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| Original file line number | Diff line number | Diff line change |
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| # Dictionary builder | ||
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| This is an *experimental* dictionary builder for Zstandard, S2, LZ4, deflate and more. | ||
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| This diverges from the Zstandard dictionary builder, and may have some failure scenarios for very small or uniform inputs. | ||
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| Dictionaries returned should all be valid, but if very little data is supplied, it may not be able to generate a dictionary. | ||
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| With a large, diverse sample set, it will generate a dictionary that can compete with the Zstandard dictionary builder, | ||
| but for very similar data it will not be able to generate a dictionary that is as good. | ||
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| Feedback is welcome. | ||
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| ## Usage | ||
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| First of all a collection of *samples* must be collected. | ||
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| These samples should be representative of the input data and should not contain any complete duplicates. | ||
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| Only the *beginning* of the samples is important, the rest can be truncated. | ||
| Beyond something like 64KB the input is not important anymore. | ||
| The commandline tool can do this truncation for you. | ||
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| ## Command line | ||
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| To install the command line tool run: | ||
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| ``` | ||
| $ go install github.com/klaupost/compress/dict/cmd/builddict@latest | ||
| ``` | ||
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| Collect the samples in a directory, for example `samples/`. | ||
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| Then run the command line tool. Basic usage is just to pass the directory with the samples: | ||
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| ``` | ||
| $ builddict samples/ | ||
| ``` | ||
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| This will build a Zstandard dictionary and write it to `dictionary.bin` in the current folder. | ||
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| The dictionary can be used with the Zstandard command line tool: | ||
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| ``` | ||
| $ zstd -D dictionary.bin input | ||
| ``` | ||
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| ### Options | ||
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| The command line tool has a few options: | ||
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| - `-format`. Output type. "zstd" "s2" or "raw". Default "zstd". | ||
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| Output a dictionary in Zstandard format, S2 format or raw bytes. | ||
| The raw bytes can be used with Deflate, LZ4, etc. | ||
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| - `-hash` Hash bytes match length. Minimum match length. Must be 4-8 (inclusive) Default 6. | ||
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| The hash bytes are used to define the shortest matches to look for. | ||
| Shorter matches can generate a more fractured dictionary with less compression, but can for certain inputs be better. | ||
| Usually lengths around 6-8 are best. | ||
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| - `-len` Specify custom output size. Default 114688. | ||
| - `-max` Max input length to index per input file. Default 32768. All inputs are truncated to this. | ||
| - `-o` Output name. Default `dictionary.bin`. | ||
| - `-q` Do not print progress | ||
| - `-dictID` zstd dictionary ID. 0 will be random. Default 0. | ||
| - `-zcompat` Generate dictionary compatible with zstd 1.5.5 and older. Default false. | ||
| - `-zlevel` Zstandard compression level. | ||
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| The Zstandard compression level to use when compressing the samples. | ||
| The dictionary will be built using the specified encoder level, | ||
| which will reflect speed and make the dictionary tailored for that level. | ||
| Default will use level 4 (best). | ||
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| Valid values are 1-4, where 1 = fastest, 2 = default, 3 = better, 4 = best. | ||
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| ## Library | ||
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| The `github.com/klaupost/compress/dict` package can be used to build dictionaries in code. | ||
| The caller must supply a collection of (pre-truncated) samples, and the options to use. | ||
| The options largely correspond to the command line options. | ||
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| ```Go | ||
| package main | ||
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| import ( | ||
| "github.com/klaupost/compress/dict" | ||
| "github.com/klauspost/compress/zstd" | ||
| ) | ||
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| func main() { | ||
| var samples [][]byte | ||
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| // ... Fill samples with representative data. | ||
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| dict, err := dict.BuildZstdDict(samples, dict.Options{ | ||
| HashLen: 6, | ||
| MaxDictSize: 114688, | ||
| ZstdDictID: 0, // Random | ||
| ZstdCompat: false, | ||
| ZstdLevel: zstd.SpeedBestCompression, | ||
| }) | ||
| // ... Handle error, etc. | ||
| } | ||
| ``` | ||
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| There are similar functions for S2 and raw dictionaries (`BuildS2Dict` and `BuildRawDict`). |
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