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// Copyright 2025 Shota FUJI
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// SPDX-License-Identifier: Apache-2.0

//! Atomic Reference Counting based on example code on `std.atomic.Value`.
//! https://ziglang.org/documentation/0.14.0/std/#std.atomic.Value

const std = @import("std");

const Rc = extern struct {
    count: std.atomic.Value(usize) = std.atomic.Value(usize).init(0),

    /// Increment reference count by one.
    pub fn ref(arc: *Rc) callconv(.C) void {
        _ = arc.count.fetchAdd(1, .monotonic);
    }

    /// Decrement reference count by one, returns `true` if no reference is alive.
    pub fn unref(arc: *Rc) callconv(.C) bool {
        if (arc.count.fetchSub(1, .release) == 1) {
            _ = arc.count.load(.acquire);
            return true;
        } else {
            return false;
        }
    }
};

pub fn RefCounted(comptime T: type) type {
    const Wrapped = if (@typeInfo(T).@"struct".layout == .@"extern") extern struct {
        pub const cname = T.cname;
        pub const Data = T;

        rc: Rc = .{},
        data: T,

        pub fn new(data: T) !*@This() {
            const self = try std.heap.c_allocator.create(@This());

            self.* = .{
                .data = data,
            };

            self.rc.ref();

            return self;
        }

        pub fn retain(ptr: *@This()) callconv(.C) *@This() {
            ptr.rc.ref();
            return ptr;
        }

        pub fn release(ptr: *@This()) callconv(.C) void {
            if (ptr.rc.unref()) {
                if (@hasDecl(T, "deinit")) {
                    T.deinit(&ptr.data);
                }

                std.heap.c_allocator.destroy(ptr);
            }
        }
    } else struct {
        pub const cname = T.cname;
        pub const Data = T;

        rc: Rc = .{},
        data: T,

        pub fn new(data: T) !*@This() {
            const self = try std.heap.c_allocator.create(@This());

            self.* = .{
                .data = data,
            };

            self.rc.ref();

            return self;
        }

        pub fn retain(ptr: *@This()) callconv(.C) *@This() {
            ptr.rc.ref();
            return ptr;
        }

        pub fn release(ptr: *@This()) callconv(.C) void {
            if (ptr.rc.unref()) {
                if (@hasDecl(T, "deinit")) {
                    T.deinit(&ptr.data);
                }

                std.heap.c_allocator.destroy(ptr);
            }
        }
    };

    comptime {
        @export(&Wrapped.retain, .{
            .name = std.fmt.comptimePrint("{s}_retain", .{T.cname}),
        });

        @export(&Wrapped.release, .{
            .name = std.fmt.comptimePrint("{s}_release", .{T.cname}),
        });
    }

    return Wrapped;
}