Zstandard Compression Format

Notices

Copyright (c) 2016-present Yann Collet, Facebook, Inc.

Permission is granted to copy and distribute this document for any purpose and without charge, including translations into other languages and incorporation into compilations, provided that the copyright notice and this notice are preserved, and that any substantive changes or deletions from the original are clearly marked. Distribution of this document is unlimited.

Version

0.2.5 (31/03/17)

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.

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.

Overall conventions

In this document:

  • square brackets i.e. [ and ] are used to indicate optional fields or parameters.
  • the naming convention for identifiers is Mixed_Case_With_Underscores

Definitions

Content compressed by Zstandard is transformed into a Zstandard frame. Multiple frames can be appended into a single file or stream. A frame is completely 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.

Overview

Frames

Zstandard compressed data is made of up one or more frames. Each frame is independent and can be decompressed indepedently of other frames. The decompressed content of multiple concatenated frames is the concatenation of each frames decompressed content.

There are two frame formats defined by Zstandard: Zstandard frames and Skippable frames. Zstandard frames contain compressed data, while skippable frames contain no data and can be used for metadata.

Zstandard frames

The structure of a single Zstandard frame is following:

Magic_NumberFrame_HeaderData_Block[More data blocks][Content_Checksum]
4 bytes2-14 bytesn bytes0-4 bytes

Magic_Number

4 Bytes, little-endian format. Value : 0xFD2FB528

Frame_Header

2 to 14 Bytes, detailed in Frame_Header.

Data_Block

Detailed in Blocks. That’s where compressed data is stored.

Content_Checksum

An optional 32-bit checksum, only present if Content_Checksum_flag is set. The content checksum is the result of xxh64() hash function digesting the original (decoded) data as input, and a seed of zero. The low 4 bytes of the checksum are stored in little-endian format.

Frame_Header

The Frame_Header has a variable size, with a minimum of 2 bytes, and up to 14 bytes depending on optional parameters. The structure of Frame_Header is following:

Frame_Header_Descriptor[Window_Descriptor][Dictionary_ID][Frame_Content_Size]
1 byte0-1 byte0-4 bytes0-8 bytes

Frame_Header_Descriptor

The first header's byte is called the Frame_Header_Descriptor. It describes which other fields are present. Decoding this byte is enough to tell the size of Frame_Header.

Bit numberField name
7-6Frame_Content_Size_flag
5Single_Segment_flag
4Unused_bit
3Reserved_bit
2Content_Checksum_flag
1-0Dictionary_ID_flag

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

Frame_Content_Size_flag

This is a 2-bits flag (= Frame_Header_Descriptor >> 6), specifying if Frame_Content_Size (the decompressed data size) is provided within the header. Flag_Value provides FCS_Field_Size, which is the number of bytes used by Frame_Content_Size according to the following table:

Flag_Value0123
FCS_Field_Size0 or 1248

When Flag_Value is 0, FCS_Field_Size depends on Single_Segment_flag : if Single_Segment_flag is set, Field_Size is 1. Otherwise, Field_Size is 0 : Frame_Content_Size is not provided.

Single_Segment_flag

If this flag is set, data must be regenerated within a single continuous memory segment.

In this case, Window_Descriptor byte is skipped, but Frame_Content_Size is necessarily present. As a consequence, the decoder must allocate a memory segment of size equal or bigger than Frame_Content_Size.

In order to preserve the decoder from unreasonable memory requirements, a decoder is allowed to reject a compressed frame which requests a memory size beyond decoder's authorized range.

For broader compatibility, decoders are recommended to support memory sizes of at least 8 MB. This is only a recommendation, each decoder is free to support higher or lower limits, depending on local limitations.

Unused_bit

The value of this bit should be set to zero. A decoder compliant with this specification version shall not interpret it. It might be used in a future version, to signal a property which is not mandatory 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 to decode the frame correctly.

Content_Checksum_flag

If this flag is set, a 32-bits Content_Checksum will be present at frame's end. See Content_Checksum paragraph.

Dictionary_ID_flag

This is a 2-bits flag (= FHD & 3), telling if a dictionary ID is provided within the header. It also specifies the size of this field as Field_Size.

Flag_Value0123
Field_Size0124

Window_Descriptor

Provides guarantees on minimum memory buffer required to decompress a frame. This information is important for decoders to allocate enough memory.

The Window_Descriptor byte is optional. When Single_Segment_flag is set, Window_Descriptor is not present. In this case, Window_Size is Frame_Content_Size, which can be any value from 0 to 2^64-1 bytes (16 ExaBytes).

Bit numbers7-32-0
Field nameExponentMantissa

The minimum memory buffer size is called Window_Size. It is described by the following formulas :

windowLog = 10 + Exponent;
windowBase = 1 << windowLog;
windowAdd = (windowBase / 8) * Mantissa;
Window_Size = windowBase + windowAdd;

The minimum Window_Size is 1 KB. The maximum Window_Size is (1<<41) + 7*(1<<38) bytes, which is 3.75 TB.

To properly decode compressed data, a decoder will need to allocate a buffer of at least Window_Size bytes.

In order to preserve decoder from unreasonable memory requirements, a decoder is allowed to reject a compressed frame which requests a memory size beyond decoder's authorized range.

For improved interoperability, decoders are recommended to be compatible with Window_Size >= 8 MB, and 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.

Dictionary_ID

This is a variable size field, which contains the ID of the dictionary required to properly decode the frame. Dictionary_ID field is optional. When it's not present, it's up to the decoder 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 0-4294967295. Format is little-endian.

It's allowed to represent a small ID (for example 13) with a large 4-bytes dictionary ID, even if it is less efficient.

Reserved ranges : If the frame is going to be distributed in a private environment, any dictionary ID can be used. However, for public distribution of compressed frames using a dictionary, the following ranges are reserved and shall not be used :

  • low range : <= 32767
  • high range : >= (1 << 31)

Frame_Content_Size

This is the original (uncompressed) size. This information is optional. Frame_Content_Size uses a variable number of bytes, provided by FCS_Field_Size. FCS_Field_Size is provided by the value of Frame_Content_Size_flag. FCS_Field_Size can be equal to 0 (not present), 1, 2, 4 or 8 bytes.

FCS_Field_SizeRange
0unknown
10 - 255
2256 - 65791
40 - 2^32-1
80 - 2^64-1

Frame_Content_Size format is little-endian. When FCS_Field_Size is 1, 4 or 8 bytes, the value is read directly. When FCS_Field_Size is 2, the offset of 256 is added. It's allowed to represent a small size (for example 18) using any compatible variant.

Blocks

After Magic_Number and Frame_Header, there are some number of blocks. Each frame must have at least one block, but there is no upper limit on the number of blocks per frame.

The structure of a block is as follows:

Block_HeaderBlock_Content
3 bytesn bytes

Block_Header uses 3 bytes, written using little-endian convention. It contains 3 fields :

Last_BlockBlock_TypeBlock_Size
bit 0bits 1-2bits 3-23

Last_Block

The lowest bit signals if this block is the last one. The frame will end after this last block. It may be followed by an optional Content_Checksum (see Zstandard Frames).

Block_Type

The next 2 bits represent the Block_Type. There are 4 block types :

Value0123
Block_TypeRaw_BlockRLE_BlockCompressed_BlockReserved

  • Raw_Block - this is an uncompressed block. Block_Content contains Block_Size bytes.

  • RLE_Block - this is a single byte, repeated Block_Size times. Block_Content consists of a single byte. On the decompression side, this byte must be repeated Block_Size times.

  • Compressed_Block - this is a Zstandard compressed block, explained later on. Block_Size is the length of Block_Content, the compressed data. The decompressed size is not known, but its maximum possible value is guaranteed (see below)

  • Reserved - this is not a block. This value cannot be used with current version of this specification.

Block_Size

The upper 21 bits of Block_Header represent the Block_Size.

Block sizes must respect a few rules :

  • For Compressed_Block, Block_Size is always strictly less than decompressed size.
  • Block decompressed size is always <= Window_Size
  • Block decompressed size is always <= 128 KB.

A block can contain any number of bytes (even empty), up to Block_Maximum_Decompressed_Size, which is the smallest of :

  • Window_Size
  • 128 KB

Compressed Blocks

To decompress a compressed block, the compressed size must be provided from Block_Size field within Block_Header.

A compressed block consists of 2 sections :

The results of the two sections are then combined to produce the decompressed data in Sequence Execution

Prerequisites

To decode a compressed block, the following elements are necessary :

  • Previous decoded data, up to a distance of Window_Size, or all previously decoded data when Single_Segment_flag is set.
  • List of "recent offsets" from previous Compressed_Block.
  • Decoding tables of previous Compressed_Block for each symbol type (literals, literals lengths, match lengths, offsets).

Literals Section

All literals are regrouped in the first part of the block. They can be decoded first, and then copied during [Sequence Execution], or they can be decoded on the flow during [Sequence Execution].

Literals can be stored uncompressed or compressed using Huffman prefix codes. When compressed, an optional tree description can be present, followed by 1 or 4 streams.

Literals_Section_Header[Huffman_Tree_Description]Stream1[Stream2][Stream3][Stream4]

Literals_Section_Header

Header is in charge of describing how literals are packed. It's a byte-aligned variable-size bitfield, ranging from 1 to 5 bytes, using little-endian convention.

Literals_Block_TypeSize_FormatRegenerated_Size[Compressed_Size]
2 bits1 - 2 bits5 - 20 bits0 - 18 bits

In this representation, bits on the left are the lowest bits.

Literals_Block_Type

This field uses 2 lowest bits of first byte, describing 4 different block types :

Literals_Block_TypeValue
Raw_Literals_Block0
RLE_Literals_Block1
Compressed_Literals_Block2
Treeless_Literals_Block3
  • Raw_Literals_Block - Literals are stored uncompressed.
  • RLE_Literals_Block - Literals consist of a single byte value repeated Regenerated_Size times.
  • Compressed_Literals_Block - This is a standard Huffman-compressed block, starting with a Huffman tree description. See details below.
  • Treeless_Literals_Block - This is a Huffman-compressed block, using Huffman tree from previous Huffman-compressed literals block. Huffman_Tree_Description will be skipped. Note: If this mode is triggered without any previous Huffman-table in the frame (or dictionary), this should be treated as data corruption.

Size_Format

Size_Format is divided into 2 families :

  • For Raw_Literals_Block and RLE_Literals_Block, it's only necessary to decode Regenerated_Size. There is no Compressed_Size field.
  • For Compressed_Block and Treeless_Literals_Block, it's required to decode both Compressed_Size and Regenerated_Size (the decompressed size). It's also necessary to decode the number of streams (1 or 4).

For values spanning several bytes, convention is little-endian.

Size_Format for Raw_Literals_Block and RLE_Literals_Block :

  • Value ?0 : Size_Format uses 1 bit. Regenerated_Size uses 5 bits (0-31). Literals_Section_Header has 1 byte. Regenerated_Size = Header[0]>>3
  • Value 01 : Size_Format uses 2 bits. Regenerated_Size uses 12 bits (0-4095). Literals_Section_Header has 2 bytes. Regenerated_Size = (Header[0]>>4) + (Header[1]<<4)
  • Value 11 : Size_Format uses 2 bits. Regenerated_Size uses 20 bits (0-1048575). Literals_Section_Header has 3 bytes. Regenerated_Size = (Header[0]>>4) + (Header[1]<<4) + (Header[2]<<12)

Only Stream1 is present for these cases. Note : it's allowed to represent a short value (for example 13) using a long format, even if it's less efficient.

Size_Format for Compressed_Literals_Block and Treeless_Literals_Block :

  • Value 00 : A single stream. Both Regenerated_Size and Compressed_Size use 10 bits (0-1023). Literals_Section_Header has 3 bytes.
  • Value 01 : 4 streams. Both Regenerated_Size and Compressed_Size use 10 bits (0-1023). Literals_Section_Header has 3 bytes.
  • Value 10 : 4 streams. Both Regenerated_Size and Compressed_Size use 14 bits (0-16383). Literals_Section_Header has 4 bytes.
  • Value 11 : 4 streams. Both Regenerated_Size and Compressed_Size use 18 bits (0-262143). Literals_Section_Header has 5 bytes.

Both Compressed_Size and Regenerated_Size fields follow little-endian convention. Note: Compressed_Size includes the size of the Huffman Tree description when it is present.

Raw Literals Block

The data in Stream1 is Regenerated_Size bytes long, it contains the raw literals data to be used during [Sequence Execution].

RLE Literals Block

Stream1 consists of a single byte which should be repeated Regenerated_Size times to generate the decoded literals.

Compressed Literals Block and Treeless Literals Block

Both of these modes contain Huffman encoded data. Treeless_Literals_Block does not have a Huffman_Tree_Description.

Huffman_Tree_Description

This section is only present when Literals_Block_Type type is Compressed_Literals_Block (2). The format of the Huffman tree description can be found at Huffman Tree description. The size of Huffman_Tree_Description is determined during decoding process, it must be used to determine where streams begin. Total_Streams_Size = Compressed_Size - Huffman_Tree_Description_Size.

For Treeless_Literals_Block, the Huffman table comes from previously compressed literals block.

Huffman compressed data consists of either 1 or 4 Huffman-coded streams.

If only one stream is present, it is a single bitstream occupying the entire remaining portion of the literals block, encoded as described within Huffman-Coded Streams.

If there are four streams, the literals section header only provides enough information to know the decompressed and compressed sizes of all four streams combined. The decompressed size of each stream is equal to (Regenerated_Size+3)/4, except for the last stream which may be up to 3 bytes smaller, to reach a total decompressed size as specified in Regenerated_Size.

The compressed size of each stream is provided explicitly: the first 6 bytes of the compressed data consist of three 2-byte little-endian fields, describing the compressed sizes of the first three streams. Stream4_Size is computed from total Total_Streams_Size minus sizes of other streams.

Stream4_Size = Total_Streams_Size - 6 - Stream1_Size - Stream2_Size - Stream3_Size.

Note: remember that Total_Streams_Size can be smaller than Compressed_Size in header, because Compressed_Size also contains Huffman_Tree_Description_Size when it is present.

Each of these 4 bitstreams is then decoded independently as a Huffman-Coded stream, as described at Huffman-Coded Streams

Sequences Section

A compressed block is a succession of sequences . A sequence is a literal copy command, followed by a match copy command. A literal copy command specifies a length. It is the number of bytes to be copied (or extracted) from the Literals Section. A match copy command specifies an offset and a length.

When all sequences are decoded, if there are literals left in the literal section, these bytes are added at the end of the block.

This is described in more detail in Sequence Execution

The Sequences_Section regroup all symbols required to decode commands. There are 3 symbol types : literals lengths, offsets and match lengths. They are encoded together, interleaved, in a single bitstream.

The Sequences_Section starts by a header, followed by optional probability tables for each symbol type, followed by the bitstream.

Sequences_Section_Header[Literals_Length_Table][Offset_Table][Match_Length_Table]bitStream

To decode the Sequences_Section, it's required to know its size. This size is deduced from Block_Size - Literals_Section_Size.

Sequences_Section_Header

Consists of 2 items:

  • Number_of_Sequences
  • Symbol compression modes

Number_of_Sequences

This is a variable size field using between 1 and 3 bytes. Let's call its first byte byte0.

  • if (byte0 == 0) : there are no sequences. The sequence section stops there. Decompressed content is defined entirely as Literals Section content.
  • if (byte0 < 128) : Number_of_Sequences = byte0 . Uses 1 byte.
  • if (byte0 < 255) : Number_of_Sequences = ((byte0-128) << 8) + byte1 . Uses 2 bytes.
  • if (byte0 == 255): Number_of_Sequences = byte1 + (byte2<<8) + 0x7F00 . Uses 3 bytes.

Symbol compression modes

This is a single byte, defining the compression mode of each symbol type.

Bit number7-65-43-21-0
Field nameLiterals_Lengths_ModeOffsets_ModeMatch_Lengths_ModeReserved

The last field, Reserved, must be all-zeroes.

Literals_Lengths_Mode, Offsets_Mode and Match_Lengths_Mode define the Compression_Mode of literals lengths, offsets, and match lengths symbols respectively.

They follow the same enumeration :

Value0123
Compression_ModePredefined_ModeRLE_ModeFSE_Compressed_ModeRepeat_Mode
  • Predefined_Mode : A predefined FSE distribution table is used, defined in default distributions. No distribution table will be present.
  • RLE_Mode : The table description consists of a single byte. This code will be repeated for all sequences.
  • Repeat_Mode : The table used in the previous compressed block will be used again. No distribution table will be present. Note: this includes RLE mode, so if Repeat_Mode follows RLE_Mode, the same symbol will be repeated. If this mode is used without any previous sequence table in the frame (or dictionary) to repeat, this should be treated as corruption.
  • FSE_Compressed_Mode : standard FSE compression. A distribution table will be present. The format of this distribution table is described in FSE Table Description. Note that the maximum allowed accuracy log for literals length and match length tables is 9, and the maximum accuracy log for the offsets table is 8.

The codes for literals lengths, match lengths, and offsets.

Each symbol is a code in its own context, which specifies Baseline and Number_of_Bits to add. Codes are FSE compressed, and interleaved with raw additional bits in the same bitstream.

Literals length codes

Literals length codes are values ranging from 0 to 35 included. They define lengths from 0 to 131071 bytes. The literals length is equal to the decoded Baseline plus the result of reading Number_of_Bits bits from the bitstream, as a little-endian value.

Literals_Length_Code0-15
lengthLiterals_Length_Code
Number_of_Bits0
Literals_Length_Code1617181920212223
Baseline1618202224283240
Number_of_Bits11112233
Literals_Length_Code2425262728293031
Baseline4864128256512102420484096
Number_of_Bits46789101112
Literals_Length_Code32333435
Baseline8192163843276865536
Number_of_Bits13141516
Match length codes

Match length codes are values ranging from 0 to 52 included. They define lengths from 3 to 131074 bytes. The match length is equal to the decoded Baseline plus the result of reading Number_of_Bits bits from the bitstream, as a little-endian value.

Match_Length_Code0-31
valueMatch_Length_Code + 3
Number_of_Bits0
Match_Length_Code3233343536373839
Baseline3537394143475159
Number_of_Bits11112233
Match_Length_Code4041424344454647
Baseline67839913125951510272051
Number_of_Bits4457891011
Match_Length_Code4849505152
Baseline40998195163873277165539
Number_of_Bits1213141516
Offset codes

Offset codes are values ranging from 0 to N.

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.

An offset code is also the number of additional bits to read in little-endian fashion, and can be translated into an Offset_Value using the following formulas :

Offset_Value = (1 << offsetCode) + readNBits(offsetCode);
if (Offset_Value > 3) offset = Offset_Value - 3;

It means that maximum Offset_Value is (2^(N+1))-1 and it supports back-reference distance up to (2^(N+1))-4 but is limited by maximum back-reference distance.

Offset_Value from 1 to 3 are special : they define "repeat codes". This is described in more detail in Repeat Offsets.

Decoding Sequences

FSE bitstreams are read in reverse direction than written. In zstd, the compressor writes bits forward into a block and the decompressor must read the bitstream backwards.

To find the start of the bitstream it is therefore necessary to know the offset of the last byte of the block which can be found by counting Block_Size bytes after the block header.

After writing the last bit containing information, the compressor writes a single 1-bit and then fills the byte with 0-7 0 bits of padding. The last byte of the compressed bitstream cannot be 0 for that reason.

When decompressing, the last byte containing the padding is the first byte to read. The decompressor needs to skip 0-7 initial 0-bits and the first 1-bit it occurs. Afterwards, the useful part of the bitstream begins.

FSE decoding requires a 'state' to be carried from symbol to symbol. For more explanation on FSE decoding, see the FSE section.

For sequence decoding, a separate state keeps track of each literal lengths, offsets, and match lengths symbols. Some FSE primitives are also used. For more details on the operation of these primitives, see the FSE section.

Starting states

The bitstream starts with initial FSE state values, each using the required number of bits in their respective accuracy, decoded previously from their normalized distribution.

It starts by Literals_Length_State, followed by Offset_State, and finally Match_Length_State.

Reminder : always keep in mind that all values are read backward, so the 'start' of the bitstream is at the highest position in memory, immediately before the last 1-bit for padding.

After decoding the starting states, a single sequence is decoded Number_Of_Sequences times. These sequences are decoded in order from first to last. Since the compressor writes the bitstream in the forward direction, this means the compressor must encode the sequences starting with the last one and ending with the first.

Decoding a sequence

For each of the symbol types, the FSE state can be used to determine the appropriate code. The code then defines the baseline and number of bits to read for each type. See the description of the codes for how to determine these values.

Decoding starts by reading the Number_of_Bits required to decode Offset. It then does the same for Match_Length, and then for Literals_Length. This sequence is then used for sequence execution.

If it is not the last sequence in the block, the next operation is to update states. Using the rules pre-calculated in the decoding tables, Literals_Length_State is updated, followed by Match_Length_State, and then Offset_State. See the FSE section for details on how to update states from the bitstream.

This operation will be repeated Number_of_Sequences times. At the end, the bitstream shall be entirely consumed, otherwise the bitstream is considered corrupted.

Default Distributions

If Predefined_Mode is selected for a symbol type, its FSE decoding table is generated from a predefined distribution table defined here. For details on how to convert this distribution into a decoding table, see the FSE section.

Literals Length

The decoding table uses an accuracy log of 6 bits (64 states).

short literalsLength_defaultDistribution[36] =
        { 4, 3, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 1, 1, 1,
          2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 2, 1, 1, 1, 1, 1,
         -1,-1,-1,-1 };
Match Length

The decoding table uses an accuracy log of 6 bits (64 states).

short matchLengths_defaultDistribution[53] =
        { 1, 4, 3, 2, 2, 2, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1,
          1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
          1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,-1,-1,
         -1,-1,-1,-1,-1 };
Offset Codes

The decoding table uses an accuracy log of 5 bits (32 states), and supports a maximum N value of 28, allowing offset values up to 536,870,908 .

If any sequence in the compressed block requires a larger offset than this, it's not possible to use the default distribution to represent it.

short offsetCodes_defaultDistribution[29] =
        { 1, 1, 1, 1, 1, 1, 2, 2, 2, 1, 1, 1, 1, 1, 1, 1,
          1, 1, 1, 1, 1, 1, 1, 1,-1,-1,-1,-1,-1 };

Sequence Execution

Once literals and sequences have been decoded, they are combined to produce the decoded content of a block.

Each sequence consists of a tuple of (literals_length, offset_value, match_length), decoded as described in the Sequences Section. To execute a sequence, first copy literals_length bytes from the literals section to the output.

Then match_length bytes are copied from previous decoded data. The offset to copy from is determined by offset_value: if offset_value > 3, then the offset is offset_value - 3. If offset_value is from 1-3, the offset is a special repeat offset value. See the repeat offset section for how the offset is determined in this case.

The offset is defined as from the current position, so an offset of 6 and a match length of 3 means that 3 bytes should be copied from 6 bytes back. Note that all offsets leading to previously decoded data must be smaller than Window_Size defined in Frame_Header_Descriptor.

Repeat offsets

As seen in Sequence Execution, the first 3 values define a repeated offset and we will call them Repeated_Offset1, Repeated_Offset2, and Repeated_Offset3. They are sorted in recency order, with Repeated_Offset1 meaning "most recent one".

If offset_value == 1, then the offset used is Repeated_Offset1, etc.

There is an exception though, when current sequence's literals_length = 0. In this case, repeated offsets are shifted by one, so an offset_value of 1 means Repeated_Offset2, an offset_value of 2 means Repeated_Offset3, and an offset_value of 3 means Repeated_Offset1 - 1_byte.

For the first block, the starting offset history is populated with the following values : 1, 4 and 8 (in order).

Then each block gets its starting offset history from the ending values of the most recent Compressed_Block. Note that blocks which are not Compressed_Block are skipped, they do not contribute to offset history.

Offset updates rules

The newest offset takes the lead in offset history, shifting others back (up to its previous place if it was already present).

This means that when Repeated_Offset1 (most recent) is used, history is unmodified. When Repeated_Offset2 is used, it's swapped with Repeated_Offset1. If any other offset is used, it becomes Repeated_Offset1 and the rest are shift back by one.

Skippable Frames

Magic_NumberFrame_SizeUser_Data
4 bytes4 bytesn 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 : 0x184D2A5?, 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). This field is represented using 4 Bytes, little-endian format, unsigned 32-bits. This means User_Data can’t be bigger than (2^32-1) bytes.

User_Data

The User_Data can be anything. Data will just be skipped by the decoder.

Entropy Encoding

Two types of entropy encoding are used by the Zstandard format: FSE, and Huffman coding.

FSE

FSE, short for Finite State Entropy, is an entropy codec based on ANS. FSE encoding/decoding involves a state that is carried over between symbols, so decoding must be done in the opposite direction as encoding. Therefore, all FSE bitstreams are read from end to beginning.

For additional details on FSE, see Finite State Entropy.

FSE decoding involves a decoding table which has a power of 2 size, and contain three elements: Symbol, Num_Bits, and Baseline. The log2 of the table size is its Accuracy_Log. The FSE state represents an index in this table.

To obtain the initial state value, consume Accuracy_Log bits from the stream as a little-endian value. The next symbol in the stream is the Symbol indicated in the table for that state. To obtain the next state value, the decoder should consume Num_Bits bits from the stream as a little-endian value and add it to Baseline.

FSE Table Description

To decode FSE streams, it is necessary to construct the decoding table. The Zstandard format encodes FSE table descriptions as follows:

An FSE distribution table describes the probabilities of all symbols from 0 to the last present one (included) on a normalized scale of 1 << Accuracy_Log .

It's a bitstream which is read forward, in little-endian fashion. It's not necessary to know its exact size, since it will be discovered and reported by the decoding process.

The bitstream starts by reporting on which scale it operates. Accuracy_Log = low4bits + 5.

Then follows each symbol value, from 0 to last present one. The number of bits used by each field is variable. It depends on :

  • Remaining probabilities + 1 : example : Presuming an Accuracy_Log of 8, and presuming 100 probabilities points have already been distributed, the decoder may read any value from 0 to 255 - 100 + 1 == 156 (inclusive). Therefore, it must read log2sup(156) == 8 bits.

  • Value decoded : small values use 1 less bit : example : Presuming values from 0 to 156 (inclusive) are possible, 255-156 = 99 values are remaining in an 8-bits field. They are used this way : first 99 values (hence from 0 to 98) use only 7 bits, values from 99 to 156 use 8 bits. This is achieved through this scheme :

    Value readValue decodedNumber of bits used
    0 - 980 - 987
    99 - 12799 - 1278
    128 - 2260 - 987
    227 - 255128 - 1568

Symbols probabilities are read one by one, in order.

Probability is obtained from Value decoded by following formula : Proba = value - 1

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 the next section. For the purpose of calculating total allocated probability points, it counts as one.

When a symbol has a probability of zero, it is followed by a 2-bits repeat flag. This repeat flag tells how many probabilities of zeroes follow the current one. It provides a number ranging from 0 to 3. If it is a 3, another 2-bits repeat flag follows, and so on.

When last symbol reaches cumulated total of 1 << Accuracy_Log, decoding is complete. If the last symbol makes cumulated total go above 1 << Accuracy_Log, distribution is considered corrupted.

Then the decoder can tell how many bytes were used in this process, and how many symbols are present. The bitstream consumes a round number of bytes. Any remaining bit within the last byte is just unused.

From normalized distribution to decoding tables

The distribution of normalized probabilities is enough to create a unique decoding table.

It follows the following build rule :

The table has a size of Table_Size = 1 << Accuracy_Log. Each cell describes the symbol decoded, and instructions to get the next state.

Symbols are scanned in their natural order for "less than 1" probabilities. Symbols with this probability are being attributed a single cell, starting from the end of the table. These symbols define a full state reset, reading Accuracy_Log bits.

All remaining symbols are sorted in their natural order. Starting from symbol 0 and table position 0, 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;

A position is skipped if already occupied by a "less than 1" probability symbol. position does not reset between symbols, it simply iterates through each position in the table, switching to the next symbol when enough states have been allocated to the current one.

The result is a list of state values. 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 more bit than higher ones.

Example : Presuming a symbol has a probability of 5. 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. Presuming the Accuracy_Log is 7, it defines 128 states. Divided by 8, each share is 16 large.

In order to reach 8, 8-5=3 lowest states will count "double", taking shares twice larger, requiring one more bit in the process.

Numbering starts from higher states using less bits.

state order01234
width3232321616
Number_of_Bits55544
range number24601
Baseline326496016
range32-6364-9596-1270-1516-31

The next state is determined from current state by reading the required Number_of_Bits, and adding the specified Baseline.

See Appendix A for the results of this process applied to the default distributions.

Huffman Coding

Zstandard Huffman-coded streams are read backwards, similar to the FSE bitstreams. Therefore, to find the start of the bitstream, it is therefore to know the offset of the last byte of the Huffman-coded stream.

After writing the last bit containing information, the compressor writes a single 1-bit and then fills the byte with 0-7 0 bits of padding. The last byte of the compressed bitstream cannot be 0 for that reason.

When decompressing, the last byte containing the padding is the first byte to read. The decompressor needs to skip 0-7 initial 0-bits and the first 1-bit it occurs. Afterwards, the useful part of the bitstream begins.

The bitstream contains Huffman-coded symbols in little-endian order, with the codes defined by the method below.

Huffman Tree Description

Prefix coding represents symbols from an a priori known alphabet by bit sequences (codewords), one codeword for each symbol, in a manner such that different symbols may be represented by bit sequences of different lengths, but a parser can always parse an encoded string unambiguously symbol-by-symbol.

Given an alphabet with known symbol frequencies, the Huffman algorithm allows the construction of an optimal prefix code using the fewest bits of any possible prefix codes for that alphabet.

Prefix code must not exceed a maximum code length. More bits improve accuracy but cost more header size, and require more memory or more complex decoding operations. This specification limits maximum code length to 11 bits.

Representation

All literal values from zero (included) to last present one (excluded) are represented by Weight with values from 0 to Max_Number_of_Bits. Transformation from Weight to Number_of_Bits follows this formula :

Number_of_Bits = Weight ? (Max_Number_of_Bits + 1 - Weight) : 0

The last symbol's Weight is deduced from previously decoded ones, by completing to the nearest power of 2. This power of 2 gives Max_Number_of_Bits, the depth of the current tree.

Example : Let's presume the following Huffman tree must be described :

literal012345
Number_of_Bits123044

The tree depth is 4, since its smallest element uses 4 bits. Value 5 will not be listed as it can be determined from the values for 0-4, nor will values above 5 as they are all 0. Values from 0 to 4 will be listed using Weight instead of Number_of_Bits. Weight formula is :

Weight = Number_of_Bits ? (Max_Number_of_Bits + 1 - Number_of_Bits) : 0

It gives the following series of weights :

literal01234
Weight43201

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 determined by advancing to the next power of 2. The sum of 2^(Weight-1) (excluding 0's) is : 8 + 4 + 2 + 0 + 1 = 15. Nearest power of 2 is 16. Therefore, Max_Number_of_Bits = 4 and Weight[5] = 1.

Huffman Tree header

This is a single byte value (0-255), which describes how to decode the list of weights.

  • if headerByte >= 128 : this is a direct representation, where each Weight is written directly as a 4 bits field (0-15). They are encoded forward, 2 weights to a byte with the first weight taking the top four bits and the second taking the bottom four (e.g. the following operations could be used to read the weights: Weight[0] = (Byte[0] >> 4), Weight[1] = (Byte[0] & 0xf), etc.). The full representation occupies ((Number_of_Symbols+1)/2) bytes, meaning it uses a last full byte even if Number_of_Symbols is odd. Number_of_Symbols = headerByte - 127. Note that maximum Number_of_Symbols is 255-127 = 128. A larger series must necessarily use FSE compression.

  • if headerByte < 128 : the series of weights is compressed by FSE. The length of the FSE-compressed series is equal to headerByte (0-127).

Finite State Entropy (FSE) compression of Huffman weights

In this case, the series of Huffman weights is compressed using FSE compression. It's a single bitstream with 2 interleaved states, sharing a single distribution table.

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 possible decompressed size, which is 255, since literal values span from 0 to 255, and last symbol's weight is not represented.

An FSE bitstream starts by a header, describing probabilities distribution. It will create a Decoding Table. For a list of Huffman weights, the maximum accuracy log is 7 bits. For more description see the FSE header description

The Huffman header compression uses 2 states, which share the same FSE distribution table. The first state (State1) encodes the even indexed symbols, and the second (State2) encodes the odd indexes. State1 is initialized first, and then State2, and they take turns decoding a single symbol and updating their state. For more details on these FSE operations, see the FSE section.

The number of symbols to decode is determined by tracking bitStream overflow condition: If updating state after decoding a symbol would require more bits than remain in the stream, it is assumed that extra bits are 0. Then, the symbols for each of the final states are decoded and the process is complete.

Conversion from weights to Huffman prefix codes

All present symbols shall now have a Weight value. It is possible to transform weights into Number_of_Bits, using this formula:

Number_of_Bits = Number_of_Bits ? Max_Number_of_Bits + 1 - Weight : 0

Symbols are sorted by Weight. Within same Weight, symbols keep natural order. Symbols with a Weight of zero are removed. Then, starting from lowest weight, prefix codes are distributed in order.

Example : Let's presume the following list of weights has been decoded :

Literal012345
Weight432011

Sorted by weight and then natural order, it gives the following distribution :

Literal345210
Weight011234
Number_of_Bits044321
prefix codesN/A00000001001011

Huffman-coded Streams

Given a Huffman decoding table, it's possible to decode a Huffman-coded stream.

Each bitstream must be read backward, that is starting from the end down to the beginning. Therefore it's necessary to know the size of each bitstream.

It's also necessary to know exactly which bit is the latest. This is detected by a final bit flag : the highest bit of latest byte is a final-bit-flag. Consequently, a last byte of 0 is not possible. And the final-bit-flag itself is not part of the useful bitstream. Hence, the last byte contains between 0 and 7 useful bits.

For example, if the literal sequence "0145" was encoded using the prefix codes above, it would be encoded as:

00000001 01110000
Symbol5410Padding
Encoding0000000101110000

Starting from the end, it's possible to read the bitstream in a little-endian fashion, keeping track of already used bits. Since the bitstream is encoded in reverse order, by starting at the end the symbols can be read in forward order.

Reading the last Max_Number_of_Bits bits, it's then possible to compare extracted value to decoding table, determining the symbol to decode and number of bits to discard.

The process continues up to reading the required number of symbols per stream. If a bitstream is not entirely and exactly consumed, hence reaching exactly its beginning position with all bits consumed, the decoding process is considered faulty.

Dictionary Format

Zstandard is compatible with "raw content" dictionaries, free of any format restriction, except that they must be at least 8 bytes. These dictionaries function as if they were just the Content part of a formatted dictionary.

But dictionaries created by zstd --train follow a format, described here.

Pre-requisites : a dictionary has a size, defined either by a buffer limit, or a file size.

Magic_NumberDictionary_IDEntropy_TablesContent

Magic_Number : 4 bytes ID, value 0xEC30A437, little-endian format

Dictionary_ID : 4 bytes, stored in little-endian format. Dictionary_ID can be any value, except 0 (which means no Dictionary_ID). It's used by decoders to check if they use the correct dictionary.

Reserved ranges : If the frame is going to be distributed in a private environment, any Dictionary_ID can be used. However, for public distribution of compressed frames, the following ranges are reserved and shall not be used :

          - low range  : <= 32767
          - high range : >= (2^31)

Entropy_Tables : following the same format as the tables in compressed blocks. See the relevant FSE and Huffman sections for how to decode these tables. They are stored in following order : Huffman tables for literals, FSE table for offsets, FSE table for match lengths, and FSE table for literals lengths. These tables populate the Repeat Stats literals mode and Repeat distribution mode for sequence decoding. It's finally followed by 3 offset values, populating recent offsets (instead of using {1,4,8}), stored in order, 4-bytes little-endian each, for a total of 12 bytes. Each recent offset must have a value < dictionary size.

Content : The rest of the dictionary is its content. The content act as a "past" in front of data to compress or decompress, so it can be referenced in sequence commands. As long as the amount of data decoded from this frame is less than or equal to Window_Size, sequence commands may specify offsets longer than the total length of decoded output so far to reference back to the dictionary. After the total output has surpassed Window_Size however, this is no longer allowed and the dictionary is no longer accessible.

Appendix A - Decoding tables for predefined codes

This appendix contains FSE decoding tables for the predefined literal length, match length, and offset codes. The tables have been constructed using the algorithm as given above in chapter "from normalized distribution to decoding tables". The tables here can be used as examples to crosscheck that an implementation build its decoding tables correctly.

Literal Length Code:

StateSymbolNumber_Of_BitsBase
0040
10416
21532
3350
4450
5650
6750
7950
81050
91250
101460
111650
121850
131950
142150
152250
162450
1725532
182650
192760
202960
213160
220432
23140
24250
254532
26550
277532
28850
2910532
301150
311360
3216532
331750
3419532
352050
3622532
372350
382540
3925416
4026532
412860
423060
430448
441416
452532
463532
475532
486532
498532
509532
5111532
5212532
531560
5417532
5518532
5620532
5721532
5823532
5924532
603560
613460
623360
633260

Match Length Code:

StateSymbolNumber_Of_BitsBase
0060
1140
22532
3350
4550
5650
6850
71060
81360
91660
101960
112260
122560
132860
143160
153360
163560
173760
183960
194160
204360
214560
221416
23240
243532
25450
266532
27750
28960
291260
301560
311860
322160
332460
342760
353060
363260
373460
383660
393860
404060
414260
424460
431432
441448
452416
464532
475532
487532
498532
501160
511460
521760
532060
542360
552660
562960
575260
585160
595060
604960
614860
624760
634660

Offset Code:

StateSymbolNumber_Of_BitsBase
0050
1640
2950
31550
42150
5350
6740
71250
81850
92350
10550
11840
121450
132050
14250
157416
161150
171750
182250
19450
208416
211350
221950
23150
246416
251050
261650
272850
282750
292650
302550
312450

Version changes

  • 0.2.5 : minor typos and clarifications
  • 0.2.4 : section restructuring, by Sean Purcell
  • 0.2.3 : clarified several details, by Sean Purcell
  • 0.2.2 : added predefined codes, by Johannes Rudolph
  • 0.2.1 : clarify field names, by Przemyslaw Skibinski
  • 0.2.0 : numerous format adjustments for zstd v0.8
  • 0.1.2 : limit Huffman tree depth to 11 bits
  • 0.1.1 : reserved dictID ranges
  • 0.1.0 : initial release