const std = @import("std.zig");
const builtin = std.builtin;
const assert = std.debug.assert;
const testing = std.testing;
const os = std.os;
const math = std.math;
const is_windows = std.Target.current.os.tag == .windows;

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

/// Spurious wakeups are possible and no precision of timing is guaranteed.
/// TODO integrate with evented I/O
pub fn sleep(nanoseconds: u64) void {
    if (is_windows) {
        const big_ms_from_ns = nanoseconds / ns_per_ms;
        const ms = math.cast(os.windows.DWORD, big_ms_from_ns) catch 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);
}

/// 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 (is_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 == os.wasi.ESUCCESS);
        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;
}

// 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;

/// A monotonic high-performance timer.
/// Timer.start() must be called to initialize the struct, which captures
/// the counter frequency on windows and darwin, records the resolution,
/// and gives the user an opportunity to check for the existnece of
/// monotonic clocks without forcing them to check for error on each read.
/// .resolution is in nanoseconds on all platforms but .start_time's meaning
/// depends on the OS. On Windows and Darwin it is a hardware counter
/// value that requires calculation to convert to a meaninful unit.
pub const Timer = struct {
    ///if we used resolution's value when performing the
    ///  performance counter calc on windows/darwin, it would
    ///  be less precise
    frequency: switch (builtin.os.tag) {
        .windows => u64,
        .macosx, .ios, .tvos, .watchos => os.darwin.mach_timebase_info_data,
        else => void,
    },
    resolution: u64,
    start_time: u64,

    pub const Error = error{TimerUnsupported};

    /// At some point we may change our minds on RAW, but for now we're
    /// sticking with posix standard MONOTONIC. For more information, see:
    /// https://github.com/ziglang/zig/pull/933
    const monotonic_clock_id = os.CLOCK_MONOTONIC;

    /// Initialize the timer structure.
    /// Can only fail when running in a hostile environment that intentionally injects
    /// error values into syscalls, such as using seccomp on Linux to intercept
    /// `clock_gettime`.
    pub fn start() Error!Timer {
        // This gives us an opportunity to grab the counter frequency in windows.
        // On Windows: QueryPerformanceCounter will succeed on anything >= XP/2000.
        // On Posix: CLOCK_MONOTONIC will only fail if the monotonic counter is not
        // supported, or if the timespec pointer is out of bounds, which should be
        // impossible here barring cosmic rays or other such occurrences of
        // incredibly bad luck.
        // On Darwin: This cannot fail, as far as I am able to tell.
        if (is_windows) {
            const freq = os.windows.QueryPerformanceFrequency();
            return Timer{
                .frequency = freq,
                .resolution = @divFloor(ns_per_s, freq),
                .start_time = os.windows.QueryPerformanceCounter(),
            };
        } else if (comptime std.Target.current.isDarwin()) {
            var freq: os.darwin.mach_timebase_info_data = undefined;
            os.darwin.mach_timebase_info(&freq);

            return Timer{
                .frequency = freq,
                .resolution = @divFloor(freq.numer, freq.denom),
                .start_time = os.darwin.mach_absolute_time(),
            };
        } else {
            // On Linux, seccomp can do arbitrary things to our ability to call
            // syscalls, including return any errno value it wants and
            // inconsistently throwing errors. Since we can't account for
            // abuses of seccomp in a reasonable way, we'll assume that if
            // seccomp is going to block us it will at least do so consistently
            var res: os.timespec = undefined;
            os.clock_getres(monotonic_clock_id, &res) catch return error.TimerUnsupported;

            var ts: os.timespec = undefined;
            os.clock_gettime(monotonic_clock_id, &ts) catch return error.TimerUnsupported;

            return Timer{
                .resolution = @intCast(u64, res.tv_sec) * ns_per_s + @intCast(u64, res.tv_nsec),
                .start_time = @intCast(u64, ts.tv_sec) * ns_per_s + @intCast(u64, ts.tv_nsec),
                .frequency = {},
            };
        }

        return self;
    }

    /// Reads the timer value since start or the last reset in nanoseconds
    pub fn read(self: Timer) u64 {
        var clock = clockNative() - self.start_time;
        return self.nativeDurationToNanos(clock);
    }

    /// Resets the timer value to 0/now.
    pub fn reset(self: *Timer) void {
        self.start_time = clockNative();
    }

    /// Returns the current value of the timer in nanoseconds, then resets it
    pub fn lap(self: *Timer) u64 {
        var now = clockNative();
        var lap_time = self.nativeDurationToNanos(now - self.start_time);
        self.start_time = now;
        return lap_time;
    }

    fn clockNative() u64 {
        if (is_windows) {
            return os.windows.QueryPerformanceCounter();
        }
        if (comptime std.Target.current.isDarwin()) {
            return os.darwin.mach_absolute_time();
        }
        var ts: os.timespec = undefined;
        os.clock_gettime(monotonic_clock_id, &ts) catch unreachable;
        return @intCast(u64, ts.tv_sec) * @as(u64, ns_per_s) + @intCast(u64, ts.tv_nsec);
    }

    fn nativeDurationToNanos(self: Timer, duration: u64) u64 {
        if (is_windows) {
            return @divFloor(duration * ns_per_s, self.frequency);
        }
        if (comptime std.Target.current.isDarwin()) {
            return @divFloor(duration * self.frequency.numer, self.frequency.denom);
        }
        return duration;
    }
};

test "sleep" {
    sleep(1);
}

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;
    testing.expect(interval > 0 and interval < margin);
}

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

    var timer = try Timer.start();
    sleep(10 * ns_per_ms);
    const time_0 = timer.read();
    testing.expect(time_0 > 0 and time_0 < margin);

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

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