Hawkynt

Reduce

PKZIP methods 2-5 (Reducing): a DLE-escaped LZ77 pre-pass (factor-controlled length/distance bit split) followed by a static, frequency-ranked probabilistic substitution stage using per-byte follower sets of up to 32 candidate successor bytes.

Properties

Property	Value
Category	Compression Algorithms
Sub-category	Dictionary + RLE
Security status	Not classified
Complexity	Advanced
Inventor	Systems Enhancement Associates (SEA); adapted by PKWARE, Inc.
Year	1989
Origin	🇺🇸 United States
Source	algorithms/compression/reduce.js

Security

Status: not classified — treat as unverified.

No vulnerabilities are recorded for this implementation.

Documentation

• .ZIP File Format Specification (APPNOTE.TXT)
• ZIP (file format) - Wikipedia (Reducing method)
• Move-to-front transform - Wikipedia (adaptive list technique)

References

• Info-ZIP unreduce.c (historical decoder notes)
• ARC archiver history (original SEA reducing algorithm)

Test vectors

5 vectors ship with this algorithm and run in the test suite. Byte values are hexadecimal.

Vector 1 — Empty input

Field	Value
input	(empty)
expected	0000000004

Vector 2 — Single byte

Field	Value
input	41
expected	01000000040000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 000000000041

Vector 3 — 256 repeated bytes

Field	Value
input	61616161616161616161616161616161 61616161616161616161616161616161 61616161616161616161616161616161 61616161616161616161616161616161 61616161616161616161616161616161 61616161616161616161616161616161 61616161616161616161616161616161 61616161616161616161616161616161 61616161616161616161616161616161 61616161616161616161616161616161 61616161616161616161616161616161 61616161616161616161616161616161 61616161616161616161616161616161 61616161616161616161616161616161 61616161616161616161616161616161 61616161616161616161616161616161
expected	00010000040000000000000000000000 04000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000004f00000000000 00000000000000000000000000000000 00000000000000000000000000104002 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 0000000000000004edc300

Vector 4 — Text sample repeated 4x

Field	Value
input	74686520717569636b2062726f776e20 666f78206a756d7073206f7665722074 6865206c617a7920646f672e20746865 20717569636b2062726f776e20666f78 206a756d7073206f7665722074686520 6c617a7920646f672e20746865207175 69636b2062726f776e20666f78206a75 6d7073206f76657220746865206c617a 7920646f672e2074686520717569636b 2062726f776e20666f78206a756d7073 206f76657220746865206c617a792064 6f672e20
expected	b4000000040000040200000000000000 00000000000000000000000000000000 00000000000000000000000000000000 00000000000000000000000000000000 … (259 bytes; the full value is in the source)

Vector 5 — All 256 byte values

Field	Value
input	000102030405060708090a0b0c0d0e0f 101112131415161718191a1b1c1d1e1f 202122232425262728292a2b2c2d2e2f 303132333435363738393a3b3c3d3e3f 404142434445464748494a4b4c4d4e4f 505152535455565758595a5b5c5d5e5f 606162636465666768696a6b6c6d6e6f 707172737475767778797a7b7c7d7e7f 808182838485868788898a8b8c8d8e8f 909192939495969798999a9b9c9d9e9f a0a1a2a3a4a5a6a7a8a9aaabacadaeaf b0b1b2b3b4b5b6b7b8b9babbbcbdbebf c0c1c2c3c4c5c6c7c8c9cacbcccdcecf d0d1d2d3d4d5d6d7d8d9dadbdcdddedf e0e1e2e3e4e5e6e7e8e9eaebecedeeef f0f1f2f3f4f5f6f7f8f9fafbfcfdfeff
expected	000100000440f01ff807fd017fb01fe8 07f9017e701fd807f5017d301fc807f1 017cf01eb807ed017bb01ea807e9017a 701e9807e50179301e8807e10178f01d … (487 bytes; the full value is in the source)

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