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use std::time::Duration;
use muldiv::MulDiv;

const NANOS_PER_SEC: u64 = 1_000_000_000;
const MILLIS_PER_SEC: u64 = 1_000;
const NANOS_PER_MILLI: u64 = NANOS_PER_SEC / MILLIS_PER_SEC;

/// Extensions to the `Duration` type.
pub trait DurationExt {
    /// Returns the number of whole milliseconds contained in this `Duration`.
    ///
    /// Returns `None` if the value exceeds the capacity of a `u64`.
    fn as_millis(&self) -> Option<u64>;

    /// Multiplies this `Duration`.
    ///
    /// Like the `Mul` implementation for `Duration`, except that it takes a
    /// `u64` instead of a `u32.
    fn mul_u64(&self, rhs: u64) -> Self;

    /// Divides this `Duration`.
    ///
    /// Like the `Div` implementation for `Duration`, except that it takes a
    /// `u64` instead of a `u32`.
    fn div_u64(&self, rhs: u64) -> Self;
}

impl DurationExt for Duration {
    fn as_millis(&self) -> Option<u64> {
        self.as_secs()
            .checked_mul(MILLIS_PER_SEC)
            .and_then(|m| m.checked_add(self.subsec_nanos() as u64 / NANOS_PER_MILLI))
    }

    fn mul_u64(&self, rhs: u64) -> Duration {
        // for nanos, treat rhs as (NANOS_PER_SEC * a + b), where b < NANOS_PER_SEC
        let a = rhs / NANOS_PER_SEC;
        let b = rhs % NANOS_PER_SEC;
        let total_nanos = self.subsec_nanos() as u64 * b; // can't overflow
        let nanos = (total_nanos % NANOS_PER_SEC as u64) as u32;

        let secs = self.as_secs()
                       .checked_mul(rhs)
                       .and_then(|s| s.checked_add(total_nanos / NANOS_PER_SEC))
                       .and_then(|s| s.checked_add(self.subsec_nanos() as u64 * a))
                       .expect("overflow when multiplying duration");
        debug_assert!(nanos < NANOS_PER_SEC as u32);
        Duration::new(secs, nanos)
    }

    fn div_u64(&self, rhs: u64) -> Duration {
        let secs = self.as_secs() / rhs;
        let carry = self.as_secs() - secs * rhs;
        let extra_nanos = carry.mul_div_floor(NANOS_PER_SEC, rhs)
                               .expect("overflow when dividing duration");
        let nanos = (self.subsec_nanos() as u64 / rhs + extra_nanos) as u32;
        debug_assert!(nanos < NANOS_PER_SEC as u32);
        Duration::new(secs, nanos)
    }
}

#[cfg(test)]
mod test {
    use std::time::Duration;

    use super::*;

    #[test]
    fn as_millis() {
        assert_eq!(Some(1100), Duration::new(1, 100_000_999).as_millis());
        assert_eq!(None, Duration::from_secs(1 << 60).as_millis());
    }

    #[test]
    fn mul_u64() {
        assert_eq!(Duration::new(0, 1).mul_u64(2), Duration::new(0, 2));
        assert_eq!(Duration::new(1, 1).mul_u64(3), Duration::new(3, 3));
        assert_eq!(Duration::new(0, 500_000_001).mul_u64(4), Duration::new(2, 4));
        assert_eq!(Duration::new(0, 500_000_001).mul_u64(4000),
                   Duration::new(2000, 4000));
        assert_eq!(Duration::new(0, 500_000_000).mul_u64(1 << 63),
                   Duration::new(1 << 62, 0));
    }

    #[test]
    fn div_u64() {
        assert_eq!(Duration::new(0, 1).div_u64(2), Duration::new(0, 0));
        assert_eq!(Duration::new(1, 1).div_u64(3), Duration::new(0, 333_333_333));
        assert_eq!(Duration::new(99, 999_999_000).div_u64(100),
                   Duration::new(0, 999_999_990));
        assert_eq!(Duration::new(1 << 62, 0).div_u64(1 << 63),
                   Duration::new(0, 500_000_000));
    }
}