Editing Deflate (section)

=== Duplicate string elimination ===
{{Further|LZ77 and LZ78|LZSS}}
Within compressed blocks, if a duplicate series of bytes is spotted (a repeated string), then a back-[[Reference (computer science)|reference]] is inserted, linking to the prior location of that identical string instead. An encoded match to an earlier string consists of an [[8-bit computing|8-bit]] length (3–258 bytes) and a 15-bit distance (1–32,768 bytes) to the start of the duplicate. Relative back-references can be made across any number of blocks, as long as the distance appears within the last 32&nbsp;[[Kibibyte|KiB]] of uncompressed data decoded (termed the ''sliding window'').

If the distance is less than the length, the duplicate overlaps itself, indicating repetition. For example, a run of 10 identical bytes can be encoded as one byte, followed by a duplicate of length 9, starting with the prior byte.

Searching the preceding text for duplicate substrings is the most computationally expensive part of the Deflate algorithm, and the operation which compression level settings affect.