| 1 | ## Edit Distance Match Finder |
| 2 | |
| 3 | ``` |
| 4 | /* This match finder leverages techniques used in file comparison algorithms |
| 5 | * to find matches between a dictionary and a source file. |
| 6 | * |
| 7 | * The original motivation for studying this approach was to try and optimize |
| 8 | * Zstandard for the use case of patching: the most common scenario being |
| 9 | * updating an existing software package with the next version. When patching, |
| 10 | * the difference between the old version of the package and the new version |
| 11 | * is generally tiny (most of the new file will be identical to |
| 12 | * the old one). In more technical terms, the edit distance (the minimal number |
| 13 | * of changes required to take one sequence of bytes to another) between the |
| 14 | * files would be small relative to the size of the file. |
| 15 | * |
| 16 | * Various 'diffing' algorithms utilize this notion of edit distance and |
| 17 | * the corresponding concept of a minimal edit script between two |
| 18 | * sequences to identify the regions within two files where they differ. |
| 19 | * The core algorithm used in this match finder is described in: |
| 20 | * |
| 21 | * "An O(ND) Difference Algorithm and its Variations", Eugene W. Myers, |
| 22 | * Algorithmica Vol. 1, 1986, pp. 251-266, |
| 23 | * <https://doi.org/10.1007/BF01840446>. |
| 24 | * |
| 25 | * Additional algorithmic heuristics for speed improvement have also been included. |
| 26 | * These we inspired from implementations of various regular and binary diffing |
| 27 | * algorithms such as GNU diff, bsdiff, and Xdelta. |
| 28 | * |
| 29 | * Note: after some experimentation, this approach proved to not provide enough |
| 30 | * utility to justify the additional CPU used in finding matches. The one area |
| 31 | * where this approach consistently outperforms Zstandard even on level 19 is |
| 32 | * when compressing small files (<10 KB) using an equally small dictionary that |
| 33 | * is very similar to the source file. For the use case that this was intended, |
| 34 | * (large similar files) this approach by itself took 5-10X longer than zstd-19 and |
| 35 | * generally resulted in 2-3X larger files. The core advantage that zstd-19 has |
| 36 | * over this approach for match finding is the overlapping matches. This approach |
| 37 | * cannot find any. |
| 38 | * |
| 39 | * I'm leaving this in the contrib section in case this ever becomes interesting |
| 40 | * to explore again. |
| 41 | * */ |
| 42 | ``` |