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updated format spec

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Yann Collet 2016-07-05 11:50:37 +02:00
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zstd_compression_format.md
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@ -76,8 +76,8 @@ allowing streaming operations.
General Structure of Zstandard Frame format
-------------------------------------------
| MagicNb | F. Header | Block | (More blocks) | EndMark |
|:-------:|:----------:| ----- | ------------- | ------- |
| MagicNb | Frame Header | Block | (More blocks) | EndMark |
|:-------:|:-------------:| ----- | ------------- | ------- |
| 4 bytes | 2-14 bytes | | | 3 bytes |
__Magic Number__
@ -87,11 +87,11 @@ Value : 0xFD2FB527
__Frame Header__
2 to 14 Bytes, to be detailed in the next part.
2 to 14 Bytes, detailed in [next part](#frame-header).
__Data Blocks__
To be detailed later on.
Detailed in [next chapter](#data-blocks).
That’s where compressed data is stored.
__EndMark__
@ -101,12 +101,12 @@ This last block header may optionally host a __Content Checksum__ .
##### __Content Checksum__
Content Checksum verify that frame content has been regenrated correctly.
Content Checksum verify that frame content has been regenerated correctly.
The content checksum is the result
of [xxh64() hash function](https://www.xxHash.com)
digesting the original (decoded) data as input, and a seed of zero.
Bits from 11 to 32 (included) are extracted to form a 22 bits checksum
stored into the last block header.
stored into the endmark body.
```
mask22bits = (1<<22)-1;
contentChecksum = (XXH64(content, size, 0) >> 11) & mask22bits;
@ -224,6 +224,10 @@ Provides guarantees on maximum back-reference distance
that will be present within compressed data.
This information is useful for decoders to allocate enough memory.
`WD` byte is optional. It's not present in `single segment` mode.
In which case, the maximum back-reference distance is the content size itself,
which can be any value from 1 to 2^64-1 bytes (16 EB).
| BitNb | 7-3 | 0-2 |
| --------- | -------- | -------- |
| FieldName | Exponent | Mantissa |
@ -236,20 +240,16 @@ windowAdd = (windowBase / 8) * Mantissa;
windowSize = windowBase + windowAdd;
```
The minimum window size is 1 KB.
The maximum size is (15*(2^38))-1 bytes, which is almost 1.875 TB.
The maximum size is `15*(1<<38)` bytes, which is 1.875 TB.
To properly decode compressed data,
a decoder will need to allocate a buffer of at least `windowSize` bytes.
Note that `WD` byte is optional. It's not present in `single segment` mode.
In which case, the maximum back-reference distance is the content size itself,
which can be any value from 1 to 2^64-1 bytes (16 EB).
In order to preserve decoder from unreasonable memory requirements,
a decoder can refuse a compressed frame
which requests a memory size beyond decoder's authorized range.
For better interoperability,
For improved interoperability,
decoders are recommended to be compatible with window sizes of 8 MB.
Encoders are recommended to not request more than 8 MB.
It's merely a recommendation though,
@ -266,10 +266,10 @@ it's up to the caller to make sure it uses the correct dictionary.
Field size depends on __Dictionary ID flag__.
1 byte can represent an ID 0-255.
2 bytes can represent an ID 0-65535.
4 bytes can represent an ID 0-(2^32-1).
4 bytes can represent an ID 0-4294967295.
It's allowed to represent a small ID (for example `13`)
with a large 4-bytes dictionary ID, losing some efficiency in the process.
with a large 4-bytes dictionary ID, losing some compacity in the process.
__Frame Content Size__
@ -331,7 +331,7 @@ There are 4 block types :
while the "compressed" block is just 1 byte (the byte to repeat).
- EndMark : this is not a block. Signal the end of the frame.
The rest of the field may be optionally filled by a checksum
(see [__Content Checksum__]).
(see [Content Checksum](#content-checksum)).
Block sizes must respect a few rules :
- In compressed mode, compressed size if always strictly `< decompressed size`.
@ -345,9 +345,9 @@ Where the actual data to decode stands.
It might be compressed or not, depending on previous field indications.
A data block is not necessarily "full" :
since an arbitrary “flush” may happen anytime,
block decompressed content can be any size, up to Block Maximum Size.
Block Maximum Decompressed Size is the smallest of :
- Max back-reference distance
block decompressed content can be any size,
up to Block Maximum Decompressed Size, which is the smallest of :
- Maximum back-reference distance
- 128 KB
@ -364,7 +364,9 @@ Its design is pretty straightforward,
with the sole objective to allow the decoder to quickly skip
over user-defined data and continue decoding.
Skippable frames defined in this specification are compatible with LZ4 ones.
Skippable frames defined in this specification are compatible with [LZ4] ones.
[LZ4]:http://www.lz4.org
__Magic Number__ :
@ -393,8 +395,8 @@ in order to properly allocate destination buffer.
See [Data Blocks](#data-blocks) for more details.
A compressed block consists of 2 sections :
- Literals section
- Sequences section
- [Literals section](#literals-section)
- [Sequences section](#sequences-section)
### Prerequisites
To decode a compressed block, the following elements are necessary :
@ -442,9 +444,8 @@ This is a 2-bits field, describing 4 different block types :
starting with a huffman tree description.
See details below.
- Repeat Stats : This is a huffman-compressed block,
using huffman tree from previous huffman-compressed block.
using huffman tree _from previous huffman-compressed literals block_.
Huffman tree description will be skipped.
Compressed stream is equivalent to "compressed" block type.
- Raw : Literals are stored uncompressed.
- RLE : Literals consist of a single byte value repeated N times.
@ -476,24 +477,24 @@ using a long format, accepting the reduced compacity.
__Sizes format for Compressed literals block__ :
Note : also applicable to "repeat-stats" blocks.
- Value : 00 : 4 streams
Compressed and regenerated sizes use 10 bits (0-1023)
Total literal header size is 3 bytes
- Value : 01 : _Single stream_
Compressed and regenerated sizes use 10 bits (0-1023)
Total literal header size is 3 bytes
- Value : 10 : 4 streams
Compressed and regenerated sizes use 14 bits (0-16383)
Total literal header size is 4 bytes
- Value : 10 : 4 streams
Compressed and regenerated sizes use 18 bits (0-262143)
Total literal header size is 5 bytes
- Value : 00 : 4 streams.
Compressed and regenerated sizes use 10 bits (0-1023).
Total literal header size is 3 bytes.
- Value : 01 : _Single stream_.
Compressed and regenerated sizes use 10 bits (0-1023).
Total literal header size is 3 bytes.
- Value : 10 : 4 streams.
Compressed and regenerated sizes use 14 bits (0-16383).
Total literal header size is 4 bytes.
- Value : 10 : 4 streams.
Compressed and regenerated sizes use 18 bits (0-262143).
Total literal header size is 5 bytes.
Compressed and regenerated size fields follow big endian convention.
#### Huffman Tree description
This section is only present when block type is `Compressed` (`0`).
This section is only present when literals block type is `Compressed` (`0`).
Prefix coding represents symbols from an a priori known alphabet
by bit sequences (codes), one code for each symbol,
@ -546,7 +547,7 @@ It gives the following serie of weights :
The decoder will do the inverse operation :
having collected weights of literals from `0` to `4`,
it knows the last literal, `5`, is present with a non-zero weight.
The weight of `5` can be deduced by joining to the nearest power of 2.
The weight of `5` can be deducted by joining to the nearest power of 2.
Sum of 2^(weight-1) (excluding 0) is :
`8 + 4 + 2 + 0 + 1 = 15`
Nearest power of 2 is 16.
@ -558,24 +559,17 @@ This is a single byte value (0-255),
which tells how to decode the list of weights.
- if headerByte >= 242 : this is one of 14 pre-defined weight distributions :
+ 242 : 1x1 (+ 1x1)
+ 243 : 2x1 (+ 1x2)
+ 244 : 3x1 (+ 1x1)
+ 245 : 4x1 (+ 1x4)
+ 246 : 7x1 (+ 1x1)
+ 247 : 8x1 (+ 1x8)
+ 248 : 15x1 (+ 1x1)
+ 249 : 16x1 (+ 1x16)
+ 250 : 31x1 (+ 1x1)
+ 251 : 32x1 (+ 1x32)
+ 252 : 63x1 (+ 1x1)
+ 253 : 64x1 (+ 1x64)
+ 254 :127x1 (+ 1x1)
+ 255 :128x1 (+ 1x128)
| value |242|243|244|245|246|247|248|249|250|251|252|253|254|255|
| -------- |
| Nb of 1s | 1 | 2 | 3 | 4 | 7 | 8 | 15| 16| 31| 32| 63| 64|127|128|
|Complement| 1 | 2 | 1 | 4 | 1 | 8 | 1 | 16| 1 | 32| 1 | 64| 1 |128|
_Note_ : complement is by using the "join to nearest power of 2" rule.
- if headerByte >= 128 : this is a direct representation,
where each weight is written directly as a 4 bits field (0-15).
The full representation occupies ((nbSymbols+1)/2) bytes,
The full representation occupies `((nbSymbols+1)/2)` bytes,
meaning it uses a last full byte even if nbSymbols is odd.
`nbSymbols = headerByte - 127;`
@ -593,13 +587,13 @@ To decode an FSE bitstream, it is necessary to know its compressed size.
Compressed size is provided by `headerByte`.
It's also necessary to know its maximum decompressed size.
In this case, it's `255`, since literal values range from `0` to `255`,
and the last symbol value is not represented.
and last symbol value is not represented.
An FSE bitstream starts by a header, describing probabilities distribution.
It will create a Decoding Table.
It is necessary to know the maximum accuracy of distribution
to properly allocate space for the Table.
For a list of huffman weights, this maximum is 8 bits.
For a list of huffman weights, this maximum is 7 bits.
FSE header and bitstreams are described in a separated chapter.
@ -652,13 +646,12 @@ presuming the CPU has enough parallelism available.
For single stream, header provides both the compressed and regenerated size.
For 4-streams though,
header only provides compressed and regenerated size of all 4 streams combined.
In order to properly decode the 4 streams,
it's necessary to know the compressed and regenerated size of each stream.
Regenerated size is easiest :
each stream has a size of `(totalSize+3)/4`,
except the last one, which is up to 3 bytes smaller, to reach totalSize.
except the last one, which is up to 3 bytes smaller, to reach `totalSize`.
Compressed size must be provided explicitly : in the 4-streams variant,
bitstreams are preceded by 3 unsigned Little Endian 16-bits values.
@ -704,11 +697,11 @@ A match copy command specifies an offset and a length.
The offset gives the position to copy from,
which can stand within a previous block.
These are 3 symbol types, `literalLength`, `matchLength` and `offset`,
There are 3 symbol types, `literalLength`, `matchLength` and `offset`,
which are encoded together, interleaved in a single _bitstream_.
Each symbol decoding consists of a _code_,
which specifies a baseline and a number of additional bits.
Each symbol is a _code_ in its own context,
which specifies a baseline and a number of bits to add.
_Codes_ are FSE compressed,
and interleaved with raw additional bits in the same bitstream.
@ -717,7 +710,7 @@ followed by optional Probability tables for each symbol type,
followed by the bitstream.
To decode the Sequence section, it's required to know its size.
This size is deducted from "blockSize - literalSectionSize".
This size is deducted from `blockSize - literalSectionSize`.
#### Sequences section header
@ -733,9 +726,9 @@ Let's call its first byte `byte0`.
- `if (byte0 == 0)` : there are no sequences.
The sequence section stops there.
Regenerated content is defined entirely by literals section.
- `if (byte0 < 128)` : nbSeqs = byte0 . Uses 1 byte.
- `if (byte0 < 255)` : nbSeqs = ((byte0-128) << 8) + byte1 . Uses 2 bytes.
- `if (byte0 == 255)`: nbSeqs = byte1 + (byte2<<8) + 0x7F00 . Uses 3 bytes.
- `if (byte0 < 128)` : `nbSeqs = byte0;` . Uses 1 byte.
- `if (byte0 < 255)` : `nbSeqs = ((byte0-128) << 8) + byte1;` . Uses 2 bytes.
- `if (byte0 == 255)`: `nbSeqs = byte1 + (byte2<<8) + 0x7F00;` . Uses 3 bytes.
__Symbol compression modes__
@ -760,8 +753,8 @@ They follow the same enumeration :
- "RLE" : it's a single code, repeated `nbSeqs` times.
- "Repeat" : re-use distribution table from previous compressed block.
- "FSE" : standard FSE compression.
Symbol type requires a distribution table,
which will be described in next part.
A distribution table will be present.
It will be described in [next part](#distribution-tables).
#### Symbols decoding
@ -772,8 +765,9 @@ They define lengths from 0 to 131071 bytes.
| Code | 0-15 |
| ------ | ---- |
| nbBits | 0 |
| value | Code |
| nbBits | 0 |
| Code | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 |
| -------- | ---- | ---- | ---- | ---- | ---- | ---- | ---- | ---- |
@ -792,7 +786,7 @@ They define lengths from 0 to 131071 bytes.
__Default distribution__
When "compression mode" is defined as "default distribution",
When "compression mode" is "predef"",
a pre-defined distribution is used for FSE compression.
Here is its definition. It uses an accuracy of 6 bits (64 states).
@ -810,8 +804,8 @@ They define lengths from 3 to 131074 bytes.
| Code | 0-31 |
| ------ | -------- |
| nbBits | 0 |
| value | Code + 3 |
| nbBits | 0 |
| Code | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 |
| -------- | ---- | ---- | ---- | ---- | ---- | ---- | ---- | ---- |
@ -847,8 +841,8 @@ short matchLengths_defaultDistribution[53] =
Offset codes are values ranging from `0` to `N`,
with `N` being limited by maximum backreference distance.
A decoder is free to limit its maximum `N` supported,
although the recommendation is to support at least up to `22`.
A decoder is free to limit its maximum `N` supported.
Recommendation is to support at least up to `22`.
For information, at the time of this writing.
the reference decoder supports a maximum `N` value of `28` in 64-bits mode.
@ -863,14 +857,15 @@ if (OFValue > 3) offset = OFValue - 3;
OFValue from 1 to 3 are special : they define "repeat codes",
which means one of the previous offsets will be repeated.
They are sorted in recency order, with 1 meaning the most recent one.
See [Repeat offsets](#repeat-offsets) paragraph.
__Default distribution__
When "compression mode" is defined as "default distribution",
When "compression mode" is defined as "predef",
a pre-defined distribution is used for FSE compression.
Here is its definition. It uses an accuracy of 5 bits (32 states),
and support a maximum `N` of 28, allowing offset values up to 536,870,908 .
and supports a maximum `N` of 28, allowing offset values up to 536,870,908 .
If any sequence in the compressed block requires an offset larger than this,
it's not possible to use the default distribution to represent it.
@ -920,7 +915,7 @@ It depends on :
__example__ :
Presuming an AccuracyLog of 8,
and presuming 100 probabilities points have already been distributed,
the decoder may discover value from `0` to `255 - 100 + 1 == 156` (included).
the decoder may read any value from `0` to `255 - 100 + 1 == 156` (included).
Therefore, it must read `log2sup(156) == 8` bits.
- Value decoded : small values use 1 less bit :
@ -946,7 +941,7 @@ Probability is obtained from Value decoded by following formulae :
It means value `0` becomes negative probability `-1`.
`-1` is a special probability, which means `less than 1`.
Its effect on distribution table is described in a later paragraph.
Its effect on distribution table is described in next paragraph.
For the purpose of calculating cumulated distribution, it counts as one.
When a symbol has a probability of `zero`,
@ -988,8 +983,10 @@ each symbol gets attributed as many cells as its probability.
Cell allocation is spreaded, not linear :
each successor position follow this rule :
`position += (tableSize>>1) + (tableSize>>3) + 3);
position &= tableSize-1;`
```
position += (tableSize>>1) + (tableSize>>3) + 3;
position &= tableSize-1;
```
A position is skipped if already occupied,
typically by a "less than 1" probability symbol.
@ -999,11 +996,11 @@ Each state will decode the current symbol.
To get the Number of bits and baseline required for next state,
it's first necessary to sort all states in their natural order.
The lower states will need 1 bit more than higher ones.
The lower states will need 1 more bit than higher ones.
__Example__ :
Presuming a symbol has a probability of 5.
It receives 5 state values.
It receives 5 state values. States are sorted in natural order.
Next power of 2 is 8.
Space of probabilities is divided into 8 equal parts.
@ -1034,8 +1031,8 @@ All sequences are stored in a single bitstream, read _backward_.
It is therefore necessary to know the bitstream size,
which is deducted from compressed block size.
The exact last bit of the stream is followed by a set-bit-flag.
The highest bit of last byte is this flag.
The bit of the stream is followed by a set-bit-flag.
Highest bit of last byte is this flag.
It does not belong to the useful part of the bitstream.
Therefore, last byte has 0-7 useful bits.
Note that it also means that last byte cannot be `0`.