Copyright (c) 2016 Yann Collet
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0.1.0 (30/06/2016 - unfinished)
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.
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, 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 parameter defined in the compressed stream, it must produce a non-ambiguous error code and associated error message explaining which parameter is unsupported.
A content compressed by Zstandard is transformed into a Zstandard frame. Multiple frames can be appended into a single file or stream. A frame is totally independent, has a defined beginning and end, and a set of parameters which tells the decoder how to decompress it.
A frame encapsulates one or multiple blocks. Each block can be compressed or not, and has a guaranteed maximum content size, which depends on frame parameters. Unlike frames, each block depends on previous blocks for proper decoding. However, each block can be decompressed without waiting for its successor, allowing streaming operations.
| MagicNb | F. Header | Block | (More blocks) | 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.
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 frame content has been regenrated correctly. The content checksum is the result of xxh64() hash function 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.
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, delivering the final decompressed result as if it was a single content.
| FHD | (WD) | (Content Size) | (dictID) |
|---|---|---|---|
| 1 byte | 0-1 byte | 0 - 8 bytes | 0-4 bytes |
Frame header has a variable size, which uses a minimum of 2 bytes, and up to 14 bytes depending on optional parameters.
FHD byte (Frame Header Descriptor)
The first Header's byte is called the Frame Header Descriptor. It tells which other fields are present. Decoding this byte is enough to get the full size of the Frame Header.
| BitNb | 7-6 | 5 | 4 | 3 | 2 | 1-0 |
|---|---|---|---|---|---|---|
| FieldName | FCSize | Segment | Unused | Reserved | Checksum | dictID |
In this table, bit 7 is highest bit, while bit 0 is lowest.
Frame Content Size flag
This is a 2-bits flag (= FHD >> 6), specifying if decompressed data size is provided within the header.
| Value | 0 | 1 | 2 | 3 |
|---|---|---|---|---|
| FieldSize | 0-1 | 2 | 4 | 8 |
Value 0 has a double meaning : it either means 0 (size not provided) if the WD byte is present, or it means 1 byte (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.
As a consequence, the decoder must allocate a memory segment of size >= Frame Content Size.
In order to preserve the decoder from unreasonable memory requirement, a decoder can refuse a compressed frame which requests a memory size beyond decoder's authorized range.
For broader compatibility, decoders are recommended to support memory sizes of 8 MB at least. However, this is merely a recommendation, and each decoder is free to support higher or lower limits, depending on local limitations.
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 present within compressed data. This information is useful for decoders 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;
The minimum window size is 1 KB. The maximum size is (15*(2^38))-1 bytes, which is almost 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, 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, decoders are free to support larger or lower limits, depending on local limitations.
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 allowed to represent a small size (ex: 18) using the 8-bytes variant. A size of 0 means content size is unknown. In which case, the WD byte will necessarily be present, and becomes 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 allowed to represent a small ID (for example 13) with a large 4-bytes dictionary ID, losing some efficiency in the process.
| 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 (compressed) block size.
There are 4 block types :
| Value | 0 | 1 | 2 | 3 |
|---|---|---|---|---|
| Block Type | Compressed | Raw | RLE | EndMark |
Block sizes must respect a few rules :
< contentSize.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” : since an arbitrary “flush” may happen anytime, block content can be any size, up to Block Maximum Size. Block Maximum Size is the smallest of :
| 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.
This specification details the content of a compressed block. A compressed block has a size, which must be known in order to decode it. It also has a guaranteed maximum regenerated size, in order to properly allocate destination buffer. See “Frame format” for more details.
A compressed block consists of 2 sections :
Literals are compressed using order-0 huffman compression. During sequence phase, literals will be entangled with match copy operations. All literals are regrouped in the first part of the block. They can be decoded first, and then copied during sequence operations, or they can be decoded on the flow, as needed by sequences.
| Header | (Tree Description) | Stream1 | (Stream2) | (Stream3) | (Stream4) |
|---|
Literals can be compressed, or uncompressed. When compressed, an optional tree description can be present, followed by 1 or 4 streams.
Header is in charge of describing precisely how literals are packed. It's a byte-aligned variable-size bitfield, ranging from 1 to 5 bytes, using big-endian convention.
| BlockType | sizes format | (compressed size) | regenerated size |
|---|---|---|---|
| 2 bits | 1 - 2 bits | 0 - 18 bits | 5 - 20 bits |
Block Type :
This is a 2-bits field, describing 4 different block types :
| Value | 0 | 1 | 2 | 3 |
|---|---|---|---|---|
| Block Type | Compressed | Repeat | Raw | RLE |
Sizes format :
Sizes format are divided into 2 families :
For values spanning several bytes, convention is Big-endian.
Sizes format for Raw or RLE block :
size = h[0] & 31;size = ((h[0] & 15) << 8) + h[1];size = ((h[0] & 15) << 16) + (h[1]<<8) + h[2];Note : it's allowed to represent a short value (ex : 13) using a long format, accepting the reduced compacity.
Sizes format for Compressed Block :
Note : also applicable to “repeat-stats” blocks.
0.1 : initial release