riegeli 0.2.1

//! `RecordPosition` — a logical position within a Riegeli file.
//!
//! A position combines a chunk's file offset with a record index within that chunk,
//! uniquely identifying any record in the file.

use std::fmt;
use std::str::FromStr;

use crate::error::RiegeliError;

/// A logical position within a Riegeli file identifying a specific record.
///
/// The position is represented as the file offset of the chunk that contains
/// the record (`chunk_begin`) and the zero-based index of the record within
/// that chunk (`record_index`).
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
pub struct RecordPosition {
    /// File offset of the chunk containing this record.
    pub chunk_begin: u64,
    /// Zero-based index of the record within its chunk.
    pub record_index: u64,
}

impl RecordPosition {
    /// Create a new `RecordPosition`.
    pub fn new(chunk_begin: u64, record_index: u64) -> Self {
        Self {
            chunk_begin,
            record_index,
        }
    }

    /// Returns a single numeric position suitable for ordering and seeking.
    ///
    /// Computed as `chunk_begin + record_index`.
    pub fn numeric(&self) -> u64 {
        // Saturating: both fields are public and freely constructible, so a
        // pathological pair must not panic a public accessor.
        self.chunk_begin.saturating_add(self.record_index)
    }
}

impl fmt::Display for RecordPosition {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "{}/{}", self.chunk_begin, self.record_index)
    }
}

impl FromStr for RecordPosition {
    type Err = RiegeliError;

    fn from_str(s: &str) -> Result<Self, Self::Err> {
        let (chunk_str, index_str) = s.split_once('/').ok_or_else(|| {
            RiegeliError::MalformedData(
                format!("invalid RecordPosition format (expected 'chunk/index'): {s}").into(),
            )
        })?;
        let chunk_begin = chunk_str.parse::<u64>().map_err(|e| {
            RiegeliError::MalformedData(
                format!("invalid chunk_begin in RecordPosition: {e}").into(),
            )
        })?;
        let record_index = index_str.parse::<u64>().map_err(|e| {
            RiegeliError::MalformedData(
                format!("invalid record_index in RecordPosition: {e}").into(),
            )
        })?;
        // Match the reference implementation: reject positions where
        // chunk_begin + record_index would overflow, so that numeric() is
        // always exact and order-consistent.
        if record_index > u64::MAX - chunk_begin {
            return Err(RiegeliError::MalformedData(
                format!(
                    "RecordPosition overflow: chunk_begin {chunk_begin} + record_index {record_index} exceeds u64::MAX"
                )
                .into(),
            ));
        }
        Ok(Self {
            chunk_begin,
            record_index,
        })
    }
}

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

    #[test]
    fn numeric() {
        let pos = RecordPosition::new(100, 5);
        assert_eq!(pos.numeric(), 105);
    }

    #[test]
    fn display() {
        let pos = RecordPosition::new(24, 0);
        assert_eq!(pos.to_string(), "24/0");
        let pos2 = RecordPosition::new(65560, 42);
        assert_eq!(pos2.to_string(), "65560/42");
    }

    #[test]
    fn from_str_roundtrip() {
        let pos = RecordPosition::new(12345, 67);
        let s = pos.to_string();
        let parsed: RecordPosition = s.parse().expect("parse ok");
        assert_eq!(parsed, pos);
    }

    #[test]
    fn from_str_invalid() {
        assert!("no-slash".parse::<RecordPosition>().is_err());
        assert!("abc/0".parse::<RecordPosition>().is_err());
        assert!("0/abc".parse::<RecordPosition>().is_err());
    }

    #[test]
    fn from_str_rejects_numeric_overflow() {
        // The C++ reference (RecordPosition::FromString) rejects any pair where
        // chunk_begin + record_index would overflow u64, so that numeric() is
        // always exact and order-consistent.
        assert!("18446744073709551615/1".parse::<RecordPosition>().is_err());
        assert!("18446744073709551615/5".parse::<RecordPosition>().is_err());
        assert!("1/18446744073709551615".parse::<RecordPosition>().is_err());
        // The boundary case chunk_begin + record_index == u64::MAX is valid.
        let pos: RecordPosition = "18446744073709551615/0".parse().expect("parse ok");
        assert_eq!(pos.numeric(), u64::MAX);
        let pos2: RecordPosition = "0/18446744073709551615".parse().expect("parse ok");
        assert_eq!(pos2.numeric(), u64::MAX);
    }
}