ona/src/coral/Box.zig

const coral = @import("./coral.zig");

const std = @import("std");

destroy: *const fn (*anyopaque) void,
erased: *anyopaque,

fn Layout(comptime Type: type) type {
    return switch (Type) {
        void => struct {
            fn destroy(_: *anyopaque) void {}
        },

        else => struct {
            allocator: std.mem.Allocator,
            value: Type,

            const TypeLayout = @This();

            fn destroy(erased: *anyopaque) void {
                const layout: *TypeLayout = @ptrCast(@alignCast(erased));

                if (@hasDecl(Type, "deinit")) {
                    const deinit_fn = switch (@typeInfo(@TypeOf(Type.deinit))) {
                        .@"fn" => |@"fn"| @"fn",

                        else => @compileError(std.fmt.comptimePrint("Declaration `{s}.deinit` must be an fn type to be boxable", .{
                            @typeName(Type),
                        })),
                    };

                    if (deinit_fn.params.len != 1 or deinit_fn.params[0].type != *Type) {
                        @compileError(std.fmt.comptimePrint("Fn `{s}.deinit` is only permitted 1 parameter to be boxable and it must be of type {s}", .{
                            @typeName(Type),
                            @typeName(*Type),
                        }));
                    }

                    layout.value.deinit();
                }

                layout.allocator.destroy(layout);
            }
        },
    };
}

const Self = @This();

pub fn deinit(self: *Self) void {
    self.destroy(self.erased);

    self.* = undefined;
}

pub fn has(self: Self, comptime Value: type) ?*Value {
    const ValueLayout = Layout(Value);

    if (self.destroy == ValueLayout.destroy) {
        const layout: *ValueLayout = @ptrCast(@alignCast(self.erased));

        return &layout.value;
    }

    return null;
}

pub fn init(allocator: std.mem.Allocator, value: anytype) std.mem.Allocator.Error!Self {
    const Value = @TypeOf(value);
    const ValueLayout = Layout(Value);
    const value_layout = try allocator.create(ValueLayout);

    errdefer {
        allocator.destroy(value_layout);
    }

    if (@hasField(ValueLayout, "allocator")) {
        value_layout.allocator = allocator;
    }

    if (@hasField(ValueLayout, "value")) {
        value_layout.value = value;
    }

    return .{
        .erased = @ptrCast(value_layout),
        .destroy = ValueLayout.destroy,
    };
}