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

use serde::{Deserialize, Serialize};

use crate::key::{
    ByteSource, CompositeKind, IncorrectByteLength, Key, KeyEncoding, KeyKind, KeyVisitor,
};

/// A timestamp relative to [`UNIX_EPOCH`].
#[derive(Serialize, Deserialize, Debug, Clone, Copy, Eq, PartialEq, PartialOrd, Ord, Default)]
pub struct Timestamp {
    /// The number of whole seconds since [`UNIX_EPOCH`].
    pub seconds: u64,
    /// The number of nanoseconds in the timestamp.
    pub nanos: u32,
}

impl Timestamp {
    /// The maximum valid value of Timestamp.
    pub const MAX: Self = Self {
        seconds: u64::MAX,
        nanos: 999_999_999,
    };
    /// The minimum representable Timestamp. This is equivalent to [`UNIX_EPOCH`].
    pub const MIN: Self = Self {
        seconds: 0,
        nanos: 0,
    };

    /// Returns the current timestamp according to the OS. Uses [`SystemTime::now()`].
    #[must_use]
    pub fn now() -> Self {
        Self::from(SystemTime::now())
    }
}

impl From<SystemTime> for Timestamp {
    fn from(time: SystemTime) -> Self {
        let duration_since_epoch = time
            .duration_since(UNIX_EPOCH)
            .expect("unrealistic system time");
        Self {
            seconds: duration_since_epoch.as_secs(),
            nanos: duration_since_epoch.subsec_nanos(),
        }
    }
}

impl From<Timestamp> for Duration {
    fn from(t: Timestamp) -> Self {
        Self::new(t.seconds, t.nanos)
    }
}

impl std::ops::Sub for Timestamp {
    type Output = Option<Duration>;

    fn sub(self, rhs: Self) -> Self::Output {
        Duration::from(self).checked_sub(Duration::from(rhs))
    }
}

impl std::ops::Add<Duration> for Timestamp {
    type Output = Self;

    fn add(self, rhs: Duration) -> Self::Output {
        let mut nanos = self.nanos + rhs.subsec_nanos();
        let mut seconds = self.seconds.saturating_add(rhs.as_secs());
        while nanos > 1_000_000_000 {
            nanos -= 1_000_000_000;
            seconds = seconds.saturating_add(1);
        }
        Self { seconds, nanos }
    }
}

impl<'k> Key<'k> for Timestamp {
    const CAN_OWN_BYTES: bool = false;

    fn from_ord_bytes<'e>(bytes: ByteSource<'k, 'e>) -> Result<Self, Self::Error> {
        if bytes.as_ref().len() != 12 {
            return Err(IncorrectByteLength);
        }

        Ok(Self {
            seconds: u64::from_ord_bytes(ByteSource::Borrowed(&bytes.as_ref()[0..8]))?,
            nanos: u32::from_ord_bytes(ByteSource::Borrowed(&bytes.as_ref()[8..12]))?,
        })
    }
}

impl KeyEncoding<Self> for Timestamp {
    type Error = IncorrectByteLength;

    const LENGTH: Option<usize> = Some(12);

    fn describe<Visitor>(visitor: &mut Visitor)
    where
        Visitor: KeyVisitor,
    {
        visitor.visit_composite(
            CompositeKind::Struct(Cow::Borrowed("std::time::Timestamp")),
            2,
        );
        visitor.visit_type(KeyKind::U64);
        visitor.visit_type(KeyKind::U32);
    }

    fn as_ord_bytes(&self) -> Result<std::borrow::Cow<'_, [u8]>, Self::Error> {
        let seconds_bytes: &[u8] = &self.seconds.to_be_bytes();
        let nanos_bytes = &self.nanos.to_be_bytes();
        Ok(Cow::Owned([seconds_bytes, nanos_bytes].concat()))
    }
}

#[test]
fn key_test() {
    let original = Timestamp::now();
    assert_eq!(
        Timestamp::from_ord_bytes(ByteSource::Borrowed(&original.as_ord_bytes().unwrap())).unwrap(),
        original
    );
}