Event

pub const Event = enum(u32) {
    unset,
    waiting,
    is_set,

    /// Returns whether the logical boolean is `true`.
    pub fn isSet(event: *const Event) bool {
        return switch (@atomicLoad(Event, event, .acquire)) {
            .unset, .waiting => false,
            .is_set => true,
        };
    }

    /// Blocks until the logical boolean is `true`.
    pub fn wait(event: *Event, io: Io) Cancelable!void {
        if (@cmpxchgStrong(Event, event, .unset, .waiting, .acquire, .acquire)) |prev| switch (prev) {
            .unset => unreachable,
            .waiting => {},
            .is_set => return,
        };
        errdefer {
            // Ideally we would restore the event back to `.unset` instead of `.waiting`, but there
            // might be other threads waiting on the event. In theory we could track the *number* of
            // waiting threads in the unused bits of the `Event`, but that has its own problem: the
            // waiters would wake up when a *new waiter* was added. So it's easiest to just leave
            // the state at `.waiting`---at worst it causes one redundant call to `futexWake`.
        }
        while (true) {
            try io.futexWait(Event, event, .waiting);
            switch (@atomicLoad(Event, event, .acquire)) {
                .unset => unreachable, // `reset` called before pending `wait` returned
                .waiting => continue,
                .is_set => return,
            }
        }
    }

    /// Same as `wait`, except does not introduce a cancelation point.
    ///
    /// For a description of cancelation and cancelation points, see `Future.cancel`.
    pub fn waitUncancelable(event: *Event, io: Io) void {
        if (@cmpxchgStrong(Event, event, .unset, .waiting, .acquire, .acquire)) |prev| switch (prev) {
            .unset => unreachable,
            .waiting => {},
            .is_set => return,
        };
        while (true) {
            io.futexWaitUncancelable(Event, event, .waiting);
            switch (@atomicLoad(Event, event, .acquire)) {
                .unset => unreachable, // `reset` called before pending `wait` returned
                .waiting => continue,
                .is_set => return,
            }
        }
    }

    pub const WaitTimeoutError = error{Timeout} || Cancelable;

    /// Blocks the calling thread until either the logical boolean is set, the timeout expires, or a
    /// spurious wakeup occurs. If the timeout expires or a spurious wakeup occurs, `error.Timeout`
    /// is returned.
    pub fn waitTimeout(event: *Event, io: Io, timeout: Timeout) WaitTimeoutError!void {
        if (@cmpxchgStrong(Event, event, .unset, .waiting, .acquire, .acquire)) |prev| switch (prev) {
            .unset => unreachable,
            .waiting => {},
            .is_set => return,
        };
        errdefer {
            // Ideally we would restore the event back to `.unset` instead of `.waiting`, but there
            // might be other threads waiting on the event. In theory we could track the *number* of
            // waiting threads in the unused bits of the `Event`, but that has its own problem: the
            // waiters would wake up when a *new waiter* was added. So it's easiest to just leave
            // the state at `.waiting`---at worst it causes one redundant call to `futexWake`.
        }
        try io.futexWaitTimeout(Event, event, .waiting, timeout);
        switch (@atomicLoad(Event, event, .acquire)) {
            .unset => unreachable, // `reset` called before pending `wait` returned
            .waiting => return error.Timeout,
            .is_set => return,
        }
    }

    /// Sets the logical boolean to true, and hence unblocks any pending calls to `wait`. The
    /// logical boolean remains true until `reset` is called, so future calls to `set` have no
    /// semantic effect.
    ///
    /// Any memory accesses prior to a `set` call are "released", so that if this `set` call causes
    /// `isSet` to return `true` or a wait to finish, those tasks will be able to observe those
    /// memory accesses.
    pub fn set(e: *Event, io: Io) void {
        switch (@atomicRmw(Event, e, .Xchg, .is_set, .release)) {
            .unset, .is_set => {},
            .waiting => io.futexWake(Event, e, math.maxInt(u32)),
        }
    }

    /// Sets the logical boolean to false.
    ///
    /// Assumes that there is no pending call to `wait` or `waitUncancelable`.
    ///
    /// However, concurrent calls to `isSet`, `set`, and `reset` are allowed.
    pub fn reset(e: *Event) void {
        @atomicStore(Event, e, .unset, .monotonic);
    }
}