zstd/zstd_compression_format.md

Zstandard Compression Format

Notices

Copyright (c) 2016 Yann Collet

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

0.2.0 (22/07/16)

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.
• a naming convention for identifiers is Mixed_Case_With_Underscores

Definitions

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.

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.

Skippable Frames

Magic_Number	Frame_Size	User_Data
4 bytes	4 bytes	n 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). 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.

General Structure of Zstandard Frame format

The structure of a single Zstandard frame is following:

Magic_Number	Frame_Header	Data_Block	[More data blocks]	[Content_Checksum]
4 bytes	2-14 bytes	n bytes		0-4 bytes

Magic_Number

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

Frame_Header

2 to 14 Bytes, detailed in next part.

Data_Block

Detailed in next chapter. 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.

The structure of Frame_Header

The Frame_Header has a variable size, which uses 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 byte	0-1 byte	0-4 bytes	0-8 bytes

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 tell the size of Frame_Header.

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

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

Frame_Content_Size_flag

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

Flag_Value	0	1	2	3
Field_Size	0 or 1	2	4	8

When Flag_Value is 0, Field_Size depends on Single_Segment_flag : if Single_Segment_flag is set, Field_Size is 1. Otherwise, Field_Size is 0 (content size not provided).

Single_Segment_flag

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

In this case, Frame_Content_Size is necessarily present, but Window_Descriptor byte is skipped. 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 requirement, a decoder can 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 just 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_Value	0	1	2	3
Field_Size	0	1	2	4

Window_Descriptor

Provides guarantees on maximum back-reference distance that will be used within compressed data. This information is important for decoders to allocate enough memory.

The Window_Descriptor byte is optional. It is absent when Single_Segment_flag is set. In this 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).

Bit numbers	7-3	0-2
Field name	Exponent	Mantissa

Maximum distance is given 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 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 Window_Size bytes.

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 improved interoperability, decoders are recommended to be compatible with window sizes of 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. Note that this field is optional. When it's not present, it's up to the caller to make sure it uses the correct dictionary. Format is little-endian.

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.

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

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 for future use and should not be used :

• low range : 1 - 32767
• high range : >= (2^31)

Frame_Content_Size

This is the original (uncompressed) size. This information is optional. The Field_Size is provided according to value of Frame_Content_Size_flag. The Field_Size can be equal to 0 (not present), 1, 2, 4 or 8 bytes. Format is little-endian.

Field_Size	Range
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, the offset of 256 is added. It's allowed to represent a small size (for example 18) using any compatible variant.

The structure of Data_Block

The structure of Data_Block is following:

Last_Block	Block_Type	Block_Size	Block_Content
1 bit	2 bits	21 bits	n bytes

The block header (Last_Block, Block_Type, and Block_Size) uses 3-bytes.

Last_Block

The lowest bit signals if this block is the last one. Frame ends right after this block. It may be followed by an optional Content_Checksum .

Block_Type and Block_Size

The next 2 bits represent the Block_Type, while the remaining 21 bits represent the Block_Size. Format is little-endian.

There are 4 block types :

Value	0	1	2	3
Block_Type	Raw_Block	RLE_Block	Compressed_Block	Reserved

• Raw_Block - this is an uncompressed block. Block_Size is the number of bytes to read and copy.
• RLE_Block - this is a single byte, repeated N times. In which case, Block_Size is the size to regenerate, while the "compressed" block is just 1 byte (the byte to repeat).
• Compressed_Block - this is a Zstandard compressed block, detailed in another section of this specification. Block_Size is the compressed size. Decompressed size is unknown, 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 sizes must respect a few rules :

• In compressed mode, compressed size if always strictly < decompressed size.
• Block decompressed size is always <= maximum back-reference distance .
• Block decompressed size is always <= 128 KB

Block_Content

The Block_Content is 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_Decompressed_Size, which is the smallest of :

• Maximum back-reference distance
• 128 KB

The format of Compressed_Block

The size of Compressed_Block must be provided using Block_Size field from Data_Block. The Compressed_Block has a guaranteed maximum regenerated size, in order to properly allocate destination buffer. See Data_Block for more details.

A compressed block consists of 2 sections :

• Literals_Section
• Sequences_Section

Prerequisites

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

• Previous decoded blocks, up to a distance of Window_Size, or all previous blocks 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

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 sequence commands.

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

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

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_Type	Size_Format	Regenerated_Size	[Compressed_Size]
2 bits	1 - 2 bits	5 - 20 bits	0 - 18 bits

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

Literals_Block_Type

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

Literals_Block_Type	Value
Raw_Literals_Block	0
RLE_Literals_Block	1
Compressed_Literals_Block	2
Repeat_Stats_Literals_Block	3

• Raw_Literals_Block - Literals are stored uncompressed.
• RLE_Literals_Block - Literals consist of a single byte value repeated N times.
• Compressed_Literals_Block - This is a standard Huffman-compressed block, starting with a Huffman tree description. See details below.
• Repeat_Stats_Literals_Block - This is a Huffman-compressed block, using Huffman tree from previous Huffman-compressed literals block. Huffman tree description will be skipped.

Size_Format

Size_Format is divided into 2 families :

• For Compressed_Block, it requires to decode both Compressed_Size and Regenerated_Size (the decompressed size). It will also decode the number of streams.
• For Raw_Literals_Block and RLE_Literals_Block it's enough to decode Regenerated_Size.

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

Size_Format for Raw_Literals_Block and RLE_Literals_Block :

• Value x0 : Regenerated_Size uses 5 bits (0-31). Literals_Section_Header has 1 byte. Regenerated_Size = Header[0]>>3
• Value 01 : Regenerated_Size uses 12 bits (0-4095). Literals_Section_Header has 2 bytes. Regenerated_Size = (Header[0]>>4) + (Header[1]<<4)
• Value 11 : Regenerated_Size uses 20 bits (0-1048575). Literals_Section_Header has 3 bytes. Regenerated_Size = (Header[0]>>4) + (Header[1]<<4) + (Header[2]<<12)

Note : it's allowed to represent a short value (for example 13) using a long format, accepting the increased compressed data size.

Size_Format for Compressed_Literals_Block and Repeat_Stats_Literals_Block :

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

Both Compressed_Size and Regenerated_Size fields follow little-endian convention.

Huffman_Tree_Description

This section is only present when Literals_Block_Type type is Compressed_Literals_Block (2).

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 :

literal	0	1	2	3	4	5
Number_of_Bits	1	2	3	0	4	4

The tree depth is 4, since its smallest element uses 4 bits. Value 5 will not be listed, nor will values above 5. 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 serie of weights :

Weight	4	3	2	0	1
literal	0	1	2	3	4

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 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. Therefore, Max_Number_of_Bits = 4 and Weight[5] = 1.

Huffman Tree header

This is a single byte value (0-255), which tells 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). 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 serie must necessarily use FSE compression.

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

Finite State Entropy (FSE) compression of Huffman weights

The serie of 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 value is not represented.

An FSE bitstream starts by a header, describing probabilities distribution. It will create a Decoding Table. Table must be pre-allocated, which requires to support a maximum accuracy. For a list of Huffman weights, maximum accuracy is 7 bits.

FSE header is described in relevant chapter, and so is FSE bitstream. The main difference is that Huffman header compression uses 2 states, which share the same FSE distribution table. Bitstream contains only FSE symbols (no interleaved "raw bitfields"). The number of symbols to decode is discovered by tracking bitStream overflow condition. When both states have overflowed the bitstream, end is reached.

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 :

Literal	0	1	2	3	4	5
Weight	4	3	2	0	1	1

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

Literal	3	4	5	2	1	0
Weight	0	1	1	2	3	4
Number_of_Bits	0	4	4	3	2	1
prefix codes	N/A	0000	0001	001	01	1

The content of Huffman-compressed literal stream

Bitstreams sizes

As seen in a previous paragraph, there are 2 types of Huffman-compressed literals : a single stream and 4 streams.

Encoding using 4 streams is useful for CPU with multiple execution units and out-of-order operations. Since each stream can be decoded independently, it's possible to decode them up to 4x faster than a single stream, 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 of each stream can be calculated by (totalSize+3)/4, except for last one, which can be up to 3 bytes smaller, to reach totalSize.

Compressed size is provided explicitly : in the 4-streams variant, bitstreams are preceded by 3 unsigned little-endian 16-bits values. Each value represents the compressed size of one stream, in order. The last stream size is deducted from total compressed size and from previously decoded stream sizes :

stream4CSize = totalCSize - 6 - stream1CSize - stream2CSize - stream3CSize.

Bitstreams read and decode

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.

Starting from the end, it's possible to read the bitstream in a little-endian fashion, keeping track of already used bits.

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.

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 literal section. A match copy command specifies an offset and a length. The offset gives the position to copy from, which can be within a previous block.

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

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 deducted from blockSize - literalSectionSize.

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. Regenerated content is defined entirely by literals section.
• 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 number	7-6	5-4	3-2	1-0
Field name	Literals_Lengths_Mode	Offsets_Mode	Match_Lengths_Mode	Reserved

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 respectively.

They follow the same enumeration :

Value	0	1	2	3
Compression_Mode	Predefined_Mode	RLE_Mode	FSE_Compressed_Mode	Repeat_Mode

• Predefined_Mode : uses a predefined distribution table.
• RLE_Mode : it's a single code, repeated Number_of_Sequences times.
• Repeat_Mode : re-use distribution table from previous compressed block.
• FSE_Compressed_Mode : standard FSE compression. A distribution table will be present. It will be described in next part.

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.

Literals_Length_Code	0-15
length	Literals_Length_Code
Number_of_Bits	0

Literals_Length_Code	16	17	18	19	20	21	22	23
Baseline	16	18	20	22	24	28	32	40
Number_of_Bits	1	1	1	1	2	2	3	3

Literals_Length_Code	24	25	26	27	28	29	30	31
Baseline	48	64	128	256	512	1024	2048	4096
Number_of_Bits	4	6	7	8	9	10	11	12

Literals_Length_Code	32	33	34	35
Baseline	8192	16384	32768	65536
Number_of_Bits	13	14	15	16

Default distribution for literals length codes

When Compression_Mode is Predefined_Mode, a predefined distribution is used for FSE compression.

Below is its definition. It uses an accuracy 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 codes

Match length codes are values ranging from 0 to 52 included. They define lengths from 3 to 131074 bytes.

Match_Length_Code	0-31
value	Match_Length_Code + 3
Number_of_Bits	0

Match_Length_Code	32	33	34	35	36	37	38	39
Baseline	35	37	39	41	43	47	51	59
Number_of_Bits	1	1	1	1	2	2	3	3

Match_Length_Code	40	41	42	43	44	45	46	47
Baseline	67	83	99	131	258	514	1026	2050
Number_of_Bits	4	4	5	7	8	9	10	11

Match_Length_Code	48	49	50	51	52
Baseline	4098	8194	16486	32770	65538
Number_of_Bits	12	13	14	15	16

Default distribution for match length codes

When Compression_Mode is defined as Predefined_Mode, a predefined distribution is used for FSE compression.

Below is its definition. It uses an accuracy 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

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, 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", 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 paragraph.

Default distribution for offset codes

When Compression_Mode is defined as Predefined_Mode, a predefined distribution is used for FSE compression.

Below is its definition. It uses an accuracy of 5 bits (32 states), 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.

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 };

Distribution tables

Following the header, up to 3 distribution tables can be described. When present, they are in this order :

• Literals lengths
• Offsets
• Match Lengths

The content to decode depends on their respective encoding mode :

• Predefined_Mode : no content. Use predefined distribution table.
• RLE_Mode : 1 byte. This is the only code to use across the whole compressed block.
• FSE_Compressed_Mode : A distribution table is present.
• Repeat_Mode : no content. Re-use distribution from previous compressed block.

FSE distribution table : condensed format

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. Note that maximum Accuracy_Log for literal and match lengths is 9, and for offsets is 8. Higher values are considered errors.

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 (included). Therefore, it must read log2sup(156) == 8 bits.

• Value decoded : small values use 1 less bit : example : Presuming values from 0 to 156 (included) 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 read	Value decoded	Number of bits used
  0 - 98	0 - 98	7
  99 - 127	99 - 127	8
  128 - 226	0 - 98	7
  227 - 255	128 - 156	8

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 next paragraph. For the purpose of calculating cumulated distribution, 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.

FSE decoding : 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 tableSize = 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, typically by a "less than 1" probability symbol.

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 order	0	1	2	3	4
width	32	32	32	16	16
Number_of_Bits	5	5	5	4	4
range number	2	4	6	0	1
Baseline	32	64	96	0	16
range	32-63	64-95	96-127	0-15	16-31

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

Bitstream

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 last useful bit of the stream is followed by an end-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.

Starting states

The bitstream starts with initial 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.

Decoding a sequence

A state gives a code. A code provides Baseline and Number_of_Bits to add. See Symbol Decoding section for details on each symbol.

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.

Offset, Match_Length, and Literals_Length define a sequence. It starts by inserting the number of literals defined by Literals_Length, then continue by copying Match_Length bytes from currentPos - Offset.

The next operation is to update states. Using rules pre-calculated in the decoding tables, Literals_Length_State is updated, followed by Match_Length_State, and then Offset_State.

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

Repeat offsets

As seen in Offset Codes, 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".

There is an exception though, when current sequence's literals length is 0. In which case, repeated offsets are "pushed by one", so Repeated_Offset1 becomes Repeated_Offset2, Repeated_Offset2 becomes Repeated_Offset3, and Repeated_Offset3 becomes Repeated_Offset1 - 1_byte.

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

Then each block receives its start value from previous compressed block. Note that non-compressed blocks are skipped, they do not contribute to offset history.

Offset updates rules

New offset take the lead in offset history, up to its previous place if it was already present.

It means that when Repeated_Offset1 (most recent) is used, history is unmodified. When Repeated_Offset2 is used, it's swapped with Repeated_Offset1.

Dictionary format

zstd is compatible with "raw content" dictionaries, free of any format restriction. 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_Number	Dictionary_ID	Entropy_Tables	Content

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 for future use and should not be used :

          - low range : 1 - 32767
          - high range : >= (2^31)

Entropy_Tables : following the same format as a compressed blocks. 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. It's finally followed by 3 offset values, populating recent offsets, 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.

Version changes

• 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