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improved zstd_compression_format.md
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inikep
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@ -29,6 +29,7 @@ Dictionary gains are mostly effective in the first few KB. Then, the compression
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will rely more and more on previously decoded content to compress the rest of the file.
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Usage of the dictionary builder and created dictionaries with CLI:
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1. Create the dictionary : `zstd --train FullPathToTrainingSet/* -o dictionaryName`
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2. Compress with dictionary: `zstd FILE -D dictionaryName`
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3. Decompress with dictionary: `zstd --decompress FILE.zst -D dictionaryName`
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@ -97,6 +97,42 @@ to decode all concatenated frames in their sequential order,
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delivering the final decompressed result as if it was a single content.
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Skippable Frames
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----------------
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| `Magic_Number` | `Frame_Size` | `User_Data` |
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|:--------------:|:------------:|:-----------:|
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| 4 bytes | 4 bytes | n bytes |
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Skippable frames allow the insertion of user-defined data
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into a flow of concatenated frames.
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Its design is pretty straightforward,
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with the sole objective to allow the decoder to quickly skip
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over user-defined data and continue decoding.
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Skippable frames defined in this specification are compatible with [LZ4] ones.
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[LZ4]:http://www.lz4.org
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__`Magic_Number`__
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4 Bytes, little-endian format.
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Value : 0x184D2A5X, which means any value from 0x184D2A50 to 0x184D2A5F.
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All 16 values are valid to identify a skippable frame.
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__`Frame_Size`__
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This is the size, in bytes, of the following `User_Data`
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(without including the magic number nor the size field itself).
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This field is represented using 4 Bytes, little-endian format, unsigned 32-bits.
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This means `User_Data` can’t be bigger than (2^32-1) bytes.
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__`User_Data`__
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The `User_Data` can be anything. Data will just be skipped by the decoder.
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General Structure of Zstandard Frame format
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-------------------------------------------
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The structure of a single Zstandard frame is following:
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@ -163,9 +199,9 @@ The `Flag_Value` can be converted into `Field_Size`,
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which is the number of bytes used by `Frame_Content_Size`
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according to the following table:
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|`Flag_Value`| 0 | 1 | 2 | 3 |
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| ---------- | --- | --- | --- | --- |
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|`Field_Size`| 0-1 | 2 | 4 | 8 |
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|`Flag_Value`| 0 | 1 | 2 | 3 |
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| ---------- | ------ | --- | --- | --- |
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|`Field_Size`| 0 or 1 | 2 | 4 | 8 |
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When `Flag_Value` is `0`, `Field_Size` depends on `Single_Segment_flag` :
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if `Single_Segment_flag` is set, `Field_Size` is 1.
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@ -361,40 +397,6 @@ up to `Block_Maximum_Decompressed_Size`, which is the smallest of :
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- 128 KB
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Skippable Frames
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----------------
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| `Magic_Number` | `Frame_Size` | `User_Data` |
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|:--------------:|:------------:|:-----------:|
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| 4 bytes | 4 bytes | n bytes |
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Skippable frames allow the insertion of user-defined data
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into a flow of concatenated frames.
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Its design is pretty straightforward,
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with the sole objective to allow the decoder to quickly skip
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over user-defined data and continue decoding.
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Skippable frames defined in this specification are compatible with [LZ4] ones.
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[LZ4]:http://www.lz4.org
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__`Magic_Number`__
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4 Bytes, little-endian format.
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Value : 0x184D2A5X, which means any value from 0x184D2A50 to 0x184D2A5F.
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All 16 values are valid to identify a skippable frame.
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__`Frame_Size`__
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This is the size, in bytes, of the following `User_Data`
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(without including the magic number nor the size field itself).
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This field is represented using 4 Bytes, little-endian format, unsigned 32-bits.
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This means `User_Data` can’t be bigger than (2^32-1) bytes.
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__`User_Data`__
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The `User_Data` can be anything. Data will just be skipped by the decoder.
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The format of `Compressed_Block`
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--------------------------------
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@ -447,9 +449,12 @@ __`Literals_Block_Type`__
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This field uses 2 lowest bits of first byte, describing 4 different block types :
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| Value | 0 | 1 | 2 | 3 |
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| --------------------- | -------------------- | -------------------- | --------------------------- | ----------------------------- |
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| `Literals_Block_Type` | `Raw_Literals_Block` | `RLE_Literals_Block` | `Compressed_Literals_Block` | `Repeat_Stats_Literals_Block` |
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| `Literals_Block_Type` | Value |
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| ----------------------------- | ----- |
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| `Raw_Literals_Block` | 0 |
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| `RLE_Literals_Block` | 1 |
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| `Compressed_Literals_Block` | 2 |
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| `Repeat_Stats_Literals_Block` | 3 |
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- `Raw_Literals_Block` - Literals are stored uncompressed.
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- `RLE_Literals_Block` - Literals consist of a single byte value repeated N times.
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@ -466,37 +471,37 @@ __`Size_Format`__
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- For `Compressed_Block`, it requires to decode both `Compressed_Size`
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and `Regenerated_Size` (the decompressed size). It will also decode the number of streams.
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- For `Raw_Block` and `RLE_Block` it's enough to decode `Regenerated_Size`.
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- For `Raw_Literals_Block` and `RLE_Literals_Block` it's enough to decode `Regenerated_Size`.
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For values spanning several bytes, convention is little-endian.
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__`Size_Format` for `Raw_Literals_Block` and `RLE_Literals_Block`__ :
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- Value : x0 : `Regenerated_Size` uses 5 bits (0-31).
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- Value x0 : `Regenerated_Size` uses 5 bits (0-31).
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`Literals_Section_Header` has 1 byte.
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`Regenerated_Size = Header[0]>>3`
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- Value : 01 : `Regenerated_Size` uses 12 bits (0-4095).
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- Value 01 : `Regenerated_Size` uses 12 bits (0-4095).
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`Literals_Section_Header` has 2 bytes.
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`Regenerated_Size = (Header[0]>>4) + (Header[1]<<4)`
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- Value : 11 : `Regenerated_Size` uses 20 bits (0-1048575).
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- Value 11 : `Regenerated_Size` uses 20 bits (0-1048575).
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`Literals_Section_Header` has 3 bytes.
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`Regenerated_Size = (Header[0]>>4) + (Header[1]<<4) + (Header[2]<<12)`
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Note : it's allowed to represent a short value (ex : `13`)
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using a long format, accepting the reduced compacity.
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Note : it's allowed to represent a short value (for example `13`)
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using a long format, accepting the increased compressed data size.
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__`Size_Format` for `Compressed_Literals_Block` and `Repeat_Stats_Literals_Block`__ :
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- Value : 00 : _Single stream_.
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- Value 00 : _A single stream_.
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Both `Compressed_Size` and `Regenerated_Size` use 10 bits (0-1023).
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`Literals_Section_Header` has 3 bytes.
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- Value : 01 : 4 streams.
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- Value 01 : 4 streams.
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Both `Compressed_Size` and `Regenerated_Size` use 10 bits (0-1023).
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`Literals_Section_Header` has 3 bytes.
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- Value : 10 : 4 streams.
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- Value 10 : 4 streams.
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Both `Compressed_Size` and `Regenerated_Size` use 14 bits (0-16383).
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`Literals_Section_Header` has 4 bytes.
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- Value : 11 : 4 streams.
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- Value 11 : 4 streams.
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Both `Compressed_Size` and `Regenerated_Size` use 18 bits (0-262143).
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`Literals_Section_Header` has 5 bytes.
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