massmap 0.1.5

Static hash table that scales via disk-backed expansion to trim memory usage while ensuring each lookup needs exactly one I/O.
Documentation 
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use std::io::Result;

/// Trait representing positional writers suitable for massmap serialization.
///
/// Writers must support writing arbitrary byte slices at fixed offsets without
/// altering shared state; this is satisfied by `FileExt` handles on both Unix
/// and Windows.
pub trait MassMapWriter {
    /// Writes `data` at the given absolute `offset`.
    fn write_all_at(&self, data: &[u8], offset: u64) -> Result<()>;
}

#[cfg(unix)]
impl<T: std::os::unix::fs::FileExt> MassMapWriter for T {
    fn write_all_at(&self, data: &[u8], offset: u64) -> Result<()> {
        self.write_all_at(data, offset)
    }
}

#[cfg(windows)]
impl<T: std::os::windows::fs::FileExt> MassMapWriter for T {
    fn write_all_at(&self, data: &[u8], offset: u64) -> Result<()> {
        let n = self.seek_write(data, offset)?;
        if n != data.len() {
            return Err(std::io::Error::new(
                std::io::ErrorKind::WriteZero,
                format!("Failed to write all bytes: {} < {}", n, data.len()),
            ));
        }
        Ok(())
    }
}