zstd/zstd_frame_format.md

Zstandard Frame Format Description
==================================

### Notices

Copyright (c) 2016 Yann Collet

Permission is granted to copy and distribute this document
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provided that the copyright notice and this notice are preserved,
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are clearly marked.
Distribution of this document is unlimited.

### Version

0.1.0 (30/06/2016)


Introduction
------------

The purpose of this document is to define a lossless compressed data format,
that is independent of CPU type, operating system,
file system and character set, suitable for
File compression, Pipe and streaming compression
using the [Zstandard algorithm](http://www.zstandard.org).

The data can be produced or consumed,
even for an arbitrarily long sequentially presented input data stream,
using only an a priori bounded amount of intermediate storage,
and hence can be used in data communications.
The format uses the Zstandard compression method,
and optional [xxHash-64 checksum method](http://www.xxhash.org),
for detection of data corruption.

The data format defined by this specification
does not attempt to allow random access to compressed data.

This specification is intended for use by implementers of software
to compress data into Zstandard format and/or decompress data from Zstandard format.
The text of the specification assumes a basic background in programming
at the level of bits and other primitive data representations.

Unless otherwise indicated below,
a compliant compressor must produce data sets
that conform to the specifications presented here.
It doesn’t need to support all options though.

A compliant decompressor must be able to decompress
at least one working set of parameters
that conforms to the specifications presented here.
It may also ignore informative fields, such as checksum.
Whenever it does not support a specific parameter within the compressed stream,
it must produce a non-ambiguous error code
and associated error message explaining which parameter is unsupported.


General Structure of Zstandard Frame format
-------------------------------------------

| MagicNb |  F. Header | Block | (...) | EndMark |
|:-------:|:----------:| ----- | ----- | ------- |
| 4 bytes | 2-14 bytes |       |       | 3 bytes |

__Magic Number__

4 Bytes, Little endian format.
Value : 0xFD2FB527

__Frame Header__

2 to 14 Bytes, to be detailed in the next part.
Most important part of the spec.

__Data Blocks__

To be detailed later on.
That’s where compressed data is stored.

__EndMark__

The flow of blocks ends when the last block header brings an _end signal_ .
This last block header may optionally host a __Content Checksum__ .

__Content Checksum__

Content Checksum verify that the full content has been decoded 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 the 22 bits checksum
stored into the last block header.
```
contentChecksum = (XXH64(content, size, 0) >> 11) & (1<<22)-1);
```
Content checksum is only present when its associated flag
is set in the frame descriptor.
Its usage is optional.

__Frame Concatenation__

In some circumstances, it may be required to append multiple frames,
for example in order to add new data to an existing compressed file
without re-framing it.

In such case, each frame brings its own set of descriptor flags.
Each frame is considered independent.
The only relation between frames is their sequential order.

The ability to decode multiple concatenated frames
within a single stream or file is left outside of this specification.
As an example, the reference `zstd` command line utility is able
to decode all concatenated frames in their sequential order,
presenting the final decompressed result as if it was a single frame.


Frame Header
----------------

| FHD     | (WD)      | (Content Size) | (dictID)  |
| ------- | --------- |:--------------:| --------- |
| 1 byte  | 0-1 byte  |  0 - 8 bytes   | 0-4 bytes |

Frame header uses a minimum of 2 bytes,
and up to 14 bytes depending on optional parameters.

__FHD byte__ (Frame Header Descriptor)

|  BitNb  |   7-6  |    5    |   4    |    3     |    2     |    1-0   |
| ------- | ------ | ------- | ------ | -------- | -------- | -------- |
|FieldName| FCSize | Segment | Unused | Reserved | Checksum |  dictID  |

In the table, bit 7 is highest bit, while bit 0 is lowest.

__Frame Content Size flag__

This is a 2-bits flag (`= FHD >> 6`),
telling if original data size is provided within the header

|  Value  |  0  |  1  |  2  |  3  |
| ------- | --- | --- | --- | --- |
|FieldSize| 0-1 |  2  |  4  |  8  |

Value 0 is special : it means `0` (data size not provided)
_if_ the `WD` byte is present.
Otherwise, it means `1` byte (data size <= 255 bytes).

__Single Segment__

If this flag is set,
data shall be regenerated within a single continuous memory segment.
In which case, `WD` byte is not present,
but `Frame Content Size` field necessarily is.
The size of the memory segment must be at least `>= Frame Content Size`.

In order to preserve decoder from unreasonable memory requirement,
a decoder can refuse a compressed frame
which requests a memory size beyond decoder's authorized range.

__Unused bit__

The value of this bit is unimportant
and not interpreted by a decoder compliant with this specification version.
It may be used in a future revision,
to signal a property which is not required to properly decode the frame.

__Reserved bit__

This bit is reserved for some future feature.
Its value must be zero.
A decoder compliant with this specification version must ensure it is not set.
This bit may be used in a future revision,
to signal a feature that must be interpreted in order to decode the frame.

__Content checksum flag__

If this flag is set, a content checksum will be present into the EndMark.
The checksum is a 22 bits value extracted from the XXH64() of data.
See __Content Checksum__ .

__Dictionary ID flag__

This is a 2-bits flag (`= FHD & 3`),
telling if a dictionary ID is provided within the header

|  Value  |  0  |  1  |  2  |  3  |
| ------- | --- | --- | --- | --- |
|FieldSize|  0  |  1  |  2  |  4  |

__WD byte__ (Window Descriptor)

Provides guarantees on maximum back-reference distance
that will be used within compressed data.
This information can then be used by decoder to allocate enough memory.

|  BitNb  |    7-3   |    0-2   |
| ------- | -------- | -------- |
|FieldName| Exponent | Mantissa |

Maximum distance is given by the following formulae :
```
windowLog = 10 + Exponent;
windowBase = 1 << windowLog;
windowAdd = (windowBase / 8) * Mantissa;
windowSize = windowBase + windowAdd;
```
`WD` byte is optional. It's not present in `single segment` mode.

In order to preserve decoder from unreasonable memory requirement,
a decoder can refuse a compressed frame
which requests a memory size beyond decoder's authorized range.

__Frame Content Size__

This is the original (uncompressed) size.
This information is optional, and only present if associated flag is set.
Content size is provided using 1, 2, 4 or 8 Bytes.
Format is Little endian.

| Field Size |    Range   |
| ---------- | ---------- |
|     0      |      0     |
|     1      |   0 - 255  |
|     2      | 256 - 65791|
|     4      | 0 - 2^32-1 |
|     8      | 0 - 2^64-1 |

When field size is 1, 4 or 8 bytes, the value is read directly.
When field size is 2, an offset of 256 is added.
It's possible to represent a small size of `18` using the 8-bytes variant.
A size of `0` means `data size is unknown`.
In which case, the `WD` byte will be the only hint
to determine memory allocation.

In order to preserve decoder from unreasonable memory requirement,
a decoder can refuse a compressed frame
which requests a memory size beyond decoder's authorized range.

__Dictionary ID__

This is a variable size field, which contains a single ID.
It checks if the correct dictionary is used for decoding.
Note that this field is optional. If it's not present,
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).

It's possible to represent a small ID (for example `13`) with a large 4-bytes dictionary ID, losing some efficiency in the process.


Data Blocks
-----------

| B. Header |  data  |
|:---------:| ------ |
|  3 bytes  |        |


__Block Header__

This field uses 3-bytes, format is big-endian.

The 2 highest bits represent the `block type`,
while the remaining 22 bits represent the block size.

There are 4 block types :

|    Value   |      0     |  1  |  2  |    3    |
| ---------- | ---------- | --- | --- | ------- |
| Block Type | Compressed | Raw | RLE | EndMark |

- Compressed : this is a compressed block,
  following Zstandard's block format specification.
  The "block size" is the compressed size.
  Decompressed size is unknown,
  but its maximum possible value is guaranteed (see later)
- Raw : this is an uncompressed block.
  "block size" is the number of bytes to read and copy.
- RLE : this is a single byte, repeated N times.
  In which case, the size of the "compressed" block is always 1,
  and the "block size" is the size to regenerate.
- 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 frame checksum).

Block Size shall never be larger than Block Maximum Size.
Block Maximum Size is the smallest of :
- Max back-reference distance
- 128 KB


__Data__

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" :
an arbitrary “flush” may happen anytime. Any block can be “partially filled”.
Therefore, data can have any size, up to Block Maximum Size.
Block Maximum Size is the smallest of :
- Max back-reference distance
- 128 KB


Skippable Frames
----------------

| Magic Number | Frame Size | User Data |
|:------------:|:----------:| --------- |
|   4 bytes    |  4 bytes   |           |

Skippable frames allow the insertion of user-defined data
into a flow of concatenated frames.
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.


__Magic Number__

4 Bytes, Little endian format.
Value : 0x184D2A5X, which means any value from 0x184D2A50 to 0x184D2A5F.
All 16 values are valid to identify a skippable frame.

__Frame Size__

This is the size, in bytes, of the following User Data
(without including the magic number nor the size field itself).
4 Bytes, Little endian format, unsigned 32-bits.
This means User Data can’t be bigger than (2^32-1) Bytes.

__User Data__

User Data can be anything. Data will just be skipped by the decoder.


Version changes
---------------

0.1 : initial release