const std = @import("../index.zig");
const builtin = @import("builtin");
const Os = builtin.Os;
const debug = std.debug;

const windows = std.os.windows;
const darwin = std.os.darwin;
const posix = std.os.posix;

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

/// Sleep for the specified duration
pub fn sleep(seconds: usize, nanoseconds: usize) void {
    switch(builtin.os) {
        Os.linux, Os.macosx, Os.ios => {
            posixSleep(u63(seconds), u63(nanoseconds));
        },
        Os.windows => {
            const milliseconds = seconds * ms_per_s + nanoseconds / (ns_per_s / ms_per_s);
            windows.Sleep(windows.DWORD(milliseconds));
        },
        else => @compileError("Unsupported OS"),
    }
}

const u63 = @IntType(false, 63);
pub fn posixSleep(seconds: u63, nanoseconds: u63) void {
    var req = posix.timespec {
        .tv_sec = seconds,
        .tv_nsec = nanoseconds,
    };
    var rem: posix.timespec = undefined;
    while (true) {
        const ret_val = posix.nanosleep(&req, &rem);
        const err = posix.getErrno(ret_val);
        if (err == 0) return;
        switch (err) {
            posix.EFAULT => unreachable,
            posix.EINVAL => {
                // Sometimes Darwin returns EINVAL for no reason.
                // We treat it as a spurious wakeup.
                return;
            },
            posix.EINTR => {
                req = rem;
                continue;
            },
            else => return,
        }
    }
}

/// Get the posix timestamp, UTC, in seconds
pub fn timestamp() u64 {
    return @divFloor(miliTimestamp(), ms_per_s);
}

/// Get the posix timestamp, UTC, in nanoseconds
pub const miliTimestamp = switch(builtin.os) {
    Os.windows => miliTimestampWindows,
    Os.linux => miliTimestampPosix,
    Os.macosx, Os.ios => miliTimestampDarwin,
    else => @compileError("Unsupported OS"),
};

fn miliTimestampWindows() u64 {
    //FileTime has a granularity of 100 nanoseconds
    //  and uses the NTFS/Windows epoch
    var ft: i64 = undefined;
    windows.GetSystemTimeAsFileTime(&ft);
    const hns_per_ms = (ns_per_s / 100) / ms_per_s;
    const epoch_adj =  epoch.windows * ms_per_s;
    return u64(@divFloor(ft, hns_per_ms) + epoch_adj);
}

fn miliTimestampDarwin() u64 {
    //Sources suggest MacOS 10.12 has support for
    //  posix clock_gettime.
    var tv: darwin.timeval = undefined;
    var err = darwin.gettimeofday(&tv, null);
    debug.assert(err == 0);
    return tv.tv_sec * ms_per_s + ts.tv_usec
        * (us_per_s / ms_per_s);
}

fn miliTimestampPosix() u64 {
    //From what I can tell there's no reason clock_gettime
    //  should ever fail for us with CLOCK_REALTIME
    var ts: posix.timespec = undefined;
    const err = posix.clock_gettime(posix.CLOCK_REALTIME, &ts);
    debug.assert(err == 0);
    const sec_ms = ts.tv_sec * ms_per_s;
    const nsec_ms = @divFloor(ts.tv_nsec, ns_per_s / ms_per_s);
    return u64(sec_ms) + u64(nsec_ms);
}

/// Divisions of a second
pub const ns_per_s = 1000000000;
pub const us_per_s = 1000000;
pub const ms_per_s = 1000;
pub const cs_per_s = 100;

/// Common time divisions
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 oportunity 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, it would be
    //  less precise
    frequency: switch(builtin.os) {
        Os.windows => u64,
        Os.macosx, Os.ios => darwin.mach_timebase_info_data,
        else => void,
    },
    resolution: u64,
    start_time: u64,
    
    //Initialize the timer structure.
    //This gives us an oportunity to grab the counter frequency in windows.
    //On Windows: QueryPerformanceCounter will succeed on anything > XP.
    //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 occurances of
    //  incredibly bad luck.
    //On Darwin: This cannot fail, as far as I am able to tell.
    pub fn start() !Timer {
        var self: Timer = undefined;
        
        switch(builtin.os) {
            Os.windows => {
                var freq: i64 = undefined;
                var err = windows.QueryPerformanceFrequency(&freq);
                if(err == 0) return error.TimerUnsupported;
                self.frequency = u64(freq);
                self.resolution = @divFloor(ns_per_s, self.frequency);
                var start_time: i64 = undefined;
                _ = windows.QueryPerformanceCounter(&start_time);
                self.start_time = u64(start_time);
            },
            Os.linux => {
                var ts: posix.timespec = undefined;
                var result = posix.clock_getres(posix.CLOCK_MONOTONIC, &ts);
                switch(posix.getErrno(result)) {
                    0 => {},
                    posix.EINVAL => return error.TimerUnsupported,
                    else => unreachable,
                }
                self.resolution = u64(ts.tv_sec * ns_per_s + ts.tv_nsec);
                _ = posix.clock_gettime(posix.CLOCK_MONOTONIC, &ts);
                self.start_time = u64(ts.tv_sec * ns_per_s + ts.tv_nsec);
            },
            Os.macosx, Os.ios => {
                darwin.c.mach_timebase_info(&self.frequency);
                self.resolution = @divFloor(self.frequency.numer, self.denom);
                self.start_time = darwin.c.mach_absolute_time();
            },
            else => @compileError("Unsupported OS"),
        }
        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 switch(builtin.os) {
            Os.windows => @divFloor(clock * ns_per_s, self.frequency),
            Os.linux => clock,
            Os.macosx, Os.ios => @divFloor(clock * self.frequency.numer, self.frequency.denom),
            else => @compileError("Unsupported OS"),
        };
    }
    
    /// 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.read();
        self.start_time = now;
        return lap_time;
    }
    
    
    const clockNative = switch(builtin.os) {
        Os.windows => clockWindows,
        Os.linux => clockLinux,
        Os.macosx, Os.ios => clockDarwin,
        else => @compileError("Unsupported OS"),
    };
    
    fn clockWindows() u64 {
        var result: i64 = undefined;
        var err = windows.QueryPerformanceCounter(&result);
        debug.assert(err != 0);
        return u64(result);
    }
    
    fn clockDarwin() u64 {
        var result: u64 = undefined;
        darwin.c.mach_absolute_time(&result);
        return result;
    }
    
    fn clockLinux() u64 {
        var ts: posix.timespec = undefined;
        var result = posix.clock_gettime(posix.CLOCK_MONOTONIC, &ts);
        debug.assert(posix.getErrno(result) == 0);
        return u64(ts.tv_sec * ns_per_s + ts.tv_nsec);
    }
};





test "os.time.sleep" {
    sleep(0, 1);
}

test "os.time.timestamp" {
    const ns_per_ms = (ns_per_s / ms_per_s);
    const margin = 50;
    
    const time_0 = miliTimestamp();
    sleep(0, ns_per_ms);
    const time_1 = miliTimestamp();
    const interval = time_1 - time_0;
    debug.assert(interval > 0 and interval < margin);
}

test "os.time.Timer" {
    const ns_per_ms = (ns_per_s / ms_per_s);
    const margin = ns_per_ms * 50;
    
    var timer = try Timer.start();
    sleep(0, 10 * ns_per_ms);
    const time_0 = timer.read();
    debug.assert(time_0 > 0 and time_0 < margin);
    
    const time_1 = timer.lap();
    debug.assert(time_1 > time_0);
    
    timer.reset();
    debug.assert(timer.read() < time_1);
}