open-wal 0.2.1

//! Log Sequence Numbers.

use std::fmt;

/// Log Sequence Number. Newtype to prevent mixing with byte offsets/counts.
///
/// `Lsn(0)` is reserved as "none"; real records are dense starting at 1
/// (§6 of `docs/wal_design_v6.md`).
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug, Hash)]
pub struct Lsn(pub u64);

impl Lsn {
    /// The reserved "none" sentinel. Never assigned to a real record.
    pub const NONE: Lsn = Lsn(0);

    /// The first valid record LSN. Records are dense starting here.
    pub const FIRST: Lsn = Lsn(1);

    /// The next LSN in sequence. Saturating, so `Lsn(u64::MAX).next()` does not
    /// wrap (a 2^64-record log is not reachable in practice, but recovery must
    /// never panic on arbitrary inputs — D11).
    #[inline]
    #[must_use]
    pub const fn next(self) -> Lsn {
        Lsn(self.0.saturating_add(1))
    }

    /// True for the reserved `Lsn(0)` sentinel.
    #[inline]
    #[must_use]
    pub const fn is_none(self) -> bool {
        self.0 == 0
    }
}

impl fmt::Display for Lsn {
    /// Formats the LSN as its underlying integer, so error and log messages can
    /// use `{lsn}` instead of poking at the `.0` field.
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}", self.0)
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn none_and_first() {
        assert_eq!(Lsn::NONE, Lsn(0));
        assert_eq!(Lsn::FIRST, Lsn(1));
        assert!(Lsn::NONE.is_none());
        assert!(!Lsn::FIRST.is_none());
    }

    #[test]
    fn next_is_monotone_and_dense() {
        assert_eq!(Lsn::NONE.next(), Lsn::FIRST);
        assert_eq!(Lsn(41).next(), Lsn(42));
    }

    #[test]
    fn next_saturates_without_panicking() {
        assert_eq!(Lsn(u64::MAX).next(), Lsn(u64::MAX));
    }

    #[test]
    fn ordering() {
        assert!(Lsn(1) < Lsn(2));
        assert!(Lsn::NONE < Lsn::FIRST);
    }

    #[test]
    fn display_is_the_underlying_integer() {
        assert_eq!(Lsn(0).to_string(), "0");
        assert_eq!(Lsn(100001).to_string(), "100001");
    }
}