| 1 | /* |
| 2 | * Copyright (c) Meta Platforms, Inc. and affiliates. |
| 3 | * All rights reserved. |
| 4 | * |
| 5 | * This source code is licensed under both the BSD-style license (found in the |
| 6 | * LICENSE file in the root directory of this source tree) and the GPLv2 (found |
| 7 | * in the COPYING file in the root directory of this source tree). |
| 8 | * You may select, at your option, one of the above-listed licenses. |
| 9 | */ |
| 10 | |
| 11 | /* This match finder leverages techniques used in file comparison algorithms |
| 12 | * to find matches between a dictionary and a source file. |
| 13 | * |
| 14 | * The original motivation for studying this approach was to try and optimize |
| 15 | * Zstandard for the use case of patching: the most common scenario being |
| 16 | * updating an existing software package with the next version. When patching, |
| 17 | * the difference between the old version of the package and the new version |
| 18 | * is generally tiny (most of the new file will be identical to |
| 19 | * the old one). In more technical terms, the edit distance (the minimal number |
| 20 | * of changes required to take one sequence of bytes to another) between the |
| 21 | * files would be small relative to the size of the file. |
| 22 | * |
| 23 | * Various 'diffing' algorithms utilize this notion of edit distance and |
| 24 | * the corresponding concept of a minimal edit script between two |
| 25 | * sequences to identify the regions within two files where they differ. |
| 26 | * The core algorithm used in this match finder is described in: |
| 27 | * |
| 28 | * "An O(ND) Difference Algorithm and its Variations", Eugene W. Myers, |
| 29 | * Algorithmica Vol. 1, 1986, pp. 251-266, |
| 30 | * <https://doi.org/10.1007/BF01840446>. |
| 31 | * |
| 32 | * Additional algorithmic heuristics for speed improvement have also been included. |
| 33 | * These we inspired from implementations of various regular and binary diffing |
| 34 | * algorithms such as GNU diff, bsdiff, and Xdelta. |
| 35 | * |
| 36 | * Note: after some experimentation, this approach proved to not provide enough |
| 37 | * utility to justify the additional CPU used in finding matches. The one area |
| 38 | * where this approach consistently outperforms Zstandard even on level 19 is |
| 39 | * when compressing small files (<10 KB) using an equally small dictionary that |
| 40 | * is very similar to the source file. For the use case that this was intended, |
| 41 | * (large similar files) this approach by itself took 5-10X longer than zstd-19 and |
| 42 | * generally resulted in 2-3X larger files. The core advantage that zstd-19 has |
| 43 | * over this approach for match finding is the overlapping matches. This approach |
| 44 | * cannot find any. |
| 45 | * |
| 46 | * I'm leaving this in the contrib section in case this ever becomes interesting |
| 47 | * to explore again. |
| 48 | * */ |
| 49 | |
| 50 | #ifndef ZSTD_EDIST_H |
| 51 | #define ZSTD_EDIST_H |
| 52 | |
| 53 | /*-************************************* |
| 54 | * Dependencies |
| 55 | ***************************************/ |
| 56 | |
| 57 | #include <stddef.h> |
| 58 | #include "zstd_internal.h" /* ZSTD_Sequence */ |
| 59 | |
| 60 | /*! ZSTD_eDist_genSequences() : |
| 61 | * Will populate the provided ZSTD_Sequence buffer with sequences |
| 62 | * based on the optimal or near-optimal (depending on 'useHeuristics') |
| 63 | * edit script between 'dict' and 'src.' |
| 64 | * @return : the number of sequences found */ |
| 65 | size_t ZSTD_eDist_genSequences(ZSTD_Sequence* sequences, |
| 66 | const void* dict, size_t dictSize, |
| 67 | const void* src, size_t srcSize, |
| 68 | int useHeuristics); |
| 69 | |
| 70 | #endif |