const std = @import("std");
const Value = @import("value.zig").Value;
const assert = std.debug.assert;
const Allocator = std.mem.Allocator;

/// This is the raw data, with no bookkeeping, no memory awareness, no de-duplication.
/// It's important for this struct to be small.
/// It is not copyable since it may contain references to its inner data.
/// Types are not de-duplicated, which helps with multi-threading since it obviates the requirement
/// of obtaining a lock on a global type table, as well as making the
/// garbage collection bookkeeping simpler.
/// This union takes advantage of the fact that the first page of memory
/// is unmapped, giving us 4096 possible enum tags that have no payload.
pub const Type = extern union {
    /// If the tag value is less than Tag.no_payload_count, then no pointer
    /// dereference is needed.
    tag_if_small_enough: usize,
    ptr_otherwise: *Payload,

    pub fn zigTypeTag(self: Type) std.builtin.TypeId {
        switch (self.tag()) {
            .@"u8",
            .@"i8",
            .@"isize",
            .@"usize",
            .@"c_short",
            .@"c_ushort",
            .@"c_int",
            .@"c_uint",
            .@"c_long",
            .@"c_ulong",
            .@"c_longlong",
            .@"c_ulonglong",
            .@"c_longdouble",
            => return .Int,

            .@"f16",
            .@"f32",
            .@"f64",
            .@"f128",
            => return .Float,

            .@"c_void" => return .Opaque,
            .@"bool" => return .Bool,
            .@"void" => return .Void,
            .@"type" => return .Type,
            .@"anyerror" => return .ErrorSet,
            .@"comptime_int" => return .ComptimeInt,
            .@"comptime_float" => return .ComptimeFloat,
            .@"noreturn" => return .NoReturn,

            .fn_naked_noreturn_no_args => return .Fn,

            .array, .array_u8_sentinel_0 => return .Array,
            .single_const_pointer => return .Pointer,
            .const_slice_u8 => return .Pointer,
        }
    }

    pub fn initTag(comptime small_tag: Tag) Type {
        comptime assert(@enumToInt(small_tag) < Tag.no_payload_count);
        return .{ .tag_if_small_enough = @enumToInt(small_tag) };
    }

    pub fn initPayload(payload: *Payload) Type {
        assert(@enumToInt(payload.tag) >= Tag.no_payload_count);
        return .{ .ptr_otherwise = payload };
    }

    pub fn tag(self: Type) Tag {
        if (self.tag_if_small_enough < Tag.no_payload_count) {
            return @intToEnum(Tag, @intCast(@TagType(Tag), self.tag_if_small_enough));
        } else {
            return self.ptr_otherwise.tag;
        }
    }

    pub fn cast(self: Type, comptime T: type) ?*T {
        if (self.tag_if_small_enough < Tag.no_payload_count)
            return null;

        const expected_tag = std.meta.fieldInfo(T, "base").default_value.?.tag;
        if (self.ptr_otherwise.tag != expected_tag)
            return null;

        return @fieldParentPtr(T, "base", self.ptr_otherwise);
    }

    pub fn format(
        self: Type,
        comptime fmt: []const u8,
        options: std.fmt.FormatOptions,
        out_stream: var,
    ) !void {
        comptime assert(fmt.len == 0);
        var ty = self;
        while (true) {
            const t = ty.tag();
            switch (t) {
                .@"u8",
                .@"i8",
                .@"isize",
                .@"usize",
                .@"c_short",
                .@"c_ushort",
                .@"c_int",
                .@"c_uint",
                .@"c_long",
                .@"c_ulong",
                .@"c_longlong",
                .@"c_ulonglong",
                .@"c_longdouble",
                .@"c_void",
                .@"f16",
                .@"f32",
                .@"f64",
                .@"f128",
                .@"bool",
                .@"void",
                .@"type",
                .@"anyerror",
                .@"comptime_int",
                .@"comptime_float",
                .@"noreturn",
                => return out_stream.writeAll(@tagName(t)),

                .const_slice_u8 => return out_stream.writeAll("[]const u8"),
                .fn_naked_noreturn_no_args => return out_stream.writeAll("fn() callconv(.Naked) noreturn"),

                .array_u8_sentinel_0 => {
                    const payload = @fieldParentPtr(Payload.Array_u8_Sentinel0, "base", ty.ptr_otherwise);
                    return out_stream.print("[{}:0]u8", .{payload.len});
                },
                .array => {
                    const payload = @fieldParentPtr(Payload.Array, "base", ty.ptr_otherwise);
                    try out_stream.print("[{}]", .{payload.len});
                    ty = payload.elem_type;
                    continue;
                },
                .single_const_pointer => {
                    const payload = @fieldParentPtr(Payload.SingleConstPointer, "base", ty.ptr_otherwise);
                    try out_stream.writeAll("*const ");
                    ty = payload.pointee_type;
                    continue;
                },
            }
            unreachable;
        }
    }

    pub fn toValue(self: Type, allocator: *Allocator) Allocator.Error!Value {
        switch (self.tag()) {
            .@"u8" => return Value.initTag(.u8_type),
            .@"i8" => return Value.initTag(.i8_type),
            .@"isize" => return Value.initTag(.isize_type),
            .@"usize" => return Value.initTag(.usize_type),
            .@"c_short" => return Value.initTag(.c_short_type),
            .@"c_ushort" => return Value.initTag(.c_ushort_type),
            .@"c_int" => return Value.initTag(.c_int_type),
            .@"c_uint" => return Value.initTag(.c_uint_type),
            .@"c_long" => return Value.initTag(.c_long_type),
            .@"c_ulong" => return Value.initTag(.c_ulong_type),
            .@"c_longlong" => return Value.initTag(.c_longlong_type),
            .@"c_ulonglong" => return Value.initTag(.c_ulonglong_type),
            .@"c_longdouble" => return Value.initTag(.c_longdouble_type),
            .@"c_void" => return Value.initTag(.c_void_type),
            .@"f16" => return Value.initTag(.f16_type),
            .@"f32" => return Value.initTag(.f32_type),
            .@"f64" => return Value.initTag(.f64_type),
            .@"f128" => return Value.initTag(.f128_type),
            .@"bool" => return Value.initTag(.bool_type),
            .@"void" => return Value.initTag(.void_type),
            .@"type" => return Value.initTag(.type_type),
            .@"anyerror" => return Value.initTag(.anyerror_type),
            .@"comptime_int" => return Value.initTag(.comptime_int_type),
            .@"comptime_float" => return Value.initTag(.comptime_float_type),
            .@"noreturn" => return Value.initTag(.noreturn_type),
            .fn_naked_noreturn_no_args => return Value.initTag(.fn_naked_noreturn_no_args_type),
            .const_slice_u8 => return Value.initTag(.const_slice_u8_type),
            else => {
                const ty_payload = try allocator.create(Value.Payload.Ty);
                ty_payload.* = .{ .ty = self };
                return Value.initPayload(&ty_payload.base);
            },
        }
    }

    pub fn isSinglePointer(self: Type) bool {
        return switch (self.tag()) {
            .@"u8",
            .@"i8",
            .@"isize",
            .@"usize",
            .@"c_short",
            .@"c_ushort",
            .@"c_int",
            .@"c_uint",
            .@"c_long",
            .@"c_ulong",
            .@"c_longlong",
            .@"c_ulonglong",
            .@"c_longdouble",
            .@"f16",
            .@"f32",
            .@"f64",
            .@"f128",
            .@"c_void",
            .@"bool",
            .@"void",
            .@"type",
            .@"anyerror",
            .@"comptime_int",
            .@"comptime_float",
            .@"noreturn",
            .array,
            .array_u8_sentinel_0,
            .const_slice_u8,
            .fn_naked_noreturn_no_args,
            => false,

            .single_const_pointer => true,
        };
    }

    pub fn isSlice(self: Type) bool {
        return switch (self.tag()) {
            .@"u8",
            .@"i8",
            .@"isize",
            .@"usize",
            .@"c_short",
            .@"c_ushort",
            .@"c_int",
            .@"c_uint",
            .@"c_long",
            .@"c_ulong",
            .@"c_longlong",
            .@"c_ulonglong",
            .@"c_longdouble",
            .@"f16",
            .@"f32",
            .@"f64",
            .@"f128",
            .@"c_void",
            .@"bool",
            .@"void",
            .@"type",
            .@"anyerror",
            .@"comptime_int",
            .@"comptime_float",
            .@"noreturn",
            .array,
            .array_u8_sentinel_0,
            .single_const_pointer,
            .fn_naked_noreturn_no_args,
            => false,

            .const_slice_u8 => true,
        };
    }

    /// Asserts the type is a pointer type.
    pub fn pointerIsConst(self: Type) bool {
        return switch (self.tag()) {
            .@"u8",
            .@"i8",
            .@"isize",
            .@"usize",
            .@"c_short",
            .@"c_ushort",
            .@"c_int",
            .@"c_uint",
            .@"c_long",
            .@"c_ulong",
            .@"c_longlong",
            .@"c_ulonglong",
            .@"c_longdouble",
            .@"f16",
            .@"f32",
            .@"f64",
            .@"f128",
            .@"c_void",
            .@"bool",
            .@"void",
            .@"type",
            .@"anyerror",
            .@"comptime_int",
            .@"comptime_float",
            .@"noreturn",
            .array,
            .array_u8_sentinel_0,
            .fn_naked_noreturn_no_args,
            => unreachable,

            .single_const_pointer, .const_slice_u8 => true,
        };
    }

    /// Asserts the type is a pointer or array type.
    pub fn elemType(self: Type) Type {
        return switch (self.tag()) {
            .@"u8",
            .@"i8",
            .@"isize",
            .@"usize",
            .@"c_short",
            .@"c_ushort",
            .@"c_int",
            .@"c_uint",
            .@"c_long",
            .@"c_ulong",
            .@"c_longlong",
            .@"c_ulonglong",
            .@"c_longdouble",
            .@"f16",
            .@"f32",
            .@"f64",
            .@"f128",
            .@"c_void",
            .@"bool",
            .@"void",
            .@"type",
            .@"anyerror",
            .@"comptime_int",
            .@"comptime_float",
            .@"noreturn",
            .fn_naked_noreturn_no_args,
            => unreachable,

            .array => self.cast(Payload.Array).?.elem_type,
            .single_const_pointer => self.cast(Payload.SingleConstPointer).?.pointee_type,
            .array_u8_sentinel_0, .const_slice_u8 => Type.initTag(.@"u8"),
        };
    }

    /// This enum does not directly correspond to `std.builtin.TypeId` because
    /// it has extra enum tags in it, as a way of using less memory. For example,
    /// even though Zig recognizes `*align(10) i32` and `*i32` both as Pointer types
    /// but with different alignment values, in this data structure they are represented
    /// with different enum tags, because the the former requires more payload data than the latter.
    /// See `zigTypeTag` for the function that corresponds to `std.builtin.TypeId`.
    pub const Tag = enum {
        // The first section of this enum are tags that require no payload.
        @"u8",
        @"i8",
        @"isize",
        @"usize",
        @"c_short",
        @"c_ushort",
        @"c_int",
        @"c_uint",
        @"c_long",
        @"c_ulong",
        @"c_longlong",
        @"c_ulonglong",
        @"c_longdouble",
        @"c_void",
        @"f16",
        @"f32",
        @"f64",
        @"f128",
        @"bool",
        @"void",
        @"type",
        @"anyerror",
        @"comptime_int",
        @"comptime_float",
        @"noreturn",
        fn_naked_noreturn_no_args,
        const_slice_u8, // See last_no_payload_tag below.
        // After this, the tag requires a payload.

        array_u8_sentinel_0,
        array,
        single_const_pointer,

        pub const last_no_payload_tag = Tag.const_slice_u8;
        pub const no_payload_count = @enumToInt(last_no_payload_tag) + 1;
    };

    pub const Payload = struct {
        tag: Tag,

        pub const Array_u8_Sentinel0 = struct {
            base: Payload = Payload{ .tag = .array_u8_sentinel_0 },

            len: u64,
        };

        pub const Array = struct {
            base: Payload = Payload{ .tag = .array },

            elem_type: Type,
            len: u64,
        };

        pub const SingleConstPointer = struct {
            base: Payload = Payload{ .tag = .single_const_pointer },

            pointee_type: Type,
        };
    };
};