34 lines
2.1 KiB
Markdown
34 lines
2.1 KiB
Markdown
# Commons Codec
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License: [APL 2.0](http://www.apache.org/licenses/LICENSE-2.0)
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This package contains a heavily modified version of the Murmur3 hash from [commons-codec](https://commons.apache.org/proper/commons-codec/)
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that cuts some corners but outperforms both Commons Codec and Guava fairly significantly for the particular use cases
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we care about being fast: Hashing ASCII/Latin1 strings into a well behaving 64-bit hash.
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The method `hashLowerBytes(String data)` performs a zero allocation and zero conversion hash of
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the *lower bytes* of the characters in the provided string. For ASCII, Latin1, or other 8 bit encodings
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this is identical to hashing the entire string. For other use cases, especially away from the
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Latin scripts, this function is possibly a foot-gun.
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The method `hashNearlyASCII(String data)` is the same as above, except it's
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seeded with Java String's hashCode(). This is a very non-standard modification that
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makes it a bit better at dealing with other encodings without measurable performance
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impact.
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The method `long hash(byte[] data)` hashes the entire byte array.
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A non-standard behavior is that the hash function folds the 128 bit
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hash into a 64 bit hash by xor:ing the 128 bit parts.
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## Performance Benchmarks
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| Algorithm | Ops/s | Remark |
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|--------------------|-------------------|-----------------------------------------------------------------|
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| Guava | 12,114 ± 439 | allocates byte buffers internally |
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| Common Codec | 29,224 ± 1,080 | String.getByte() penalty, long\[2\] allocation, possibly elided |
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| MH hash | 30,885 ± 847 | String.getByte() penalty, zero allocations |
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| MH hashNearlyASCII | 50,018 ± 399 | Zero allocations, worse characteristics outside Latin1/ASCII |
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| MH hashLowerBytes | 50,533 ± 478 | Zero allocations, only works for Latin1/ASCII |
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| String.hashCode() | 567,381 ± 136,185 | Zero allocations, much weaker algo |
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