Internet Engineering Task Force (IETF) Y. Collet
Request for Comments: 8878 M. Kucherawy, Ed.
Obsoletes: 8478 Facebook
Category: Informational February 2021
ISSN: 2070-1721
Zstandard Compression and the 'application/zstd' Media Type
Abstract
Zstandard, or "zstd" (pronounced "zee standard"), is a lossless data
compression mechanism. This document describes the mechanism and
registers a media type, content encoding, and a structured syntax
suffix to be used when transporting zstd-compressed content via MIME.
Despite use of the word "standard" as part of Zstandard, readers are
advised that this document is not an Internet Standards Track
specification; it is being published for informational purposes only.
This document replaces and obsoletes RFC 8478.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8878.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Definitions
3. Compression Algorithm
3.1. Frames
3.1.1. Zstandard Frames
3.1.1.1. Frame Header
3.1.1.2. Blocks
3.1.1.3. Compressed Blocks
3.1.1.4. Sequence Execution
3.1.1.5. Repeat Offsets
3.1.2. Skippable Frames
4. Entropy Encoding
4.1. FSE
4.1.1. FSE Table Description
4.2. Huffman Coding
4.2.1. Huffman Tree Description
4.2.1.1. Huffman Tree Header
4.2.1.2. FSE Compression of Huffman Weights
4.2.1.3. Conversion from Weights to Huffman Prefix Codes
4.2.2. Huffman-Coded Streams
5. Dictionary Format
6. Use of Dictionaries
7. IANA Considerations
7.1. The 'application/zstd' Media Type
7.2. Content Encoding
7.3. Structured Syntax Suffix
7.4. Dictionaries
8. Security Considerations
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Decoding Tables for Predefined Codes
A.1. Literals Length Code Table
A.2. Match Length Code Table
A.3. Offset Code Table
Appendix B. Changes since RFC 8478
Acknowledgments
Authors' Addresses
1. Introduction
Zstandard, or "zstd" (pronounced "zee standard"), is a data
compression mechanism, akin to gzip [RFC1952].
Despite use of the word "standard" as part of its name, readers are
advised that this document is not an Internet Standards Track
specification; it is being published for informational purposes only.
This document describes the Zstandard format. Also, to enable the
transport of a data object compressed with Zstandard, this document
registers a media type, content encoding, and structured syntax
suffix that can be used to identify such content when it is used in a
payload.
2. Definitions
Some terms used elsewhere in this document are defined here for
clarity.
uncompressed: Describes an arbitrary set of bytes in their original
form, prior to being subjected to compression.
compressed: Describes the result of passing a set of bytes through
this mechanism. The original input has thus been compressed.
decompressed: Describes the result of passing a set of bytes through
the reverse of this mechanism. When this is successful, the
decompressed payload and the uncompressed payload are
indistinguishable.
encode: The process of translating data from one form to another;
this may include compression, or it may refer to other
translations done as part of this specification.
decode: The reverse of "encode"; describes a process of reversing a
prior encoding to recover the original content.
frame: 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 has a set of parameters that tells the
decoder how to decompress it.
block: A frame encapsulates one or multiple blocks. Each block
contains arbitrary content, which is described by its header, and
has a guaranteed maximum content size that depends upon 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.
natural order: A sequence or ordering of objects or values that is
typical of that type of object or value. A set of unique
integers, for example, is in "natural order" if, when progressing
from one element in the set or sequence to the next, there is
never a decrease in value.
The naming convention for identifiers within the specification is
Mixed_Case_With_Underscores. Identifiers inside square brackets
indicate that the identifier is optional in the presented context.
3. Compression Algorithm
This section describes the Zstandard algorithm.
The purpose of this document is to define a lossless compressed data
format that is a) independent of the CPU type, operating system, file
system, and character set and b) suitable for file compression and
pipe and streaming compression, using the Zstandard algorithm. The
text of the specification assumes a basic background in programming
at the level of bits and other primitive data representations.
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; hence, it can be used in data
communications. The format uses the Zstandard compression method,
and an optional xxHash-64 checksum method [XXHASH], for detection of
data corruption.
The data format defined by this specification does not attempt to
allow random access to compressed data.
Unless otherwise indicated below, a compliant compressor must produce
data sets that conform to the specifications presented here.
However, it does not need to support all options.
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 the
checksum. Whenever it does not support a parameter defined in the
compressed stream, it must produce an unambiguous error code and
associated error message explaining which parameter is unsupported.
This specification is intended for use by implementers of software to
compress data into Zstandard format and/or decompress data from
Zstandard format. The Zstandard format is supported by an open-
source reference implementation, written in portable C, and available
at [ZSTD].
3.1. Frames
Zstandard compressed data is made up of one or more frames. Each
frame is independent and can be decompressed independently of other
frames. The decompressed content of multiple concatenated frames is
the concatenation of each frame's decompressed content.
There are two frame formats defined for Zstandard: Zstandard frames
and skippable frames. Zstandard frames contain compressed data,
while skippable frames contain custom user metadata.
3.1.1. Zstandard Frames
The structure of a single Zstandard frame is as follows:
+--------------------+------------+
| Magic_Number | 4 bytes |
+--------------------+------------+
| Frame_Header | 2-14 bytes |
+--------------------+------------+
| Data_Block | n bytes |
+--------------------+------------+
| [More Data_Blocks] | |
+--------------------+------------+
| [Content_Checksum] | 4 bytes |
+--------------------+------------+
Table 1: The Structure of a
Single Zstandard Frame
Magic_Number: 4 bytes, little-endian format. Value: 0xFD2FB528.
Frame_Header: 2 to 14 bytes, detailed in Section 3.1.1.1.
Data_Block: Detailed in Section 3.1.1.2. This is where data
appears.
Content_Checksum: An optional 32-bit checksum, only present if
Content_Checksum_Flag is set. The content checksum is the result
of the XXH64() hash function [XXHASH] 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 magic number was selected to be less probable to find at the
beginning of an arbitrary file. It avoids trivial patterns (0x00,
0xFF, repeated bytes, increasing bytes, etc.), contains byte values
outside of the ASCII range, and doesn't map into UTF-8 space, all of
which reduce the likelihood of its appearance at the top of a text
file.
3.1.1.1. Frame Header
The frame header has a variable size, with a minimum of 2 bytes up to
a maximum of 14 bytes depending on optional parameters. The
structure of Frame_Header is as follows:
+-------------------------+-----------+
| Frame_Header_Descriptor | 1 byte |
+-------------------------+-----------+
| [Window_Descriptor] | 0-1 byte |
+-------------------------+-----------+
| [Dictionary_ID] | 0-4 bytes |
+-------------------------+-----------+
| [Frame_Content_Size] | 0-8 bytes |
+-------------------------+-----------+
Table 2: The Structure of Frame_Header
3.1.1.1.1. 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 Number | Field Name |
+============+=========================+
| 7-6 | Frame_Content_Size_Flag |
+------------+-------------------------+
| 5 | Single_Segment_Flag |
+------------+-------------------------+
| 4 | (unused) |
+------------+-------------------------+
| 3 | (reserved) |
+------------+-------------------------+
| 2 | Content_Checksum_Flag |
+------------+-------------------------+
| 1-0 | Dictionary_ID_Flag |
+------------+-------------------------+
Table 3: The Frame_Header_Descriptor
In Table 3, bit 7 is the highest bit, while bit 0 is the lowest one.
3.1.1.1.1.1. Frame_Content_Size_Flag
This is a 2-bit flag (equivalent to Frame_Header_Descriptor right-
shifted 6 bits) specifying whether Frame_Content_Size (the
decompressed data size) is provided within the header.
Frame_Content_Size_Flag provides FCS_Field_Size, which is the number
of bytes used by Frame_Content_Size according to Table 4:
+-------------------------+--------+---+---+---+
| Frame_Content_Size_Flag | 0 | 1 | 2 | 3 |
+-------------------------+--------+---+---+---+
| FCS_Field_Size | 0 or 1 | 2 | 4 | 8 |
+-------------------------+--------+---+---+---+
Table 4: Frame_Content_Size_Flag Provides
FCS_Field_Size
When Frame_Content_Size_Flag is 0, FCS_Field_Size depends on
Single_Segment_Flag: if Single_Segment_Flag is set, FCS_Field_Size is
1. Otherwise, FCS_Field_Size is 0; Frame_Content_Size is not
provided.
3.1.1.1.1.2. 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 a size equal to or larger
than Frame_Content_Size.
In order to protect the decoder from unreasonable memory
requirements, a decoder is allowed to reject a compressed frame that
requests a memory size beyond the 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.
3.1.1.1.1.3. Unused Bit
A decoder compliant with this specification version shall not
interpret this bit. It might be used in a future version to signal a
property that is not mandatory to properly decode the frame. An
encoder compliant with this specification must set this bit to zero.
3.1.1.1.1.4. 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.
3.1.1.1.1.5. Content_Checksum_Flag
If this flag is set, a 32-bit Content_Checksum will be present at the
frame's end. See the description of Content_Checksum above.
3.1.1.1.1.6. Dictionary_ID_Flag
This is a 2-bit flag (= Frame_Header_Descriptor & 0x3) indicating
whether a dictionary ID is provided within the header. It also
specifies the size of this field as DID_Field_Size:
+--------------------+---+---+---+---+
| Dictionary_ID_Flag | 0 | 1 | 2 | 3 |
+--------------------+---+---+---+---+
| DID_Field_Size | 0 | 1 | 2 | 4 |
+--------------------+---+---+---+---+
Table 5: Dictionary_ID_Flag
3.1.1.1.2. Window Descriptor
This provides guarantees about the 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 Number | 7-3 | 2-0 |
+------------+----------+----------+
| Field Name | Exponent | Mantissa |
+------------+----------+----------+
Table 6: Window_Descriptor
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.
In general, larger Window_Size values tend to improve the compression
ratio, but at the cost of increased memory usage.
To properly decode compressed data, a decoder will need to allocate a
buffer of at least Window_Size bytes.
In order to protect decoders from unreasonable memory requirements, a
decoder is allowed to reject a compressed frame that requests a
memory size beyond the decoder's authorized range.
For improved interoperability, it's recommended for decoders to
support values of Window_Size up to 8 MB and for encoders not to
generate frames requiring a Window_Size larger than 8 MB. It's
merely a recommendation though, and decoders are free to support
higher or lower limits, depending on local limitations.
3.1.1.1.3. Dictionary_ID
This is a field of variable size, which contains the ID of the
dictionary required to properly decode the frame. This field is
optional. When it's not present, it's up to the decoder to know
which dictionary to use.
Dictionary_ID field size is provided by DID_Field_Size.
DID_Field_Size is directly derived from the value of
Dictionary_ID_Flag. One 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 is permitted to represent a small ID (for example, 13) with a
large 4-byte dictionary ID, even if it is less efficient.
Within private environments, any dictionary ID can be used. However,
for frames and dictionaries distributed in public space,
Dictionary_ID must be attributed carefully. The following ranges are
reserved for use only with dictionaries that have been registered
with IANA (see Section 7.4):
low range: <= 32767
high range: >= (1 << 31)
Any other value for Dictionary_ID can be used by private arrangement
between participants.
Any payload presented for decompression that references an
unregistered reserved dictionary ID results in an error.
3.1.1.1.4. 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 Size | Range |
+================+================+
| 0 | unknown |
+----------------+----------------+
| 1 | 0 - 255 |
+----------------+----------------+
| 2 | 256 - 65791 |
+----------------+----------------+
| 4 | 0 - 2^(32) - 1 |
+----------------+----------------+
| 8 | 0 - 2^(64) - 1 |
+----------------+----------------+
Table 7: Frame_Content_Size
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.
3.1.1.2. Blocks
After Magic_Number and Frame_Header, there are some number of blocks.
Each frame must have at least 1 block, but there is no upper limit on
the number of blocks per frame.
The structure of a block is as follows:
+==============+===============+
| Block_Header | Block_Content |
+==============+===============+
| 3 bytes | n bytes |
+--------------+---------------+
Table 8: The Structure of a
Block
Block_Header uses 3 bytes, written using little-endian convention.
It contains three fields:
+============+============+============+
| Last_Block | Block_Type | Block_Size |
+============+============+============+
| bit 0 | bits 1-2 | bits 3-23 |
+------------+------------+------------+
Table 9: Block_Header
3.1.1.2.1. Last_Block
The lowest bit (Last_Block) signals whether this block is the last
one. The frame will end after this last block. It may be followed
by an optional Content_Checksum (see Section 3.1.1).
3.1.1.2.2. Block_Type
The next 2 bits represent the Block_Type. There are four block
types:
+=======+==================+
| Value | Block_Type |
+=======+==================+
| 0 | Raw_Block |
+-------+------------------+
| 1 | RLE_Block |
+-------+------------------+
| 2 | Compressed_Block |
+-------+------------------+
| 3 | Reserved |
+-------+------------------+
Table 10: The Four Block
Types
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 compressed block as described in
Section 3.1.1.3. Block_Size is the length of Block_Content,
namely 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 the
current specification. If such a value is present, it is
considered to be corrupt data, and a compliant decoder must reject
it.
3.1.1.2.3. Block_Size
The upper 21 bits of Block_Header represent the Block_Size.
When Block_Type is Compressed_Block or Raw_Block, Block_Size is the
size of Block_Content (hence excluding Block_Header).
When Block_Type is RLE_Block, since Block_Content's size is always 1,
Block_Size represents the number of times this byte must be repeated.
Block_Size is limited by Block_Maximum_Size (see below).
3.1.1.2.4. Block_Content and Block_Maximum_Size
The size of Block_Content is limited by Block_Maximum_Size, which is
the smallest of:
Block_Maximum_Size is constant for a given frame. This maximum is
applicable to both the decompressed size and the compressed size of
any block in the frame.
The reasoning for this limit is that a decoder can read this
information at the beginning of a frame and use it to allocate
buffers. The guarantees on the size of blocks ensure that the
buffers will be large enough for any following block of the valid
frame.
If the compressed block is larger than the uncompressed one, sending
the uncompressed block (i.e., a Raw_Block) is recommended instead.
3.1.1.3. Compressed Blocks
To decompress a compressed block, the compressed size must be
provided from the Block_Size field within Block_Header.
A compressed block consists of two sections: a
Literals_Section (Section 3.1.1.3.1) and a
Sequences_Section (Section 3.1.1.3.2). The results of the two
sections are then combined to produce the decompressed data in
Sequence Execution (Section 3.1.1.4).
To decode a compressed block, the following elements are necessary:
beginning of the Frame, whichever is smaller. Single_Segment_Flag
will be set in the latter case.
type.
Repeat_Mode, for each symbol type (literals length codes, match
length codes, offset codes).
Note that decoding tables are not always from the previous
Compressed_Block:
Compressed_Literals_Block.
3.1.1.3.1. Literals_Section_Header
All literals are regrouped in the first part of the block. They can
be decoded first and then copied during Sequence Execution (see
Section 3.1.1.4), 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] |
+----------------------------+
| [Jump_Table] |
+----------------------------+
| Stream_1 |
+----------------------------+
| [Stream_2] |
+----------------------------+
| [Stream_3] |
+----------------------------+
| [Stream_4] |
+----------------------------+
Table 11: Compressed Literals
3.1.1.3.1.1. Literals_Section_Header
This field describes how literals are packed. It's a byte-aligned
variable-size bit field, ranging from 1 to 5 bytes, using little-
endian convention.
+---------------------+-----------+
| Literals_Block_Type | 2 bits |
+---------------------+-----------+
| Size_Format | 1-2 bits |
+---------------------+-----------+
| Regenerated_Size | 5-20 bits |
+---------------------+-----------+
| [Compressed_Size] | 0-18 bits |
+---------------------+-----------+
Table 12: Literals_Section_Header
In this representation, bits at the top are the lowest bits.
The Literals_Block_Type field uses the two lowest bits of the first
byte, describing four different block types:
+===========================+=======+
| Literals_Block_Type | Value |
+===========================+=======+
| Raw_Literals_Block | 0 |
+---------------------------+-------+
| RLE_Literals_Block | 1 |
+---------------------------+-------+
| Compressed_Literals_Block | 2 |
+---------------------------+-------+
| Treeless_Literals_Block | 3 |
+---------------------------+-------+
Table 13: Literals_Block_Type
Raw_Literals_Block: Literals are stored uncompressed.
Literals_Section_Content is Regenerated_Size.
RLE_Literals_Block: Literals consist of a single-byte value repeated
Regenerated_Size times. Literals_Section_Content is 1.
Compressed_Literals_Block: This is a standard Huffman-compressed
block, starting with a Huffman tree description. See details
below. Literals_Section_Content is Compressed_Size.
Treeless_Literals_Block: This is a Huffman-compressed block, using
the Huffman tree from the previous Compressed_Literals_Block or a
dictionary if there is no previous Huffman-compressed literals
block. Huffman_Tree_Description will be skipped. Note that if
this mode is triggered without any previous Huffman table in the
frame (or dictionary, per Section 5), it should be treated as data
corruption. Literals_Section_Content is Compressed_Size.
The Size_Format is divided into two families:
to decode Regenerated_Size. There is no Compressed_Size field.
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, the convention is little endian.
Size_Format for Raw_Literals_Block and RLE_Literals_Block uses 1 or 2
bits. Its value is (Literals_Section_Header[0]>>2) & 0x3.
Size_Format == 00 or 10: Size_Format uses 1 bit. Regenerated_Size
uses 5 bits (value 0-31). Literals_Section_Header uses 1 byte.
Regenerated_Size = Literal_Section_Header[0]>>3.
Size_Format == 01: Size_Format uses 2 bits. Regenerated_Size uses
12 bits (values 0-4095). Literals_Section_Header uses 2 bytes.
Regenerated_Size = (Literals_Section_Header[0]>>4) +
(Literals_Section_Header[1]<<4).
Size_Format == 11: Size_Format uses 2 bits. Regenerated_Size uses
20 bits (values 0-1048575). Literals_Section_Header uses 3 bytes.
Regenerated_Size = (Literals_Section_Header[0]>>4) +
(Literals_Section_Header[1]<<4) +
(Literals_Section_Header[2]<<12).
Only Stream_1 is present for these cases. Note that it is permitted
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
always uses 2 bits.
Size_Format == 00: A single stream. Both Regenerated_Size and
Compressed_Size use 10 bits (values 0-1023).
Literals_Section_Header uses 3 bytes.
Size_Format == 01: 4 streams. Both Regenerated_Size and
Compressed_Size use 10 bits (values 0-1023).
Literals_Section_Header uses 3 bytes.
Size_Format == 10: 4 streams. Both Regenerated_Size and
Compressed_Size use 14 bits (values 0-16383).
Literals_Section_Header uses 4 bytes.
Size_Format == 11: 4 streams. Both Regenerated_Size and
Compressed_Size use 18 bits (values 0-262143).
Literals_Section_Header uses 5 bytes.
Both the Compressed_Size and Regenerated_Size fields follow little-
endian convention. Note that Compressed_Size includes the size of
the Huffman_Tree_Description when it is present.
3.1.1.3.1.2. Raw_Literals_Block
The data in Stream_1 is Regenerated_Size bytes long. It contains the
raw literals data to be used during Sequence Execution
(Section 3.1.1.3.2).
3.1.1.3.1.3. RLE_Literals_Block
Stream_1 consists of a single byte that should be repeated
Regenerated_Size times to generate the decoded literals.
3.1.1.3.1.4. Compressed_Literals_Block and Treeless_Literals_Block
Both of these modes contain Huffman-coded data. For
Treeless_Literals_Block, the Huffman table comes from the previously
compressed literals block, or from a dictionary; see Section 5.
3.1.1.3.1.5. Huffman_Tree_Description
This section is only present when the Literals_Block_Type type is
Compressed_Literals_Block (2). The format of
Huffman_Tree_Description can be found in Section 4.2.1. The size of
Huffman_Tree_Description is determined during the decoding process.
It must be used to determine where streams begin.
Total_Streams_Size = Compressed_Size
- Huffman_Tree_Description_Size
3.1.1.3.1.6. Jump_Table
The Jump_Table is only present when there are 4 Huffman-coded
streams.
(Reminder: Huffman-compressed data consists of either 1 or 4 Huffman-
coded streams.)
If only 1 stream is present, it is a single bitstream occupying the
entire remaining portion of the literals block, encoded as described
within Section 4.2.2.
If there are 4 streams, Literals_Section_Header only provides enough
information to know the decompressed and compressed sizes of all 4
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 in the
Jump_Table. The Jump_Table is 6 bytes long and consists of three
2-byte little-endian fields, describing the compressed sizes of the
first 3 streams. Stream4_Size is computed from Total_Streams_Size
minus the sizes of other streams.
Stream4_Size = Total_Streams_Size - 6
- Stream1_Size - Stream2_Size
- Stream3_Size
Note that if Stream1_Size + Stream2_Size + Stream3_Size exceeds
Total_Streams_Size, the data are considered corrupted.
Each of these 4 bitstreams is then decoded independently as a
Huffman-coded stream, as described in Section 4.2.2.
3.1.1.3.2. 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
Literals_Section, these bytes are added at the end of the block.
This is described in more detail in Section 3.1.1.4.
The Sequences_Section regroups all symbols required to decode
commands. There are three symbol types: literals length codes,
offset codes, and match length codes. 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 necessary to know its size.
This size is deduced from the size of the Literals_Section:
Sequences_Section_Size = Block_Size - Literals_Section_Header -
Literals_Section_Content.
3.1.1.3.2.1. Sequences_Section_Header
This header consists of two items:
Number_of_Sequences is a variable size field using between 1 and 3
bytes. If the first byte is "byte0":
stops here. Decompressed content is defined entirely as
Literals_Section content. The FSE tables used in Repeat_Mode are
not updated.
if (byte0 < 255): Number_of_Sequences =
1)