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

    /// Many producer, many consumer, non-allocating, thread-safe.
    /// Uses a mutex to protect access.
    /// The queue does not manage ownership and the user is responsible to
    /// manage the storage of the nodes.
    pub fn Queue(comptime T: type) type {
        return struct {
            head: ?*Node,
            tail: ?*Node,
            mutex: std.Thread.Mutex,

            pub const Self = @This();
            pub const Node = std.TailQueue(T).Node;

            /// Initializes a new queue. The queue does not provide a `deinit()`
            /// function, so the user must take care of cleaning up the queue elements.
            pub fn init() Self {
                return Self{
                    .head = null,
                    .tail = null,
                    .mutex = std.Thread.Mutex{},
                };
            }

            /// Appends `node` to the queue.
            /// The lifetime of `node` must be longer than lifetime of queue.
            pub fn put(self: *Self, node: *Node) void {
                node.next = null;

                self.mutex.lock();
                defer self.mutex.unlock();

                node.prev = self.tail;
                self.tail = node;
                if (node.prev) |prev_tail| {
                    prev_tail.next = node;
                } else {
                    assert(self.head == null);
                    self.head = node;
                }
            }

            /// Gets a previously inserted node or returns `null` if there is none.
            /// It is safe to `get()` a node from the queue while another thread tries
            /// to `remove()` the same node at the same time.
            pub fn get(self: *Self) ?*Node {
                self.mutex.lock();
                defer self.mutex.unlock();

                const head = self.head orelse return null;
                self.head = head.next;
                if (head.next) |new_head| {
                    new_head.prev = null;
                } else {
                    self.tail = null;
                }
                // This way, a get() and a remove() are thread-safe with each other.

                head.prev = null;
                head.next = null;
                return head;
            }

            /// Prepends `node` to the front of the queue.
            /// The lifetime of `node` must be longer than the lifetime of the queue.
            pub fn unget(self: *Self, node: *Node) void {
                node.prev = null;

                self.mutex.lock();
                defer self.mutex.unlock();

                const opt_head = self.head;
                self.head = node;
                if (opt_head) |old_head| {
                    node.next = old_head;
                } else {
                    assert(self.tail == null);
                    self.tail = node;
                }
            }

            /// Removes a node from the queue, returns whether node was actually removed.
            /// It is safe to `remove()` a node from the queue while another thread tries
            /// to `get()` the same node at the same time.
            pub fn remove(self: *Self, node: *Node) bool {
                self.mutex.lock();
                defer self.mutex.unlock();

                if (node.prev == null and node.next == null and self.head != node) {
                    return false;
                }

                if (node.prev) |prev| {
                    prev.next = node.next;
                } else {
                    self.head = node.next;
                }
                if (node.next) |next| {
                    next.prev = node.prev;
                } else {
                    self.tail = node.prev;
                }
                node.prev = null;
                node.next = null;
                return true;
            }

            /// Returns `true` if the queue is currently empty.
            /// Note that in a multi-consumer environment a return value of `false`
            /// does not mean that `get` will yield a non-`null` value!
            pub fn isEmpty(self: *Self) bool {
                self.mutex.lock();
                defer self.mutex.unlock();
                return self.head == null;
            }

            /// Dumps the contents of the queue to `stderr`.
            pub fn dump(self: *Self) void {
                self.dumpToStream(std.io.getStdErr().writer()) catch return;
            }

            /// Dumps the contents of the queue to `stream`.
            /// Up to 4 elements from the head are dumped and the tail of the queue is
            /// dumped as well.
            pub fn dumpToStream(self: *Self, stream: anytype) !void {
                const S = struct {
                    fn dumpRecursive(
                        s: anytype,
                        optional_node: ?*Node,
                        indent: usize,
                        comptime depth: comptime_int,
                    ) !void {
                        try s.writeByteNTimes(' ', indent);
                        if (optional_node) |node| {
                            try s.print("0x{x}={}\n", .{ @intFromPtr(node), node.data });
                            if (depth == 0) {
                                try s.print("(max depth)\n", .{});
                                return;
                            }
                            try dumpRecursive(s, node.next, indent + 1, depth - 1);
                        } else {
                            try s.print("(null)\n", .{});
                        }
                    }
                };
                self.mutex.lock();
                defer self.mutex.unlock();

                try stream.print("head: ", .{});
                try S.dumpRecursive(stream, self.head, 0, 4);
                try stream.print("tail: ", .{});
                try S.dumpRecursive(stream, self.tail, 0, 4);
            }
        };
    }

    const Context = struct {
        allocator: std.mem.Allocator,
        queue: *Queue(i32),
        put_sum: isize,
        get_sum: isize,
        get_count: usize,
        puts_done: bool,
    };

    // TODO add lazy evaluated build options and then put puts_per_thread behind

    // some option such as: "AggressiveMultithreadedFuzzTest". In the AppVeyor

    // CI we would use a less aggressive setting since at 1 core, while we still

    // want this test to pass, we need a smaller value since there is so much thrashing

    // we would also use a less aggressive setting when running in valgrind

    const puts_per_thread = 500;
    const put_thread_count = 3;

    test "std.atomic.Queue" {
        var plenty_of_memory = try std.heap.page_allocator.alloc(u8, 300 * 1024);
        defer std.heap.page_allocator.free(plenty_of_memory);

        var fixed_buffer_allocator = std.heap.FixedBufferAllocator.init(plenty_of_memory);
        var a = fixed_buffer_allocator.threadSafeAllocator();

        var queue = Queue(i32).init();
        var context = Context{
            .allocator = a,
            .queue = &queue,
            .put_sum = 0,
            .get_sum = 0,
            .puts_done = false,
            .get_count = 0,
        };

        if (builtin.single_threaded) {
            try expect(context.queue.isEmpty());
            {
                var i: usize = 0;
                while (i < put_thread_count) : (i += 1) {
                    try expect(startPuts(&context) == 0);
                }
            }
            try expect(!context.queue.isEmpty());
            context.puts_done = true;
            {
                var i: usize = 0;
                while (i < put_thread_count) : (i += 1) {
                    try expect(startGets(&context) == 0);
                }
            }
            try expect(context.queue.isEmpty());
        } else {
            try expect(context.queue.isEmpty());

            var putters: [put_thread_count]std.Thread = undefined;
            for (&putters) |*t| {
                t.* = try std.Thread.spawn(.{}, startPuts, .{&context});
            }
            var getters: [put_thread_count]std.Thread = undefined;
            for (&getters) |*t| {
                t.* = try std.Thread.spawn(.{}, startGets, .{&context});
            }

            for (putters) |t|
                t.join();
            @atomicStore(bool, &context.puts_done, true, .SeqCst);
            for (getters) |t|
                t.join();

            try expect(context.queue.isEmpty());
        }

        if (context.put_sum != context.get_sum) {
            std.debug.panic("failure\nput_sum:{} != get_sum:{}", .{ context.put_sum, context.get_sum });
        }

        if (context.get_count != puts_per_thread * put_thread_count) {
            std.debug.panic("failure\nget_count:{} != puts_per_thread:{} * put_thread_count:{}", .{
                context.get_count,
                @as(u32, puts_per_thread),
                @as(u32, put_thread_count),
            });
        }
    }

    fn startPuts(ctx: *Context) u8 {
        var put_count: usize = puts_per_thread;
        var prng = std.rand.DefaultPrng.init(0xdeadbeef);
        const random = prng.random();
        while (put_count != 0) : (put_count -= 1) {
            std.time.sleep(1); // let the os scheduler be our fuzz

            const x = @as(i32, @bitCast(random.int(u32)));
            const node = ctx.allocator.create(Queue(i32).Node) catch unreachable;
            node.* = .{
                .prev = undefined,
                .next = undefined,
                .data = x,
            };
            ctx.queue.put(node);
            _ = @atomicRmw(isize, &ctx.put_sum, .Add, x, .SeqCst);
        }
        return 0;
    }

    fn startGets(ctx: *Context) u8 {
        while (true) {
            const last = @atomicLoad(bool, &ctx.puts_done, .SeqCst);

            while (ctx.queue.get()) |node| {
                std.time.sleep(1); // let the os scheduler be our fuzz

                _ = @atomicRmw(isize, &ctx.get_sum, .Add, node.data, .SeqCst);
                _ = @atomicRmw(usize, &ctx.get_count, .Add, 1, .SeqCst);
            }

            if (last) return 0;
        }
    }

    test "std.atomic.Queue single-threaded" {
        var queue = Queue(i32).init();
        try expect(queue.isEmpty());

        var node_0 = Queue(i32).Node{
            .data = 0,
            .next = undefined,
            .prev = undefined,
        };
        queue.put(&node_0);
        try expect(!queue.isEmpty());

        var node_1 = Queue(i32).Node{
            .data = 1,
            .next = undefined,
            .prev = undefined,
        };
        queue.put(&node_1);
        try expect(!queue.isEmpty());

        try expect(queue.get().?.data == 0);
        try expect(!queue.isEmpty());

        var node_2 = Queue(i32).Node{
            .data = 2,
            .next = undefined,
            .prev = undefined,
        };
        queue.put(&node_2);
        try expect(!queue.isEmpty());

        var node_3 = Queue(i32).Node{
            .data = 3,
            .next = undefined,
            .prev = undefined,
        };
        queue.put(&node_3);
        try expect(!queue.isEmpty());

        try expect(queue.get().?.data == 1);
        try expect(!queue.isEmpty());

        try expect(queue.get().?.data == 2);
        try expect(!queue.isEmpty());

        var node_4 = Queue(i32).Node{
            .data = 4,
            .next = undefined,
            .prev = undefined,
        };
        queue.put(&node_4);
        try expect(!queue.isEmpty());

        try expect(queue.get().?.data == 3);
        node_3.next = null;
        try expect(!queue.isEmpty());

        queue.unget(&node_3);
        try expect(queue.get().?.data == 3);
        try expect(!queue.isEmpty());

        try expect(queue.get().?.data == 4);
        try expect(queue.isEmpty());

        try expect(queue.get() == null);
        try expect(queue.isEmpty());

        // unget an empty queue

        queue.unget(&node_4);
        try expect(queue.tail == &node_4);
        try expect(queue.head == &node_4);

        try expect(queue.get().?.data == 4);

        try expect(queue.get() == null);
        try expect(queue.isEmpty());
    }

    test "std.atomic.Queue dump" {
        const mem = std.mem;
        var buffer: [1024]u8 = undefined;
        var expected_buffer: [1024]u8 = undefined;
        var fbs = std.io.fixedBufferStream(&buffer);

        var queue = Queue(i32).init();

        // Test empty stream

        fbs.reset();
        try queue.dumpToStream(fbs.writer());
        try expect(mem.eql(u8, buffer[0..fbs.pos],
            \\head: (null)

            \\tail: (null)

            \\

        ));

        // Test a stream with one element

        var node_0 = Queue(i32).Node{
            .data = 1,
            .next = undefined,
            .prev = undefined,
        };
        queue.put(&node_0);

        fbs.reset();
        try queue.dumpToStream(fbs.writer());

        var expected = try std.fmt.bufPrint(expected_buffer[0..],
            \\head: 0x{x}=1

            \\ (null)

            \\tail: 0x{x}=1

            \\ (null)

            \\

        , .{ @intFromPtr(queue.head), @intFromPtr(queue.tail) });
        try expect(mem.eql(u8, buffer[0..fbs.pos], expected));

        // Test a stream with two elements

        var node_1 = Queue(i32).Node{
            .data = 2,
            .next = undefined,
            .prev = undefined,
        };
        queue.put(&node_1);

        fbs.reset();
        try queue.dumpToStream(fbs.writer());

        expected = try std.fmt.bufPrint(expected_buffer[0..],
            \\head: 0x{x}=1

            \\ 0x{x}=2

            \\  (null)

            \\tail: 0x{x}=2

            \\ (null)

            \\

        , .{ @intFromPtr(queue.head), @intFromPtr(queue.head.?.next), @intFromPtr(queue.tail) });
        try expect(mem.eql(u8, buffer[0..fbs.pos], expected));
    }