648db22b |
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 | ``` |