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