use std::time::{Duration, SystemTime, UNIX_EPOCH};

#[derive(Clone, Copy, Debug, PartialEq, PartialOrd)]
pub struct Timestamp(pub SystemTime);

impl Timestamp {
    pub const ZERO: Timestamp = Timestamp(UNIX_EPOCH);

    pub fn now() -> Timestamp {
        return Timestamp(SystemTime::now());
    }

    pub fn duration_since(self, earlier: Timestamp) -> Result<Duration, std::time::SystemTimeError> {
        return self.0.duration_since(earlier.0);
    }
}

impl std::ops::Add<Duration> for Timestamp {
    type Output = Timestamp;
    fn add(self, rhs: Duration) -> Timestamp {
        return Timestamp(self.0 + rhs);
    }
}

impl std::ops::Sub<Duration> for Timestamp {
    type Output = Timestamp;
    fn sub(self, rhs: Duration) -> Timestamp {
        return Timestamp(self.0 - rhs);
    }
}

fn is_leap_year(y: u64) -> bool {
    return (y % 4 == 0 && y % 100 != 0) || (y % 400 == 0);
}

fn days_in_month(y: u64, m: u64) -> u64 {
    match m {
        1 | 3 | 5 | 7 | 8 | 10 | 12 => 31,
        4 | 6 | 9 | 11 => 30,
        2 => if is_leap_year(y) { 29 } else { 28 },
        _ => panic!("invalid month"),
    }
}

pub fn secs_to_ymd_hms(mut secs: u64) -> (u64, u64, u64, u64, u64, u64) {
    let s = secs % 60;
    secs /= 60;
    let min = secs % 60;
    secs /= 60;
    let h = secs % 24;
    let mut days = secs / 24;

    let mut y = 1970u64;
    loop {
        let days_in_year = if is_leap_year(y) { 366 } else { 365 };
        if days < days_in_year {
            break;
        }
        days -= days_in_year;
        y += 1;
    }

    let mut mo = 1u64;
    loop {
        let dim = days_in_month(y, mo);
        if days < dim {
            break;
        }
        days -= dim;
        mo += 1;
    }

    let day = days + 1;
    return (y, mo, day, h, min, s);
}

pub fn ymd_hms_to_secs(y: u64, m: u64, d: u64, h: u64, min: u64, s: u64) -> u64 {
    let mut days = 0u64;
    for year in 1970..y {
        days += if is_leap_year(year) { 366 } else { 365 };
    }
    for month in 1..m {
        days += days_in_month(y, month);
    }
    days += d - 1;
    return days * 86400 + h * 3600 + min * 60 + s;
}