const std = @import("std"); const work_quota: usize = 4096; pub const BuildCheckpoint = struct {    context: *anyopaque,    input_pending: *const fn (context: *anyopaque) bool,    remaining: usize = work_quota,     pub fn init(        context: *anyopaque,        input_pending: *const fn (context: *anyopaque) bool,    ) BuildCheckpoint {        return .{ .context = context, .input_pending = input_pending };    }     fn tick(self: *BuildCheckpoint) error{InputPending}!void {        try self.consume(1);    }     fn consume(self: *BuildCheckpoint, units: usize) error{InputPending}!void {        if (units < self.remaining) {            self.remaining -= units;            return;        }        const overflow = units - self.remaining;        const used_in_next_quota = overflow % work_quota;        self.remaining = if (used_in_next_quota == 0)            work_quota        else            work_quota - used_in_next_quota;        try self.poll();    }     fn poll(self: *BuildCheckpoint) error{InputPending}!void {        if (self.input_pending(self.context)) return error.InputPending;    }}; pub fn tick(checkpoint: ?*BuildCheckpoint) !void {    if (checkpoint) |active| try active.tick();} pub fn consume(checkpoint: ?*BuildCheckpoint, units: usize) !void {    if (checkpoint) |active| try active.consume(units);} pub fn poll(checkpoint: ?*BuildCheckpoint) !void {    if (checkpoint) |active| try active.poll();} test "checkpoint polls only when its quota is exhausted" {    const Probe = struct {        polls: usize = 0,         fn pending(context: *anyopaque) bool {            const self: *@This() = @ptrCast(@alignCast(context));            self.polls += 1;            return false;        }    };     var probe = Probe{};    var checkpoint = BuildCheckpoint.init(&probe, Probe.pending);    try consume(&checkpoint, work_quota - 1);    try std.testing.expectEqual(@as(usize, 0), probe.polls);    try tick(&checkpoint);    try std.testing.expectEqual(@as(usize, 1), probe.polls);    try consume(&checkpoint, work_quota * 2 + 7);    try std.testing.expectEqual(@as(usize, 2), probe.polls);    try consume(&checkpoint, work_quota - 8);    try std.testing.expectEqual(@as(usize, 2), probe.polls);    try tick(&checkpoint);    try std.testing.expectEqual(@as(usize, 3), probe.polls);    try consume(null, work_quota);}