    const std = @import("std.zig");
    const builtin = @import("builtin");
    const assert = std.debug.assert;
    const testing = std.testing;
    const os = std.os;
    const math = std.math;

    pub const epoch = @import("time/epoch.zig");

    /// Spurious wakeups are possible and no precision of timing is guaranteed.
    pub fn sleep(nanoseconds: u64) void {
        // TODO: opting out of async sleeping?

        if (std.io.is_async) {
            return std.event.Loop.instance.?.sleep(nanoseconds);
        }

        if (builtin.os.tag == .windows) {
            const big_ms_from_ns = nanoseconds / ns_per_ms;
            const ms = math.cast(os.windows.DWORD, big_ms_from_ns) orelse math.maxInt(os.windows.DWORD);
            os.windows.kernel32.Sleep(ms);
            return;
        }

        if (builtin.os.tag == .wasi) {
            const w = std.os.wasi;
            const userdata: w.userdata_t = 0x0123_45678;
            const clock = w.subscription_clock_t{
                .id = w.CLOCK.MONOTONIC,
                .timeout = nanoseconds,
                .precision = 0,
                .flags = 0,
            };
            const in = w.subscription_t{
                .userdata = userdata,
                .u = w.subscription_u_t{
                    .tag = w.EVENTTYPE_CLOCK,
                    .u = w.subscription_u_u_t{
                        .clock = clock,
                    },
                },
            };

            var event: w.event_t = undefined;
            var nevents: usize = undefined;
            _ = w.poll_oneoff(&in, &event, 1, &nevents);
            return;
        }

        const s = nanoseconds / ns_per_s;
        const ns = nanoseconds % ns_per_s;
        std.os.nanosleep(s, ns);
    }

    test "sleep" {
        sleep(1);
    }

    /// Get a calendar timestamp, in seconds, relative to UTC 1970-01-01.
    /// Precision of timing depends on the hardware and operating system.
    /// The return value is signed because it is possible to have a date that is
    /// before the epoch.
    /// See `std.os.clock_gettime` for a POSIX timestamp.
    pub fn timestamp() i64 {
        return @divFloor(milliTimestamp(), ms_per_s);
    }

    /// Get a calendar timestamp, in milliseconds, relative to UTC 1970-01-01.
    /// Precision of timing depends on the hardware and operating system.
    /// The return value is signed because it is possible to have a date that is
    /// before the epoch.
    /// See `std.os.clock_gettime` for a POSIX timestamp.
    pub fn milliTimestamp() i64 {
        return @intCast(i64, @divFloor(nanoTimestamp(), ns_per_ms));
    }

    /// Get a calendar timestamp, in nanoseconds, relative to UTC 1970-01-01.
    /// Precision of timing depends on the hardware and operating system.
    /// On Windows this has a maximum granularity of 100 nanoseconds.
    /// The return value is signed because it is possible to have a date that is
    /// before the epoch.
    /// See `std.os.clock_gettime` for a POSIX timestamp.
    pub fn nanoTimestamp() i128 {
        if (builtin.os.tag == .windows) {
            // FileTime has a granularity of 100 nanoseconds and uses the NTFS/Windows epoch,

            // which is 1601-01-01.

            const epoch_adj = epoch.windows * (ns_per_s / 100);
            var ft: os.windows.FILETIME = undefined;
            os.windows.kernel32.GetSystemTimeAsFileTime(&ft);
            const ft64 = (@as(u64, ft.dwHighDateTime) << 32) | ft.dwLowDateTime;
            return @as(i128, @bitCast(i64, ft64) + epoch_adj) * 100;
        }

        if (builtin.os.tag == .wasi and !builtin.link_libc) {
            var ns: os.wasi.timestamp_t = undefined;
            const err = os.wasi.clock_time_get(os.wasi.CLOCK.REALTIME, 1, &ns);
            assert(err == .SUCCESS);
            return ns;
        }

        var ts: os.timespec = undefined;
        os.clock_gettime(os.CLOCK.REALTIME, &ts) catch |err| switch (err) {
            error.UnsupportedClock, error.Unexpected => return 0, // "Precision of timing depends on hardware and OS".

        };
        return (@as(i128, ts.tv_sec) * ns_per_s) + ts.tv_nsec;
    }

    test "timestamp" {
        const margin = ns_per_ms * 50;

        const time_0 = milliTimestamp();
        sleep(ns_per_ms);
        const time_1 = milliTimestamp();
        const interval = time_1 - time_0;
        try testing.expect(interval > 0);
        // Tests should not depend on timings: skip test if outside margin.

        if (!(interval < margin)) return error.SkipZigTest;
    }

    // Divisions of a nanosecond.

    pub const ns_per_us = 1000;
    pub const ns_per_ms = 1000 * ns_per_us;
    pub const ns_per_s = 1000 * ns_per_ms;
    pub const ns_per_min = 60 * ns_per_s;
    pub const ns_per_hour = 60 * ns_per_min;
    pub const ns_per_day = 24 * ns_per_hour;
    pub const ns_per_week = 7 * ns_per_day;

    // Divisions of a microsecond.

    pub const us_per_ms = 1000;
    pub const us_per_s = 1000 * us_per_ms;
    pub const us_per_min = 60 * us_per_s;
    pub const us_per_hour = 60 * us_per_min;
    pub const us_per_day = 24 * us_per_hour;
    pub const us_per_week = 7 * us_per_day;

    // Divisions of a millisecond.

    pub const ms_per_s = 1000;
    pub const ms_per_min = 60 * ms_per_s;
    pub const ms_per_hour = 60 * ms_per_min;
    pub const ms_per_day = 24 * ms_per_hour;
    pub const ms_per_week = 7 * ms_per_day;

    // Divisions of a second.

    pub const s_per_min = 60;
    pub const s_per_hour = s_per_min * 60;
    pub const s_per_day = s_per_hour * 24;
    pub const s_per_week = s_per_day * 7;

    /// An Instant represents a timestamp with respect to the currently
    /// executing program that ticks during suspend and can be used to
    /// record elapsed time unlike `nanoTimestamp`.
    ///
    /// It tries to sample the system's fastest and most precise timer available.
    /// It also tries to be monotonic, but this is not a guarantee due to OS/hardware bugs.
    /// If you need monotonic readings for elapsed time, consider `Timer` instead.
    pub const Instant = struct {
        timestamp: if (is_posix) os.timespec else u64,

        // true if we should use clock_gettime()

        const is_posix = switch (builtin.os.tag) {
            .wasi => builtin.link_libc,
            .windows => false,
            else => true,
        };

        /// Queries the system for the current moment of time as an Instant.
        /// This is not guaranteed to be monotonic or steadily increasing, but for most implementations it is.
        /// Returns `error.Unsupported` when a suitable clock is not detected.
        pub fn now() error{Unsupported}!Instant {
            // QPC on windows doesn't fail on >= XP/2000 and includes time suspended.

            if (builtin.os.tag == .windows) {
                return Instant{ .timestamp = os.windows.QueryPerformanceCounter() };
            }

            // On WASI without libc, use clock_time_get directly.

            if (builtin.os.tag == .wasi and !builtin.link_libc) {
                var ns: os.wasi.timestamp_t = undefined;
                const rc = os.wasi.clock_time_get(os.wasi.CLOCK.MONOTONIC, 1, &ns);
                if (rc != .SUCCESS) return error.Unsupported;
                return Instant{ .timestamp = ns };
            }

            // On darwin, use UPTIME_RAW instead of MONOTONIC as it ticks while suspended.

            // On linux, use BOOTTIME instead of MONOTONIC as it ticks while suspended.

            // On freebsd derivatives, use MONOTONIC_FAST as currently there's no precision tradeoff.

            // On other posix systems, MONOTONIC is generally the fastest and ticks while suspended.

            const clock_id = switch (builtin.os.tag) {
                .macos, .ios, .tvos, .watchos => os.CLOCK.UPTIME_RAW,
                .freebsd, .dragonfly => os.CLOCK.MONOTONIC_FAST,
                .linux => os.CLOCK.BOOTTIME,
                else => os.CLOCK.MONOTONIC,
            };

            var ts: os.timespec = undefined;
            os.clock_gettime(clock_id, &ts) catch return error.Unsupported;
            return Instant{ .timestamp = ts };
        }

        /// Quickly compares two instances between each other.
        pub fn order(self: Instant, other: Instant) std.math.Order {
            // windows and wasi timestamps are in u64 which is easily comparible

            if (!is_posix) {
                return std.math.order(self.timestamp, other.timestamp);
            }

            var ord = std.math.order(self.timestamp.tv_sec, other.timestamp.tv_sec);
            if (ord == .eq) {
                ord = std.math.order(self.timestamp.tv_nsec, other.timestamp.tv_nsec);
            }
            return ord;
        }

        /// Returns elapsed time in nanoseconds since the `earlier` Instant.
        /// This assumes that the `earlier` Instant represents a moment in time before or equal to `self`.
        /// This also assumes that the time that has passed between both Instants fits inside a u64 (~585 yrs).
        pub fn since(self: Instant, earlier: Instant) u64 {
            if (builtin.os.tag == .windows) {
                // We don't need to cache QPF as it's internally just a memory read to KUSER_SHARED_DATA

                // (a read-only page of info updated and mapped by the kernel to all processes):

                // https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/ntddk/ns-ntddk-kuser_shared_data

                // https://www.geoffchappell.com/studies/windows/km/ntoskrnl/inc/api/ntexapi_x/kuser_shared_data/index.htm

                const qpc = self.timestamp - earlier.timestamp;
                const qpf = os.windows.QueryPerformanceFrequency();

                // 10Mhz (1 qpc tick every 100ns) is a common enough QPF value that we can optimize on it.

                // https://github.com/microsoft/STL/blob/785143a0c73f030238ef618890fd4d6ae2b3a3a0/stl/inc/chrono#L694-L701

                const common_qpf = 10_000_000;
                if (qpf == common_qpf) {
                    return qpc * (ns_per_s / common_qpf);
                }

                // Convert to ns using fixed point.

                const scale = @as(u64, std.time.ns_per_s << 32) / @intCast(u32, qpf);
                const result = (@as(u96, qpc) * scale) >> 32;
                return @truncate(u64, result);
            }

            // WASI timestamps are directly in nanoseconds

            if (builtin.os.tag == .wasi and !builtin.link_libc) {
                return self.timestamp - earlier.timestamp;
            }

            // Convert timespec diff to ns

            const seconds = @intCast(u64, self.timestamp.tv_sec - earlier.timestamp.tv_sec);
            const elapsed = (seconds * ns_per_s) + @intCast(u32, self.timestamp.tv_nsec);
            return elapsed - @intCast(u32, earlier.timestamp.tv_nsec);
        }
    };

    /// A monotonic, high performance timer.
    ///
    /// Timer.start() is used to initalize the timer
    /// and gives the caller an opportunity to check for the existence of a supported clock.
    /// Once a supported clock is discovered,
    /// it is assumed that it will be available for the duration of the Timer's use.
    ///
    /// Monotonicity is ensured by saturating on the most previous sample.
    /// This means that while timings reported are monotonic,
    /// they're not guaranteed to tick at a steady rate as this is up to the underlying system.
    pub const Timer = struct {
        started: Instant,
        previous: Instant,

        pub const Error = error{TimerUnsupported};

        /// Initialize the timer by querying for a supported clock.
        /// Returns `error.TimerUnsupported` when such a clock is unavailable.
        /// This should only fail in hostile environments such as linux seccomp misuse.
        pub fn start() Error!Timer {
            const current = Instant.now() catch return error.TimerUnsupported;
            return Timer{ .started = current, .previous = current };
        }

        /// Reads the timer value since start or the last reset in nanoseconds.
        pub fn read(self: *Timer) u64 {
            const current = self.sample();
            return current.since(self.started);
        }

        /// Resets the timer value to 0/now.
        pub fn reset(self: *Timer) void {
            const current = self.sample();
            self.started = current;
        }

        /// Returns the current value of the timer in nanoseconds, then resets it.
        pub fn lap(self: *Timer) u64 {
            const current = self.sample();
            defer self.started = current;
            return current.since(self.started);
        }

        /// Returns an Instant sampled at the callsite that is
        /// guaranteed to be monotonic with respect to the timer's starting point.
        fn sample(self: *Timer) Instant {
            const current = Instant.now() catch unreachable;
            if (current.order(self.previous) == .gt) {
                self.previous = current;
            }
            return self.previous;
        }
    };

    test "Timer + Instant" {
        const margin = ns_per_ms * 150;

        var timer = try Timer.start();
        sleep(10 * ns_per_ms);
        const time_0 = timer.read();
        try testing.expect(time_0 > 0);
        // Tests should not depend on timings: skip test if outside margin.

        if (!(time_0 < margin)) return error.SkipZigTest;

        const time_1 = timer.lap();
        try testing.expect(time_1 >= time_0);

        timer.reset();
        try testing.expect(timer.read() < time_1);
    }

    test {
        _ = epoch;
    }