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use super::{
    result,
    util,
};

#[derive(Clone, Copy, Debug, PartialEq, PartialOrd)]
pub struct UnixTimestamp(i64);

impl UnixTimestamp {
    pub const MIN: UnixTimestamp = Self::new(0);
    pub const MAX: UnixTimestamp = Self::new(253_402_300_800);

    const fn new(value: i64) -> Self {
        Self(value)
    }

    #[cfg(feature = "std")]
    pub fn now() -> Self {
        super::std_support::system_time_now()
    }

    pub const fn checked_from_unix_timestamp(timestamp: i64) -> Option<Self> {
        Self::from_unix_timestamp(timestamp).ok()
    }

    pub const fn from_unix_timestamp(timestamp: i64) -> result::TimestampResult {
        if util::is_supported_unix_timestamp(timestamp) {
            result::TimestampResult::TimestampOk(Self::new(timestamp))
        } else {
            result::TimestampResult::OverflowErr(timestamp)
        }
    }

    pub const fn unix_timestamp(self) -> i64 {
        self.0
    }

    pub const fn midnight(self) -> Self {
        Self::new(self.unix_timestamp() - self.seconds_since_midnight())
    }

    pub const fn seconds_since_midnight(self) -> i64 {
        (self.unix_timestamp() as u64 % util::SECONDS_PER_DAY as u64) as i64
    }
    
    pub const fn checked_add(self, seconds: i64) -> Option<Self> {
        let timestamp = self.unix_timestamp().wrapping_add(seconds);
        Self::checked_from_unix_timestamp(timestamp)
    }

    pub const fn checked_sub(self, seconds: i64) -> Option<Self> {
        let timestamp = self.unix_timestamp().wrapping_sub(seconds);
        Self::checked_from_unix_timestamp(timestamp)
    }

    pub const fn saturating_add(self, seconds: i64) -> Self {
        let timestamp = self.unix_timestamp().saturating_add(seconds); // MSRV 1.47
        Self::from_unix_timestamp(timestamp).unwrap()
    }

    pub const fn saturating_sub(self, seconds: i64) -> Self {
        let timestamp = self.unix_timestamp().saturating_sub(seconds); // MSRV 1.47
        Self::from_unix_timestamp(timestamp).unwrap()
    }

    pub const fn checked_from_year_month_day(year: u16, month: u8, day: u8) -> Option<Self> {
        if util::is_valid_year_month_day(year, month, day) {
            Self::from_year_month_day(year, month, day).ok()
        } else {
            None
        }
    }

    // Only valid for dates greater than or equal to 0000-1-1
    // [section 2.2.1](https://www.researchgate.net/publication/316558298_Date_Algorithms)
    pub const fn from_year_month_day(year: u16, month: u8, day: u8) -> result::TimestampResult {
        let (adj_year, adj_month, day) = if month < 3 {
            (year as i32 + 399, month as i32 + 12, day as i32)
        } else {
            (year as i32 + 400, month as i32, day as i32)
        };
        // f = (153 * adj_month - 457) / 5
        let f = (979 * adj_month - 2_918) >> 5;
        let julian_day_number = day + f + 365 * adj_year + adj_year / 4 - adj_year / 100 + adj_year / 400 + 1_575_022;
        Self::from_julian_day_number(julian_day_number)
    }

    // Only valid for dates greater than or equal to 0000-1-1
    // [section 3.2.1/3.3.1](https://www.researchgate.net/publication/316558298_Date_Algorithms)
    pub const fn to_year_month_day(self) -> (u16, u8, u8) {
        let julian_day_number = self.julian_day_number() as u32;
        let z = julian_day_number - 1_575_022;
        let h = 100 * z - 25;
        let a = h / 3_652_425;
        let b = a - a / 4;
        let adj_year = (100 * b + h) / 36_525;
        let c = b + z - 365 * adj_year - adj_year / 4;
        // adj_month = (5 * c + 456) / 153
        let adj_month = (535 * c + 48_950) >> 14;
        // f = (153 * adj_month - 457) / 5
        let f = (979 * adj_month - 2_918) >> 5;
        let day = c - f;
        let (year, month) = if adj_month > 12 {
            (adj_year - 399, adj_month - 12)
        } else {
            (adj_year - 400, adj_month)
        };
        (year as u16, month as u8, day as u8)
    }

    pub const fn checked_from_year_ordinal(year: u16, ordinal: u16) -> Option<Self> {
        if util::is_valid_year_ordinal(year, ordinal) {
            Self::from_year_ordinal(year, ordinal).ok()
        } else {
            None
        }
    }

    // I derived this algorithm based on the doy_from_month equation
    // [Computing day-of-year...](http://howardhinnant.github.io/date_algorithms.html#days_from_civil)
    pub const fn from_year_ordinal(year: u16, ordinal: u16) -> result::TimestampResult {
        let ordinal = ordinal as u64;
        let last_day_of_february = 59 + util::is_leap_year(year) as u64;
        // adj_month = (10 * adj_ordinal - 5) / 306
        let (adj_ordinal, adj_month, month) = if ordinal > last_day_of_february {
            let adj_ordinal = ordinal - last_day_of_february;
            let adj_month = (1_071 * adj_ordinal - 535) >> 15;
            (adj_ordinal, adj_month, (adj_month + 3) as u8)
        } else {
            let adj_ordinal = ordinal + 306;
            let adj_month = (1_071 * adj_ordinal - 535) >> 15;
            (adj_ordinal, adj_month, (adj_month - 9) as u8)
        };
        // f = (306 * adj_month + 5) / 10
        let f = (979 * adj_month + 16) >> 5;
        let day = adj_ordinal - f;
        Self::from_year_month_day(year, month, day as u8)
    }

    // [Eliminating the Lookup Table](https://blog.reverberate.org/2020/05/12/optimizing-date-algorithms.html)
    pub const fn to_year_ordinal(self) -> (u16, u16) {
        let (year, month, day) = self.to_year_month_day();
        let (month, day) = (month as u64, day as u64);
        // f = (306 * adj_month + 5) / 10
        let ordinal = if month >= 3 {
            ((979 * (month - 3) + 16) >> 5) + day + 59 + util::is_leap_year(year) as u64
        } else {
            ((979 * (month + 9) + 16) >> 5) + day - 306
        };
        (year, ordinal as u16)
    }

    pub const fn checked_from_julian_day_number(julian_day_number: i32) -> Option<Self> {
        Self::from_julian_day_number(julian_day_number).ok()
    }

    pub const fn from_julian_day_number(julian_day_number: i32) -> result::TimestampResult {
        let timestamp = (julian_day_number as i64 - util::UNIX_EPOCH_JULIAN_DAY_NUMBER as i64) * util::SECONDS_PER_DAY;
        Self::from_unix_timestamp(timestamp)
    }
    
    pub const fn julian_day_number(self) -> i32 {
        (self.unix_timestamp() as u64 / util::SECONDS_PER_DAY as u64) as i32 + util::UNIX_EPOCH_JULIAN_DAY_NUMBER
    }

    pub const fn weekday(self) -> util::Weekday {
        // (days_since_epoch + 3) % 7
        let adj_days = self.unix_timestamp() as u64 / util::SECONDS_PER_DAY as u64 + 3;
        let wd = adj_days - (((adj_days * 613_566_757) >> 32) * 7);
        util::Weekday::new(wd)
    }
}