Fiber

const Fiber = struct {
    required_align: void align(4),
    evented: *Evented,
    context: Io.fiber.Context,
    link: union {
        awaiter: ?*Fiber,
        group: struct { prev: ?*Fiber, next: ?*Fiber },
    },
    awaiting_group: Group,
    cancel_status: CancelStatus,
    cancel_protection: CancelProtection,

    var next_name: u64 = 0;

    const CancelStatus = packed struct(usize) {
        requested: bool,
        awaiting: Awaiting,

        const unrequested: CancelStatus = .{ .requested = false, .awaiting = .nothing };

        const Awaiting = enum(@Int(.unsigned, @bitSizeOf(usize) - shift)) {
            nothing = 0,
            group = 1,
            _,

            const shift = 1;

            fn subWrap(lhs: Awaiting, rhs: Awaiting) Awaiting {
                return @fromBackingInt(@intCast(@backingInt(lhs) -% @backingInt(rhs)));
            }

            fn fromCancelable(cancelable: *Cancelable) Awaiting {
                return @fromBackingInt(@intCast(@shrExact(@intFromPtr(cancelable), shift)));
            }

            fn toCancelable(awaiting: Awaiting) *Cancelable {
                return @ptrFromInt(@shlExact(@as(usize, @backingInt(awaiting)), shift));
            }
        };

        fn changeAwaiting(
            cancel_status: *CancelStatus,
            old_awaiting: Awaiting,
            new_awaiting: Awaiting,
        ) bool {
            const old_cancel_status = @atomicRmw(CancelStatus, cancel_status, .Add, .{
                .requested = false,
                .awaiting = new_awaiting.subWrap(old_awaiting),
            }, .release);
            assert(old_cancel_status.awaiting == old_awaiting);
            return old_cancel_status.requested;
        }
    };

    const CancelProtection = packed struct {
        user: Io.CancelProtection,
        acknowledged: bool,

        const unblocked: CancelProtection = .{ .user = .unblocked, .acknowledged = false };

        fn check(cancel_protection: CancelProtection) Io.CancelProtection {
            return @fromBackingInt(@intCast(@intFromBool(cancel_protection != unblocked)));
        }

        fn acknowledge(cancel_protection: *CancelProtection) void {
            assert(!cancel_protection.acknowledged);
            cancel_protection.acknowledged = true;
        }

        fn recancel(cancel_protection: *CancelProtection) void {
            assert(cancel_protection.acknowledged);
            cancel_protection.acknowledged = false;
        }

        test check {
            try std.testing.expectEqual(Io.CancelProtection.unblocked, check(.unblocked));
            try std.testing.expectEqual(Io.CancelProtection.blocked, check(.{
                .user = .unblocked,
                .acknowledged = true,
            }));
            try std.testing.expectEqual(Io.CancelProtection.blocked, check(.{
                .user = .blocked,
                .acknowledged = false,
            }));
            try std.testing.expectEqual(Io.CancelProtection.blocked, check(.{
                .user = .blocked,
                .acknowledged = true,
            }));
        }
    };

    const finished: ?*Fiber = @ptrFromInt(@alignOf(Fiber));

    const max_result_align: Alignment = .@"16";
    const max_result_size = max_result_align.forward(512);
    /// This includes any stack realignments that need to happen, and also the
    /// initial frame return address slot and argument frame, depending on target.
    const min_stack_size = 60 * 1024 * 1024;
    const max_context_align: Alignment = .@"16";
    const max_context_size = max_context_align.forward(1024);
    const max_closure_size: usize = @sizeOf(AsyncClosure);
    const max_closure_align: Alignment = .of(AsyncClosure);
    const allocation_size = std.mem.alignForward(
        usize,
        max_closure_align.max(max_context_align).forward(
            max_result_align.forward(@sizeOf(Fiber)) + max_result_size + min_stack_size,
        ) + max_closure_size + max_context_size,
        std.heap.page_size_max,
    );

    fn create(ev: *Evented) error{OutOfMemory}!*Fiber {
        return @ptrCast(try ev.allocator().alignedAlloc(u8, .of(Fiber), allocation_size));
    }

    fn destroy(fiber: *Fiber, ev: *Evented) void {
        ev.allocator().free(fiber.allocatedSlice());
    }

    fn allocatedSlice(f: *Fiber) []align(@alignOf(Fiber)) u8 {
        return @as([*]align(@alignOf(Fiber)) u8, @ptrCast(f))[0..allocation_size];
    }

    fn allocatedEnd(f: *Fiber) [*]u8 {
        const allocated_slice = f.allocatedSlice();
        return allocated_slice[allocated_slice.len..].ptr;
    }

    fn resultPointer(f: *Fiber, comptime Result: type) *Result {
        return @ptrCast(@alignCast(f.resultBytes(.of(Result))));
    }

    fn resultBytes(f: *Fiber, alignment: Alignment) [*]u8 {
        return @ptrFromInt(alignment.forward(@intFromPtr(f) + @sizeOf(Fiber)));
    }

    const Queue = struct { head: *Fiber, tail: *Fiber };

    /// Like a `*Fiber`, but 2 bits smaller than a pointer (because the LSBs are always 0 due to
    /// alignment) so that those two bits can be used in a `packed struct`.
    const PackedPtr = enum(@Int(.unsigned, @bitSizeOf(usize) - 2)) {
        null = 0,
        all_ones = std.math.maxInt(@Int(.unsigned, @bitSizeOf(usize) - 2)),
        _,

        const Split = packed struct(usize) { low: u2, high: PackedPtr };
        fn pack(ptr: ?*Fiber) PackedPtr {
            const split: Split = @bitCast(@intFromPtr(ptr));
            assert(split.low == 0);
            return split.high;
        }
        fn unpack(ptr: PackedPtr) ?*Fiber {
            const split: Split = .{ .low = 0, .high = ptr };
            return @ptrFromInt(@as(usize, @bitCast(split)));
        }
    };

    fn requestCancel(fiber: *Fiber, ev: *Evented) void {
        const cancel_status = @atomicRmw(
            Fiber.CancelStatus,
            &fiber.cancel_status,
            .Or,
            .{ .requested = true, .awaiting = .nothing },
            .acquire,
        );
        assert(!cancel_status.requested);
        switch (cancel_status.awaiting) {
            .nothing => {},
            .group => {
                // The awaiter received a cancelation request while awaiting a group,
                // so propagate the cancelation to the group.
                if (fiber.awaiting_group.cancel(ev, null)) {
                    fiber.awaiting_group = undefined;
                    ev.queue.async(fiber, &Fiber.@"resume");
                }
            },
            _ => |awaiting| awaiting.toCancelable().async(),
        }
    }

    fn @"resume"(context: ?*anyopaque) callconv(.c) void {
        const fiber: *Fiber = @ptrCast(@alignCast(context));
        const thread: *Thread = .current();
        const message: SwitchMessage = .{
            .contexts = .{
                .old = &thread.main_context,
                .new = &fiber.context,
            },
            .pending_task = .nothing,
        };
        contextSwitch(&message).handle(fiber.evented);
    }
}