    const std = @import("../std.zig");
    const testing = std.testing;

    /// Performs multiple async functions in parallel, without heap allocation.
    /// Async function frames are managed externally to this abstraction, and
    /// passed in via the `add` function. Once all the jobs are added, call `wait`.
    /// This API is *not* thread-safe. The object must be accessed from one thread at
    /// a time, however, it need not be the same thread.
    pub fn Batch(
        /// The return value for each job.
        /// If a job slot was re-used due to maxed out concurrency, then its result
        /// value will be overwritten. The values can be accessed with the `results` field.
        comptime Result: type,
        /// How many jobs to run in parallel.
        comptime max_jobs: comptime_int,
        /// Controls whether the `add` and `wait` functions will be async functions.
        comptime async_behavior: enum {
            /// Observe the value of `std.io.is_async` to decide whether `add`
            /// and `wait` will be async functions. Asserts that the jobs do not suspend when
            /// `std.io.mode == .blocking`. This is a generally safe assumption, and the
            /// usual recommended option for this parameter.
            auto_async,

            /// Always uses the `nosuspend` keyword when using `await` on the jobs,
            /// making `add` and `wait` non-async functions. Asserts that the jobs do not suspend.
            never_async,

            /// `add` and `wait` use regular `await` keyword, making them async functions.
            always_async,
        },
    ) type {
        return struct {
            jobs: [max_jobs]Job,
            next_job_index: usize,
            collected_result: CollectedResult,

            const Job = struct {
                frame: ?anyframe->Result,
                result: Result,
            };

            const Self = @This();

            const CollectedResult = switch (@typeInfo(Result)) {
                .ErrorUnion => Result,
                else => void,
            };

            const async_ok = switch (async_behavior) {
                .auto_async => std.io.is_async,
                .never_async => false,
                .always_async => true,
            };

            pub fn init() Self {
                return Self{
                    .jobs = [1]Job{
                        .{
                            .frame = null,
                            .result = undefined,
                        },
                    } ** max_jobs,
                    .next_job_index = 0,
                    .collected_result = {},
                };
            }

            /// Add a frame to the Batch. If all jobs are in-flight, then this function
            /// waits until one completes.
            /// This function is *not* thread-safe. It must be called from one thread at
            /// a time, however, it need not be the same thread.
            /// TODO: "select" language feature to use the next available slot, rather than
            /// awaiting the next index.
            pub fn add(self: *Self, frame: anyframe->Result) void {
                const job = &self.jobs[self.next_job_index];
                self.next_job_index = (self.next_job_index + 1) % max_jobs;
                if (job.frame) |existing| {
                    job.result = if (async_ok) await existing else nosuspend await existing;
                    if (CollectedResult != void) {
                        job.result catch |err| {
                            self.collected_result = err;
                        };
                    }
                }
                job.frame = frame;
            }

            /// Wait for all the jobs to complete.
            /// Safe to call any number of times.
            /// If `Result` is an error union, this function returns the last error that occurred, if any.
            /// Unlike the `results` field, the return value of `wait` will report any error that occurred;
            /// hitting max parallelism will not compromise the result.
            /// This function is *not* thread-safe. It must be called from one thread at
            /// a time, however, it need not be the same thread.
            pub fn wait(self: *Self) CollectedResult {
                for (self.jobs) |*job|
                    if (job.frame) |f| {
                        job.result = if (async_ok) await f else nosuspend await f;
                        if (CollectedResult != void) {
                            job.result catch |err| {
                                self.collected_result = err;
                            };
                        }
                        job.frame = null;
                    };
                return self.collected_result;
            }
        };
    }

    test "std.event.Batch" {
        if (@import("builtin").zig_backend != .stage1) return error.SkipZigTest;
        var count: usize = 0;
        var batch = Batch(void, 2, .auto_async).init();
        batch.add(&async sleepALittle(&count));
        batch.add(&async increaseByTen(&count));
        batch.wait();
        try testing.expect(count == 11);

        var another = Batch(anyerror!void, 2, .auto_async).init();
        another.add(&async somethingElse());
        another.add(&async doSomethingThatFails());
        try testing.expectError(error.ItBroke, another.wait());
    }

    fn sleepALittle(count: *usize) void {
        std.time.sleep(1 * std.time.ns_per_ms);
        _ = @atomicRmw(usize, count, .Add, 1, .SeqCst);
    }

    fn increaseByTen(count: *usize) void {
        var i: usize = 0;
        while (i < 10) : (i += 1) {
            _ = @atomicRmw(usize, count, .Add, 1, .SeqCst);
        }
    }

    fn doSomethingThatFails() anyerror!void {}
    fn somethingElse() anyerror!void {
        return error.ItBroke;
    }