend16_s = 0xC3,
        i64_extend32_s = 0xC4,
        _,
    };

    /// Returns the integer value of an `Opcode`. Used by the Zig compiler
    /// to write instructions to the wasm binary file
    pub fn opcode(op: Opcode) u8 {
        return @enumToInt(op);
    }

    test "Wasm - opcodes" {
        // Ensure our opcodes values remain intact as certain values are skipped due to them being reserved

        const i32_const = opcode(.i32_const);
        const end = opcode(.end);
        const drop = opcode(.drop);
        const local_get = opcode(.local_get);
        const i64_extend32_s = opcode(.i64_extend32_s);

        try testing.expectEqual(@as(u16, 0x41), i32_const);
        try testing.expectEqual(@as(u16, 0x0B), end);
        try testing.expectEqual(@as(u16, 0x1A), drop);
        try testing.expectEqual(@as(u16, 0x20), local_get);
        try testing.expectEqual(@as(u16, 0xC4), i64_extend32_s);
    }

    /// Opcodes that require a prefix `0xFC`
    pub const PrefixedOpcode = enum(u8) {
        i32_trunc_sat_f32_s = 0x00,
        i32_trunc_sat_f32_u = 0x01,
        i32_trunc_sat_f64_s = 0x02,
        i32_trunc_sat_f64_u = 0x03,
        i64_trunc_sat_f32_s = 0x04,
        i64_trunc_sat_f32_u = 0x05,
        i64_trunc_sat_f64_s = 0x06,
        i64_trunc_sat_f64_u = 0x07,
        memory_init = 0x08,
        data_drop = 0x09,
        memory_copy = 0x0A,
        memory_fill = 0x0B,
        table_init = 0x0C,
        elem_drop = 0x0D,
        table_copy = 0x0E,
        table_grow = 0x0F,
        table_size = 0x10,
        table_fill = 0x11,
    };

    /// Enum representing all Wasm value types as per spec:
    /// https://webassembly.github.io/spec/core/binary/types.html
    pub const Valtype = enum(u8) {
        i32 = 0x7F,
        i64 = 0x7E,
        f32 = 0x7D,
        f64 = 0x7C,
    };

    /// Returns the integer value of a `Valtype`
    pub fn valtype(value: Valtype) u8 {
        return @enumToInt(value);
    }

    /// Reference types, where the funcref references to a function regardless of its type
    /// and ref references an object from the embedder.
    pub const RefType = enum(u8) {
        funcref = 0x70,
        externref = 0x6F,
    };

    /// Returns the integer value of a `Reftype`
    pub fn reftype(value: RefType) u8 {
        return @enumToInt(value);
    }

    test "Wasm - valtypes" {
        const _i32 = valtype(.i32);
        const _i64 = valtype(.i64);
        const _f32 = valtype(.f32);
        const _f64 = valtype(.f64);

        try testing.expectEqual(@as(u8, 0x7F), _i32);
        try testing.expectEqual(@as(u8, 0x7E), _i64);
        try testing.expectEqual(@as(u8, 0x7D), _f32);
        try testing.expectEqual(@as(u8, 0x7C), _f64);
    }

    /// Limits classify the size range of resizeable storage associated with memory types and table types.
    pub const Limits = struct {
        min: u32,
        max: ?u32,
    };

    /// Initialization expressions are used to set the initial value on an object
    /// when a wasm module is being loaded.
    pub const InitExpression = union(enum) {
        i32_const: i32,
        i64_const: i64,
        f32_const: f32,
        f64_const: f64,
        global_get: u32,
    };

    /// Represents a function entry, holding the index to its type
    pub const Func = struct {
        type_index: u32,
    };

    /// Tables are used to hold pointers to opaque objects.
    /// This can either by any function, or an object from the host.
    pub const Table = struct {
        limits: Limits,
        reftype: RefType,
    };

    /// Describes the layout of the memory where `min` represents
    /// the minimal amount of pages, and the optional `max` represents
    /// the max pages. When `null` will allow the host to determine the
    /// amount of pages.
    pub const Memory = struct {
        limits: Limits,
    };

    /// Represents the type of a `Global` or an imported global.
    pub const GlobalType = struct {
        valtype: Valtype,
        mutable: bool,
    };

    pub const Global = struct {
        global_type: GlobalType,
        init: InitExpression,
    };

    /// Notates an object to be exported from wasm
    /// to the host.
    pub const Export = struct {
        name: []const u8,
        kind: ExternalKind,
        index: u32,
    };

    /// Element describes the layout of the table that can
    /// be found at `table_index`
    pub const Element = struct {
        table_index: u32,
        offset: InitExpression,
        func_indexes: []const u32,
    };

    /// Imports are used to import objects from the host
    pub const Import = struct {
        module_name: []const u8,
        name: []const u8,
        kind: Kind,

        pub const Kind = union(ExternalKind) {
            function: u32,
            table: Table,
            memory: Limits,
            global: GlobalType,
        };
    };

    /// `Type` represents a function signature type containing both
    /// a slice of parameters as well as a slice of return values.
    pub const Type = struct {
        params: []const Valtype,
        returns: []const Valtype,

        pub fn format(self: Type, comptime fmt: []const u8, opt: std.fmt.FormatOptions, writer: anytype) !void {
            _ = fmt;
            _ = opt;
            try writer.writeByte('(');
            for (self.params) |param, i| {
                try writer.print("{s}", .{@tagName(param)});
                if (i + 1 != self.params.len) {
                    try writer.writeAll(", ");
                }
            }
            try writer.writeAll(") -> ");
            if (self.returns.len == 0) {
                try writer.writeAll("nil");
            } else {
                for (self.returns) |return_ty, i| {
                    try writer.print("{s}", .{@tagName(return_ty)});
                    if (i + 1 != self.returns.len) {
                        try writer.writeAll(", ");
                    }
                }
            }
        }

        pub fn eql(self: Type, other: Type) bool {
            return std.mem.eql(Valtype, self.params, other.params) and
                std.mem.eql(Valtype, self.returns, other.returns);
        }

        pub fn deinit(self: *Type, gpa: std.mem.Allocator) void {
            gpa.free(self.params);
            gpa.free(self.returns);
            self.* = undefined;
        }
    };

    /// Wasm module sections as per spec:
    /// https://webassembly.github.io/spec/core/binary/modules.html
    pub const Section = enum(u8) {
        custom,
        type,
        import,
        function,
        table,
        memory,
        global,
        @"export",
        start,
        element,
        code,
        data,
        data_count,
        _,
    };

    /// Returns the integer value of a given `Section`
    pub fn section(val: Section) u8 {
        return @enumToInt(val);
    }

    /// The kind of the type when importing or exporting to/from the host environment
    /// https://webassembly.github.io/spec/core/syntax/modules.html
    pub const ExternalKind = enum(u8) {
        function,
        table,
        memory,
        global,
    };

    /// Returns the integer value of a given `ExternalKind`
    pub fn externalKind(val: ExternalKind) u8 {
        return @enumToInt(val);
    }

    /// Defines the enum values for each subsection id for the "Names" custom section
    /// as described by:
    /// https://webassembly.github.io/spec/core/appendix/custom.html?highlight=name#name-section
    pub const NameSubsection = enum(u8) {
        module,
        function,
        local,
        label,
        type,
        table,
        memory,
        global,
        elem_segment,
        data_segment,
    };

    // type constants

    pub const element_type: u8 = 0x70;
    pub const function_type: u8 = 0x60;
    pub const result_type: u8 = 0x40;

    /// Represents a block which will not return a value
    pub const block_empty: u8 = 0x40;

    // binary constants

    pub const magic = [_]u8{ 0x00, 0x61, 0x73, 0x6D }; // \0asm

    pub const version = [_]u8{ 0x01, 0x00, 0x00, 0x00 }; // version 1 (MVP)

    // Each wasm page size is 64kB

    pub const page_size = 64 * 1024;