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//! Utils used for local time managment.

use num_traits::ops::checked::CheckedAdd;
use num_traits::Num;

use core::cmp::PartialOrd;
use core::ops::{Div, Neg, Sub, SubAssign};

/// Increment the given ``ip`` with ``j`` if it doesn't overflow.
///
/// If the operation overflow, this return true otherwise false.
#[must_use]
pub fn increment_overflow<T: Num + CheckedAdd + Copy>(ip: &mut T, j: T) -> bool {
    let res = ip.checked_add(&j);

    if let Some(value) = res {
        *ip = value;
        false
    } else {
        true
    }
}

/// Normalize and increment the given ``ip`` with the given ``unit`` and ``base`` if it doesn't overflow.
///
/// If the operation overflow, this return true otherwise false.
/// 
/// Note:
/// 
/// The normalization part allows to remove (or in negative case, add) the amount of ``unit`` that we are going to add (or in negative case, remove) to ``ip``.
///
/// e.g: This can be used to get the number of minutes in a given number of seconds and permit to catches possible overflow on the number of minutes.
pub fn normalize_overflow<T>(
    ip: &mut T,
    unit: &mut T,
    base: T,
) -> bool where T: Num
        + Sub<Output = T>
        + Div<Output = T>
        + Neg<Output = T>
        + CheckedAdd
        + SubAssign
        + PartialOrd
        + Copy {
    let time_delta = if *unit >= T::zero() {
        *unit / base
    } else {
        -T::one() - (-T::one() - *unit) / base
    };

    *unit -= time_delta * base;

    increment_overflow(ip, time_delta)
}

/// Return true if it's a leap year.
#[inline]
pub fn is_leap_year(y: i64) -> bool {
    ((y) % 4) == 0 && (((y) % 100) != 0 || ((y) % 400) == 0)
}

/// Actual implementation of get_leap_days.
#[inline]
fn get_leap_days_not_neg(y: i64) -> i64 {
    y / 4 - y / 100 + y / 400
}

/// Get the total count of leap days since year 1.
/// 
/// For BC years, the amount of days will be negative. 
///
/// Note:
///
/// Year 0 by convention doesn't exist.
#[inline]
pub fn get_leap_days(y: i64) -> i64 {
    if y < 0 {
        -1 - get_leap_days_not_neg(-1 - y)
    } else {
        get_leap_days_not_neg(y)
    }
}

#[cfg(test)]
mod test {
    #[test]
    fn test_leap_year() {
        use crate::utils::is_leap_year;

        assert_eq!(is_leap_year(-100), false);
        assert_eq!(is_leap_year(0), true);
        assert_eq!(is_leap_year(1970), false);
        assert_eq!(is_leap_year(1980), true);
        assert_eq!(is_leap_year(1990), false);
        assert_eq!(is_leap_year(2000), true);
        assert_eq!(is_leap_year(2010), false);
        assert_eq!(is_leap_year(2020), true);
    }

    #[test]
    fn test_get_leap_days() {
        use crate::utils::get_leap_days;

        assert_eq!(get_leap_days(12), 3);
        assert_eq!(get_leap_days(8), 2);
        assert_eq!(get_leap_days(4), 1);
        assert_eq!(get_leap_days(1), 0);

        // This is an invalid case, that SHOULD return 0.
        assert_eq!(get_leap_days(0), 0);

        assert_eq!(get_leap_days(-1), -1);
    }
}