Function tac_bytes 

pub fn tac_bytes(
    data: &[u8],
    separator: u8,
    before: bool,
    out: &mut impl Write,
) -> Result<()>

Reverse the records in data separated by a single byte separator and write to out. If before is true, the separator is attached before the record instead of after. Uses vectored I/O (writev) to write directly from mmap’d data — zero intermediate copies.