    const std = @import("../std.zig");
    const builtin = @import("builtin");
    const assert = std.debug.assert;
    const testing = std.testing;
    const Loop = std.event.Loop;

    /// Many producer, many consumer, thread-safe, runtime configurable buffer size.
    /// When buffer is empty, consumers suspend and are resumed by producers.
    /// When buffer is full, producers suspend and are resumed by consumers.
    pub fn Channel(comptime T: type) type {
        return struct {
            getters: std.atomic.Queue(GetNode),
            or_null_queue: std.atomic.Queue(*std.atomic.Queue(GetNode).Node),
            putters: std.atomic.Queue(PutNode),
            get_count: usize,
            put_count: usize,
            dispatch_lock: bool,
            need_dispatch: bool,

            // simple fixed size ring buffer

            buffer_nodes: []T,
            buffer_index: usize,
            buffer_len: usize,

            const SelfChannel = @This();
            const GetNode = struct {
                tick_node: *Loop.NextTickNode,
                data: Data,

                const Data = union(enum) {
                    Normal: Normal,
                    OrNull: OrNull,
                };

                const Normal = struct {
                    ptr: *T,
                };

                const OrNull = struct {
                    ptr: *?T,
                    or_null: *std.atomic.Queue(*std.atomic.Queue(GetNode).Node).Node,
                };
            };
            const PutNode = struct {
                data: T,
                tick_node: *Loop.NextTickNode,
            };

            const global_event_loop = Loop.instance orelse
                @compileError("std.event.Channel currently only works with event-based I/O");

            /// Call `deinit` to free resources when done.
            /// `buffer` must live until `deinit` is called.
            /// For a zero length buffer, use `[0]T{}`.
            /// TODO https://github.com/ziglang/zig/issues/2765
            pub fn init(self: *SelfChannel, buffer: []T) void {
                // The ring buffer implementation only works with power of 2 buffer sizes

                // because of relying on subtracting across zero. For example (0 -% 1) % 10 == 5

                assert(buffer.len == 0 or @popCount(buffer.len) == 1);

                self.* = SelfChannel{
                    .buffer_len = 0,
                    .buffer_nodes = buffer,
                    .buffer_index = 0,
                    .dispatch_lock = false,
                    .need_dispatch = false,
                    .getters = std.atomic.Queue(GetNode).init(),
                    .putters = std.atomic.Queue(PutNode).init(),
                    .or_null_queue = std.atomic.Queue(*std.atomic.Queue(GetNode).Node).init(),
                    .get_count = 0,
                    .put_count = 0,
                };
            }

            /// Must be called when all calls to put and get have suspended and no more calls occur.
            /// This can be omitted if caller can guarantee that the suspended putters and getters
            /// do not need to be run to completion. Note that this may leave awaiters hanging.
            pub fn deinit(self: *SelfChannel) void {
                while (self.getters.get()) |get_node| {
                    resume get_node.data.tick_node.data;
                }
                while (self.putters.get()) |put_node| {
                    resume put_node.data.tick_node.data;
                }
                self.* = undefined;
            }

            /// puts a data item in the channel. The function returns when the value has been added to the
            /// buffer, or in the case of a zero size buffer, when the item has been retrieved by a getter.
            /// Or when the channel is destroyed.
            pub fn put(self: *SelfChannel, data: T) void {
                var my_tick_node = Loop.NextTickNode{ .data = @frame() };
                var queue_node = std.atomic.Queue(PutNode).Node{
                    .data = PutNode{
                        .tick_node = &my_tick_node,
                        .data = data,
                    },
                };

                suspend {
                    self.putters.put(&queue_node);
                    _ = @atomicRmw(usize, &self.put_count, .Add, 1, .SeqCst);

                    self.dispatch();
                }
            }

            /// await this function to get an item from the channel. If the buffer is empty, the frame will
            /// complete when the next item is put in the channel.
            pub fn get(self: *SelfChannel) callconv(.Async) T {
                // TODO https://github.com/ziglang/zig/issues/2765

                var result: T = undefined;
                var my_tick_node = Loop.NextTickNode{ .data = @frame() };
                var queue_node = std.atomic.Queue(GetNode).Node{
                    .data = GetNode{
                        .tick_node = &my_tick_node,
                        .data = GetNode.Data{
                            .Normal = GetNode.Normal{ .ptr = &result },
                        },
                    },
                };

                suspend {
                    self.getters.put(&queue_node);
                    _ = @atomicRmw(usize, &self.get_count, .Add, 1, .SeqCst);

                    self.dispatch();
                }
                return result;
            }

            //pub async fn select(comptime EnumUnion: type, channels: ...) EnumUnion {

            //    assert(@memberCount(EnumUnion) == channels.len); // enum union and channels mismatch

            //    assert(channels.len != 0); // enum unions cannot have 0 fields

            //    if (channels.len == 1) {

            //        const result = await (async channels[0].get() catch unreachable);

            //        return @unionInit(EnumUnion, @memberName(EnumUnion, 0), result);

            //    }

            //}

            /// Get an item from the channel. If the buffer is empty and there are no
            /// puts waiting, this returns `null`.
            pub fn getOrNull(self: *SelfChannel) ?T {
                // TODO integrate this function with named return values

                // so we can get rid of this extra result copy

                var result: ?T = null;
                var my_tick_node = Loop.NextTickNode{ .data = @frame() };
                var or_null_node = std.atomic.Queue(*std.atomic.Queue(GetNode).Node).Node{ .data = undefined };
                var queue_node = std.atomic.Queue(GetNode).Node{
                    .data = GetNode{
                        .tick_node = &my_tick_node,
                        .data = GetNode.Data{
                            .OrNull = GetNode.OrNull{
                                .ptr = &result,
                                .or_null = &or_null_node,
                            },
                        },
                    },
                };
                or_null_node.data = &queue_node;

                suspend {
                    self.getters.put(&queue_node);
                    _ = @atomicRmw(usize, &self.get_count, .Add, 1, .SeqCst);
                    self.or_null_queue.put(&or_null_node);

                    self.dispatch();
                }
                return result;
            }

            fn dispatch(self: *SelfChannel) void {
                // set the "need dispatch" flag

                @atomicStore(bool, &self.need_dispatch, true, .SeqCst);

                lock: while (true) {
                    // set the lock flag

                    if (@atomicRmw(bool, &self.dispatch_lock, .Xchg, true, .SeqCst)) return;

                    // clear the need_dispatch flag since we're about to do it

                    @atomicStore(bool, &self.need_dispatch, false, .SeqCst);

                    while (true) {
                        one_dispatch: {
                            // later we correct these extra subtractions

                            var get_count = @atomicRmw(usize, &self.get_count, .Sub, 1, .SeqCst);
                            var put_count = @atomicRmw(usize, &self.put_count, .Sub, 1, .SeqCst);

                            // transfer self.buffer to self.getters

                            while (self.buffer_len != 0) {
                                if (get_count == 0) break :one_dispatch;

                                const get_node = &self.getters.get().?.data;
                                switch (get_node.data) {
                                    GetNode.Data.Normal => |info| {
                                        info.ptr.* = self.buffer_nodes[(self.buffer_index -% self.buffer_len) % self.buffer_nodes.len];
                                    },
                                    GetNode.Data.OrNull => |info| {
                                        _ = self.or_null_queue.remove(info.or_null);
                                        info.ptr.* = self.buffer_nodes[(self.buffer_index -% self.buffer_len) % self.buffer_nodes.len];
                                    },
                                }
                                global_event_loop.onNextTick(get_node.tick_node);
                                self.buffer_len -= 1;

                                get_count = @atomicRmw(usize, &self.get_count, .Sub, 1, .SeqCst);
                            }

                            // direct transfer self.putters to self.getters

                            while (get_count != 0 and put_count != 0) {
                                const get_node = &self.getters.get().?.data;
                                const put_node = &self.putters.get().?.data;

                                switch (get_node.data) {
                                    GetNode.Data.Normal => |info| {
                                        info.ptr.* = put_node.data;
                                    },
                                    GetNode.Data.OrNull => |info| {
                                        _ = self.or_null_queue.remove(info.or_null);
                                        info.ptr.* = put_node.data;
                                    },
                                }
                                global_event_loop.onNextTick(get_node.tick_node);
                                global_event_loop.onNextTick(put_node.tick_node);

                                get_count = @atomicRmw(usize, &self.get_count, .Sub, 1, .SeqCst);
                                put_count = @atomicRmw(usize, &self.put_count, .Sub, 1, .SeqCst);
                            }

                            // transfer self.putters to self.buffer

                            while (self.buffer_len != self.buffer_nodes.len and put_count != 0) {
                                const put_node = &self.putters.get().?.data;

                                self.buffer_nodes[self.buffer_index % self.buffer_nodes.len] = put_node.data;
                                global_event_loop.onNextTick(put_node.tick_node);
                                self.buffer_index +%= 1;
                                self.buffer_len += 1;

                                put_count = @atomicRmw(usize, &self.put_count, .Sub, 1, .SeqCst);
                            }
                        }

                        // undo the extra subtractions

                        _ = @atomicRmw(usize, &self.get_count, .Add, 1, .SeqCst);
                        _ = @atomicRmw(usize, &self.put_count, .Add, 1, .SeqCst);

                        // All the "get or null" functions should resume now.

                        var remove_count: usize = 0;
                        while (self.or_null_queue.get()) |or_null_node| {
                            remove_count += @boolToInt(self.getters.remove(or_null_node.data));
                            global_event_loop.onNextTick(or_null_node.data.data.tick_node);
                        }
                        if (remove_count != 0) {
                            _ = @atomicRmw(usize, &self.get_count, .Sub, remove_count, .SeqCst);
                        }

                        // clear need-dispatch flag

                        if (@atomicRmw(bool, &self.need_dispatch, .Xchg, false, .SeqCst)) continue;

                        assert(@atomicRmw(bool, &self.dispatch_lock, .Xchg, false, .SeqCst));

                        // we have to check again now that we unlocked

                        if (@atomicLoad(bool, &self.need_dispatch, .SeqCst)) continue :lock;

                        return;
                    }
                }
            }
        };
    }

    test "std.event.Channel" {
        if (!std.io.is_async) return error.SkipZigTest;

        // https://github.com/ziglang/zig/issues/1908

        if (builtin.single_threaded) return error.SkipZigTest;

        // https://github.com/ziglang/zig/issues/3251

        if (builtin.os.tag == .freebsd) return error.SkipZigTest;

        var channel: Channel(i32) = undefined;
        channel.init(&[0]i32{});
        defer channel.deinit();

        var handle = async testChannelGetter(&channel);
        var putter = async testChannelPutter(&channel);

        await handle;
        await putter;
    }

    test "std.event.Channel wraparound" {

        // TODO provide a way to run tests in evented I/O mode

        if (!std.io.is_async) return error.SkipZigTest;

        const channel_size = 2;

        var buf: [channel_size]i32 = undefined;
        var channel: Channel(i32) = undefined;
        channel.init(&buf);
        defer channel.deinit();

        // add items to channel and pull them out until

        // the buffer wraps around, make sure it doesn't crash.

        channel.put(5);
        try testing.expectEqual(@as(i32, 5), channel.get());
        channel.put(6);
        try testing.expectEqual(@as(i32, 6), channel.get());
        channel.put(7);
        try testing.expectEqual(@as(i32, 7), channel.get());
    }
    fn testChannelGetter(channel: *Channel(i32)) callconv(.Async) void {
        const value1 = channel.get();
        try testing.expect(value1 == 1234);

        const value2 = channel.get();
        try testing.expect(value2 == 4567);

        const value3 = channel.getOrNull();
        try testing.expect(value3 == null);

        var last_put = async testPut(channel, 4444);
        const value4 = channel.getOrNull();
        try testing.expect(value4.? == 4444);
        await last_put;
    }
    fn testChannelPutter(channel: *Channel(i32)) callconv(.Async) void {
        channel.put(1234);
        channel.put(4567);
    }
    fn testPut(channel: *Channel(i32), value: i32) callconv(.Async) void {
        channel.put(value);
    }