Deflate backreference ($(\ell ,d)$)

A Deflate backreference specifies a length $\ell$ and backward distance $d$, measured in decoded bytes. If $n$ bytes have already been produced, append $\ell$ bytes starting at output index $n-d$, counting indices from zero. The permitted ranges are $3\le \ell \le 258$ and $1\le d\le 32768$. Without a supplied initial history, $d\le n$ is also required. The distance refers to decoded history, not a position in the compressed input.^[ref]

For the twelve bytes ABCABCABCABC, an illustrative instruction sequence is three Deflate literal bytes A, B, C, followed by the Deflate backreference $(\ell ,d)=(9,3)$. Run-length encoding sees twelve single-byte runs because neighbouring values differ; the Deflate backreference describes a repeated three-byte pattern instead. Conversely, a Deflate literal byte A followed by $(7,1)$ produces AAAAAAAA: distance one can express a single-value repetition. These are chosen instructional examples, not claims about an encoder's chosen instructions or complete serialized streams.