--- /dev/null
+Zstandard Compression Format
+============================
+
+### Notices
+
+Copyright (c) Meta Platforms, Inc. and affiliates.
+
+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
+
+0.3.9 (2023-03-08)
+
+
+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](https://facebook.github.io/zstd/).
+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,
+and hence can be used in data communications.
+The format uses the Zstandard compression method,
+and optional [xxHash-64 checksum method](https://cyan4973.github.io/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.
+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.
+
+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 : https://github.com/facebook/zstd .
+
+
+### 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 contains arbitrary content, which is described by its header,
+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](#frames)
+ - [Zstandard frames](#zstandard-frames)
+ - [Blocks](#blocks)
+ - [Literals Section](#literals-section)
+ - [Sequences Section](#sequences-section)
+ - [Sequence Execution](#sequence-execution)
+ - [Skippable frames](#skippable-frames)
+- [Entropy Encoding](#entropy-encoding)
+ - [FSE](#fse)
+ - [Huffman Coding](#huffman-coding)
+- [Dictionary Format](#dictionary-format)
+
+Frames
+------
+Zstandard compressed data is made 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 decompressed content.
+
+There are two frame formats defined by Zstandard:
+ Zstandard frames and Skippable frames.
+Zstandard frames contain compressed data, while
+skippable frames contain custom user metadata.
+
+## Zstandard frames
+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
+Note: This value was selected to be less probable to find at the beginning of some random file.
+It avoids trivial patterns (0x00, 0xFF, repeated bytes, increasing bytes, etc.),
+contains byte values outside of ASCII range,
+and doesn't map into UTF8 space.
+It reduces the chances that a text file represent this value by accident.
+
+__`Frame_Header`__
+
+2 to 14 Bytes, detailed in [`Frame_Header`](#frame_header).
+
+__`Data_Block`__
+
+Detailed in [`Blocks`](#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](https://cyan4973.github.io/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.
+
+### `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 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 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_bit` |
+| 3 | `Reserved_bit` |
+| 2 | `Content_Checksum_flag` |
+| 1-0 | `Dictionary_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_Value` | 0 | 1 | 2 | 3 |
+| -------------- | ------ | --- | --- | --- |
+|`FCS_Field_Size`| 0 or 1 | 2 | 4 | 8 |
+
+When `Flag_Value` 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.
+
+__`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 larger 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`__
+
+A decoder compliant with this specification version shall not interpret this bit.
+It might be used in any future version,
+to signal a property which is transparent to properly decode the frame.
+An encoder compliant with this specification version must set this bit to zero.
+
+__`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 `DID_Field_Size`.
+
+|`Flag_Value` | 0 | 1 | 2 | 3 |
+| -------------- | --- | --- | --- | --- |
+|`DID_Field_Size`| 0 | 1 | 2 | 4 |
+
+#### `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 numbers | 7-3 | 2-0 |
+| ----------- | ---------- | ---------- |
+| Field name | `Exponent` | `Mantissa` |
+
+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` tend to improve compression ratio,
+but at the cost of memory usage.
+
+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,
+it's recommended for decoders to support `Window_Size` of up to 8 MB,
+and it's recommended for encoders to not generate frame requiring `Window_Size` larger 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 know which dictionary to use.
+
+`Dictionary_ID` field size is provided by `DID_Field_Size`.
+`DID_Field_Size` is directly derived from value of `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.
+
+A value of `0` has same meaning as no `Dictionary_ID`,
+in which case the frame may or may not need a dictionary to be decoded,
+and the ID of such a dictionary is not specified.
+The decoder must know this information by other means.
+
+#### `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 |
+
+`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_Header` | `Block_Content` |
+|:--------------:|:---------------:|
+| 3 bytes | n bytes |
+
+__`Block_Header`__
+
+`Block_Header` uses 3 bytes, written using __little-endian__ convention.
+It contains 3 fields :
+
+| `Last_Block` | `Block_Type` | `Block_Size` |
+|:------------:|:------------:|:------------:|
+| bit 0 | bits 1-2 | bits 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](#zstandard-frames)).
+
+__`Block_Type`__
+
+The next 2 bits represent the `Block_Type`.
+`Block_Type` influences the meaning of `Block_Size`.
+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_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](#compressed-blocks),
+ 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.
+ If such a value is present, it is considered corrupted data.
+
+__`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).
+
+__`Block_Content`__ and __`Block_Maximum_Size`__
+
+The size of `Block_Content` is limited by `Block_Maximum_Size`,
+which is the smallest of:
+- `Window_Size`
+- 128 KB
+
+`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.
+
+
+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 :
+- [Literals Section](#literals-section)
+- [Sequences Section](#sequences-section)
+
+The results of the two sections are then combined to produce the decompressed
+data in [Sequence Execution](#sequence-execution)
+
+#### Prerequisites
+To decode a compressed block, the following elements are necessary :
+- Previous decoded data, up to a distance of `Window_Size`,
+ or beginning of the Frame, whichever is smaller.
+- List of "recent offsets" from previous `Compressed_Block`.
+- The previous Huffman tree, required by `Treeless_Literals_Block` type
+- Previous FSE decoding tables, required by `Repeat_Mode`
+ for each symbol type (literals lengths, match lengths, offsets)
+
+Note that decoding tables aren't always from the previous `Compressed_Block`.
+
+- Every decoding table can come from a dictionary.
+- The Huffman tree comes from the previous `Compressed_Literals_Block`.
+
+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, a tree description may optionally be present,
+followed by 1 or 4 streams.
+
+| `Literals_Section_Header` | [`Huffman_Tree_Description`] | [jumpTable] | 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_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 the lowest 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 |
+| `Treeless_Literals_Block` | 3 |
+
+- `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.
+ In this mode, there are at least 2 different literals represented in the 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](#dictionary-format)), 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`__ :
+
+`Size_Format` uses 1 _or_ 2 bits.
+Its value is : `Size_Format = (Literals_Section_Header[0]>>2) & 3`
+
+- `Size_Format` == 00 or 10 : `Size_Format` uses 1 bit.
+ `Regenerated_Size` uses 5 bits (0-31).
+ `Literals_Section_Header` uses 1 byte.
+ `Regenerated_Size = Literals_Section_Header[0]>>3`
+- `Size_Format` == 01 : `Size_Format` uses 2 bits.
+ `Regenerated_Size` uses 12 bits (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 (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 Stream1 is present for these cases.
+Note : it's allowed to represent a short value (for example `27`)
+using a long format, even if it's less efficient.
+
+__`Size_Format` for `Compressed_Literals_Block` and `Treeless_Literals_Block`__ :
+
+`Size_Format` always uses 2 bits.
+
+- `Size_Format` == 00 : _A single stream_.
+ Both `Regenerated_Size` and `Compressed_Size` use 10 bits (0-1023).
+ `Literals_Section_Header` uses 3 bytes.
+- `Size_Format` == 01 : 4 streams.
+ Both `Regenerated_Size` and `Compressed_Size` use 10 bits (6-1023).
+ `Literals_Section_Header` uses 3 bytes.
+- `Size_Format` == 10 : 4 streams.
+ Both `Regenerated_Size` and `Compressed_Size` use 14 bits (6-16383).
+ `Literals_Section_Header` uses 4 bytes.
+- `Size_Format` == 11 : 4 streams.
+ Both `Regenerated_Size` and `Compressed_Size` use 18 bits (6-262143).
+ `Literals_Section_Header` uses 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.
+Note 2: `Compressed_Size` can never be `==0`.
+Even in single-stream scenario, assuming an empty content, it must be `>=1`,
+since it contains at least the final end bit flag.
+In 4-streams scenario, a valid `Compressed_Size` is necessarily `>= 10`
+(6 bytes for the jump table, + 4x1 bytes for the 4 streams).
+
+4 streams is faster than 1 stream in decompression speed,
+by exploiting instruction level parallelism.
+But it's also more expensive,
+costing on average ~7.3 bytes more than the 1 stream mode, mostly from the jump table.
+
+In general, use the 4 streams mode when there are more literals to decode,
+to favor higher decompression speeds.
+Note that beyond >1KB of literals, the 4 streams mode is compulsory.
+
+Note that a minimum of 6 bytes is required for the 4 streams mode.
+That's a technical minimum, but it's not recommended to employ the 4 streams mode
+for such a small quantity, that would be wasteful.
+A more practical lower bound would be around ~256 bytes.
+
+#### 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.
+
+For `Treeless_Literals_Block`,
+the Huffman table comes from previously compressed literals block,
+or from a dictionary.
+
+
+### `Huffman_Tree_Description`
+This section is only present when `Literals_Block_Type` type is `Compressed_Literals_Block` (`2`).
+The tree describes the weights of all literals symbols that can be present in the literals block, at least 2 and up to 256.
+The format of the Huffman tree description can be found at [Huffman Tree description](#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`.
+
+
+### 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 streams.
+
+If only one stream is present, it is a single bitstream occupying the entire
+remaining portion of the literals block, encoded as described in
+[Huffman-Coded Streams](#huffman-coded-streams).
+
+If there are four streams, `Literals_Section_Header` only provided
+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 in the Jump Table.
+Jump Table is 6 bytes long, and consists of three 2-byte __little-endian__ fields,
+describing the compressed sizes of the first three streams.
+`Stream4_Size` is computed from `Total_Streams_Size` minus sizes of other streams:
+
+`Stream4_Size = Total_Streams_Size - 6 - Stream1_Size - Stream2_Size - Stream3_Size`.
+
+`Stream4_Size` is necessarily `>= 1`. Therefore,
+if `Total_Streams_Size < Stream1_Size + Stream2_Size + Stream3_Size + 6 + 1`,
+data is considered corrupted.
+
+Each of these 4 bitstreams is then decoded independently as a Huffman-Coded stream,
+as described in [Huffman-Coded Streams](#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 _literals section_,
+these bytes are added at the end of the block.
+
+This is described in more detail in [Sequence Execution](#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.
+Its size is deduced from the size of `Literals_Section`:
+`Sequences_Section_Size = 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.
+ The FSE tables used in `Repeat_Mode` aren't updated.
+- `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 symbols respectively.
+
+They follow the same enumeration :
+
+| Value | 0 | 1 | 2 | 3 |
+| ------------------ | ----------------- | ---------- | --------------------- | ------------- |
+| `Compression_Mode` | `Predefined_Mode` | `RLE_Mode` | `FSE_Compressed_Mode` | `Repeat_Mode` |
+
+- `Predefined_Mode` : A predefined FSE distribution table is used, defined in
+ [default distributions](#default-distributions).
+ No distribution table will be present.
+- `RLE_Mode` : The table description consists of a single byte, which contains the symbol's value.
+ This symbol will be used for all sequences.
+- `FSE_Compressed_Mode` : standard FSE compression.
+ A distribution table will be present.
+ The format of this distribution table is described in [FSE Table Description](#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.
+ `FSE_Compressed_Mode` must not be used when only one symbol is present,
+ `RLE_Mode` should be used instead (although any other mode will work).
+- `Repeat_Mode` : The table used in the previous `Compressed_Block` with `Number_of_Sequences > 0` will be used again,
+ or if this is the first block, table in the dictionary will be used.
+ Note that this includes `RLE_mode`, so if `Repeat_Mode` follows `RLE_Mode`, the same symbol will be repeated.
+ It also includes `Predefined_Mode`, in which case `Repeat_Mode` will have same outcome as `Predefined_Mode`.
+ No distribution table will be present.
+ If this mode is used without any previous sequence table in the frame
+ (nor [dictionary](#dictionary-format)) to repeat, this should be treated as corruption.
+
+#### 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_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 |
+
+
+##### 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_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 | 259 | 515 | 1027 | 2051 |
+| `Number_of_Bits` | 4 | 4 | 5 | 7 | 8 | 9 | 10 | 11 |
+
+| `Match_Length_Code` | 48 | 49 | 50 | 51 | 52 |
+| ------------------- | ---- | ---- | ---- | ---- | ---- |
+| `Baseline` | 4099 | 8195 |16387 |32771 |65539 |
+| `Number_of_Bits` | 12 | 13 | 14 | 15 | 16 |
+
+##### 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 `31`.
+
+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`
+supporting back-reference distances up to `(2^(N+1))-4`,
+but is limited by [maximum back-reference distance](#window_descriptor).
+
+`Offset_Value` from 1 to 3 are special : they define "repeat codes".
+This is described in more detail in [Repeat Offsets](#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](#fse).
+
+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](#fse).
+
+##### 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.
+
+[description of the codes]: #the-codes-for-literals-lengths-match-lengths-and-offsets
+
+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](#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](#fse) 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].
+
+[FSE section]: #from-normalized-distribution-to-decoding-tables
+
+##### 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](#sequences-section).
+To execute a sequence, first copy `literals_length` bytes
+from the decoded literals 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](#repeat-offsets) 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](#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 following values :
+`Repeated_Offset1`=1, `Repeated_Offset2`=4, `Repeated_Offset3`=8,
+unless a dictionary is used, in which case they come from the dictionary.
+
+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 Codes]: #offset-codes
+
+###### Offset updates rules
+
+During the execution of the sequences of a `Compressed_Block`, the
+`Repeated_Offsets`' values are kept up to date, so that they always represent
+the three most-recently used offsets. In order to achieve that, they are
+updated after executing each sequence in the following way:
+
+When the sequence's `offset_value` does not refer to one of the
+`Repeated_Offsets`--when it has value greater than 3, or when it has value 3
+and the sequence's `literals_length` is zero--the `Repeated_Offsets`' values
+are shifted back one, and `Repeated_Offset1` takes on the value of the
+just-used offset.
+
+Otherwise, when the sequence's `offset_value` refers to one of the
+`Repeated_Offsets`--when it has value 1 or 2, or when it has value 3 and the
+sequence's `literals_length` is non-zero--the `Repeated_Offsets` are re-ordered
+so that `Repeated_Offset1` takes on the value of the used Repeated_Offset, and
+the existing values are pushed back from the first `Repeated_Offset` through to
+the `Repeated_Offset` selected by the `offset_value`. This effectively performs
+a single-stepped wrapping rotation of the values of these offsets, so that
+their order again reflects the recency of their use.
+
+The following table shows the values of the `Repeated_Offsets` as a series of
+sequences are applied to them:
+
+| `offset_value` | `literals_length` | `Repeated_Offset1` | `Repeated_Offset2` | `Repeated_Offset3` | Comment |
+|:--------------:|:-----------------:|:------------------:|:------------------:|:------------------:|:-----------------------:|
+| | | 1 | 4 | 8 | starting values |
+| 1114 | 11 | 1111 | 1 | 4 | non-repeat |
+| 1 | 22 | 1111 | 1 | 4 | repeat 1: no change |
+| 2225 | 22 | 2222 | 1111 | 1 | non-repeat |
+| 1114 | 111 | 1111 | 2222 | 1111 | non-repeat |
+| 3336 | 33 | 3333 | 1111 | 2222 | non-repeat |
+| 2 | 22 | 1111 | 3333 | 2222 | repeat 2: swap 1 & 2 |
+| 3 | 33 | 2222 | 1111 | 3333 | repeat 3: rotate 3 to 1 |
+| 3 | 0 | 2221 | 2222 | 1111 | special case : insert `repeat1 - 1` |
+| 1 | 0 | 2222 | 2221 | 1111 | == repeat 2 |
+
+
+Skippable Frames
+----------------
+
+| `Magic_Number` | `Frame_Size` | `User_Data` |
+|:--------------:|:------------:|:-----------:|
+| 4 bytes | 4 bytes | n bytes |
+
+Skippable frames allow the insertion of user-defined metadata
+into a flow of concatenated frames.
+
+Skippable frames defined in this specification are compatible with [LZ4] ones.
+
+[LZ4]:https://lz4.github.io/lz4/
+
+From a compliant decoder perspective, skippable frames need just be skipped,
+and their content ignored, resuming decoding after the skippable frame.
+
+It can be noted that a skippable frame
+can be used to watermark a stream of concatenated frames
+embedding any kind of tracking information (even just a UUID).
+Users wary of such possibility should scan the stream of concatenated frames
+in an attempt to detect such frame for analysis or removal.
+
+__`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.
+This specification doesn't detail any specific tagging for skippable frames.
+
+__`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.
+Huffman is used to compress literals,
+while FSE is used for all other symbols
+(`Literals_Length_Code`, `Match_Length_Code`, offset codes)
+and to compress Huffman headers.
+
+
+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.
+Note that the order of the bits in the stream is not reversed,
+we just read the elements in the reverse order they are written.
+
+For additional details on FSE, see [Finite State Entropy].
+
+[Finite State Entropy]:https://github.com/Cyan4973/FiniteStateEntropy/
+
+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`.
+An FSE state value 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`.
+
+[ANS]: https://en.wikipedia.org/wiki/Asymmetric_Numeral_Systems
+
+### 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` .
+Note that there must be two or more symbols with nonzero probability.
+
+It's a bitstream which is read forward, in __little-endian__ fashion.
+It's not necessary to know bitstream exact size,
+it will be discovered and reported by the decoding process.
+
+The bitstream starts by reporting on which scale it operates.
+Let's `low4Bits` designate the lowest 4 bits of the first byte :
+`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 `256 - 100 + 1 == 157` (inclusive).
+ Therefore, it must read `log2sup(157) == 8` bits.
+
+- Value decoded : small values use 1 less bit :
+ __example__ :
+ Presuming values from 0 to 157 (inclusive) are possible,
+ 255-157 = 98 values are remaining in an 8-bits field.
+ They are used this way :
+ first 98 values (hence from 0 to 97) use only 7 bits,
+ values from 98 to 157 use 8 bits.
+ This is achieved through this scheme :
+
+ | Value read | Value decoded | Number of bits used |
+ | ---------- | ------------- | ------------------- |
+ | 0 - 97 | 0 - 97 | 7 |
+ | 98 - 127 | 98 - 127 | 8 |
+ | 128 - 225 | 0 - 97 | 7 |
+ | 226 - 255 | 128 - 157 | 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 the [next section].
+For the purpose of calculating total allocated probability points, it counts as one.
+
+[next section]:#from-normalized-distribution-to-decoding-tables
+
+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 (`Number_of_Bits` and `Baseline`).
+
+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 and retreating.
+These symbols define a full state reset, reading `Accuracy_Log` bits.
+
+Then, all remaining symbols, sorted in natural order, are allocated cells.
+Starting from symbol `0` (if it exists), and table position `0`,
+each symbol gets allocated as many cells as its probability.
+Cell allocation is spread, not linear :
+each successor position follows 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 process guarantees that the table is entirely filled.
+Each cell corresponds to a state value, which contains the symbol being decoded.
+
+To add the `Number_of_Bits` and `Baseline` required to retrieve next state,
+it's first necessary to sort all occurrences of each symbol in state order.
+Lower states will need 1 more bit than higher ones.
+The process is repeated for each symbol.
+
+__Example__ :
+Presuming a symbol has a probability of 5,
+it receives 5 cells, corresponding to 5 state values.
+These state values are then sorted in natural order.
+
+Next power of 2 after 5 is 8.
+Space of probabilities must be divided into 8 equal parts.
+Presuming the `Accuracy_Log` is 7, it defines a space of 128 states.
+Divided by 8, each share is 16 large.
+
+In order to reach 8 shares, 8-5=3 lowest states will count "double",
+doubling their shares (32 in width), hence requiring one more bit.
+
+Baseline is assigned starting from the higher states using fewer bits,
+increasing at each state, then resuming at the first state,
+each state takes its allocated width from Baseline.
+
+| state value | 1 | 39 | 77 | 84 | 122 |
+| 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 |
+
+During decoding, the next state value is determined from current state value,
+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.
+
+[Appendix A]: #appendix-a---decoding-tables-for-predefined-codes
+
+
+Huffman Coding
+--------------
+Zstandard Huffman-coded streams are read backwards,
+similar to the FSE bitstreams.
+Therefore, to find the start of the bitstream, it is required 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
+```
+When a literal value is not present, it receives a `Weight` of 0.
+The least frequent symbol receives a `Weight` of 1.
+Consequently, the `Weight` 1 is necessarily present.
+The most frequent symbol receives a `Weight` anywhere between 1 and 11 (max).
+The last symbol's `Weight` is deduced from previously retrieved Weights,
+by completing to the nearest power of 2. It's necessarily non 0.
+If it's not possible to reach a clean power of 2 with a single `Weight` value,
+the Huffman Tree Description is considered invalid.
+This final power of 2 gives `Max_Number_of_Bits`, the depth of the current tree.
+`Max_Number_of_Bits` must be <= 11,
+otherwise the representation is considered corrupted.
+
+__Example__ :
+Let's presume the following Huffman tree must be described :
+
+| literal value | 0 | 1 | 2 | 3 | 4 | 5 |
+| ---------------- | --- | --- | --- | --- | --- | --- |
+| `Number_of_Bits` | 1 | 2 | 3 | 0 | 4 | 4 |
+
+The tree depth is 4, since its longest elements uses 4 bits
+(longest elements are the one with smallest frequency).
+Literal value `5` will not be listed, as it can be determined from previous values 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 :
+
+| literal value | 0 | 1 | 2 | 3 | 4 |
+| ------------- | --- | --- | --- | --- | --- |
+| `Weight` | 4 | 3 | 2 | 0 | 1 |
+
+The decoder will do the inverse operation :
+having collected weights of literal symbols 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 larger power of 2 value is 16.
+Therefore, `Max_Number_of_Bits = 4` and `Weight[5] = log_2(16 - 15) + 1 = 1`.
+
+#### Huffman Tree header
+
+This is a single byte value (0-255),
+which describes how the series of weights is encoded.
+
+- if `headerByte` < 128 :
+ the series of weights is compressed using FSE (see below).
+ The length of the FSE-compressed series is equal to `headerByte` (0-127).
+
+- if `headerByte` >= 128 :
+ + the series of weights uses a direct representation,
+ where each `Weight` is encoded directly as a 4 bits field (0-15).
+ + They are encoded forward, 2 weights to a byte,
+ first weight taking the top four bits and second one 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 `Ceiling(Number_of_Weights/2)` bytes,
+ meaning it uses only full bytes even if `Number_of_Weights` is odd.
+ + `Number_of_Weights = headerByte - 127`.
+ * Note that maximum `Number_of_Weights` is 255-127 = 128,
+ therefore, only up to 128 `Weight` can be encoded using direct representation.
+ * Since the last non-zero `Weight` is _not_ encoded,
+ this scheme is compatible with alphabet sizes of up to 129 symbols,
+ hence including literal symbol 128.
+ * If any literal symbol > 128 has a non-zero `Weight`,
+ direct representation is not possible.
+ In such case, it's necessary to use FSE compression.
+
+
+#### 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 6 bits.
+For more description see the [FSE header description](#fse-table-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 indexed symbols.
+`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](#fse).
+
+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,
+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 = (Weight>0) ? Max_Number_of_Bits + 1 - Weight : 0
+```
+Symbols are sorted by `Weight`.
+Within same `Weight`, symbols keep natural sequential order.
+Symbols with a `Weight` of zero are removed.
+Then, starting from lowest `Weight`, prefix codes are distributed in sequential 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 sequential 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 |
+
+### 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 last one.
+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. Since the bitstream is encoded in reverse
+order, starting from the end read symbols in forward order.
+
+For example, if the literal sequence "0145" was encoded using above prefix code,
+it would be encoded (in reverse order) as:
+
+|Symbol | 5 | 4 | 1 | 0 | Padding |
+|--------|------|------|----|---|---------|
+|Encoding|`0000`|`0001`|`01`|`1`| `00001` |
+
+Resulting in following 2-bytes bitstream :
+```
+00010000 00001101
+```
+
+Here is an alternative representation with the symbol codes separated by underscore:
+```
+0001_0000 00001_1_01
+```
+
+Reading highest `Max_Number_of_Bits` bits,
+it's 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_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 dictionary is going to be distributed in a public environment,
+the following ranges of `Dictionary_ID` are reserved for some future registrar
+and shall not be used :
+
+ - low range : <= 32767
+ - high range : >= (2^31)
+
+Outside of these ranges, any value of `Dictionary_ID`
+which is both `>= 32768` and `< (1<<31)` can be used freely,
+even in public environment.
+
+
+__`Entropy_Tables`__ : follow the same format as tables in [compressed blocks].
+ See the relevant [FSE](#fse-table-description)
+ and [Huffman](#huffman-tree-description) 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 content size, and cannot equal 0.
+
+__`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, even parts of the dictionary with offsets larger than `Window_Size`.
+ After the total output has surpassed `Window_Size` however,
+ this is no longer allowed and the dictionary is no longer accessible.
+
+[compressed blocks]: #the-format-of-compressed_block
+
+If a dictionary is provided by an external source,
+it should be loaded with great care, its content considered untrusted.
+
+
+
+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:
+
+| State | Symbol | Number_Of_Bits | Base |
+| ----- | ------ | -------------- | ---- |
+| 0 | 0 | 4 | 0 |
+| 1 | 0 | 4 | 16 |
+| 2 | 1 | 5 | 32 |
+| 3 | 3 | 5 | 0 |
+| 4 | 4 | 5 | 0 |
+| 5 | 6 | 5 | 0 |
+| 6 | 7 | 5 | 0 |
+| 7 | 9 | 5 | 0 |
+| 8 | 10 | 5 | 0 |
+| 9 | 12 | 5 | 0 |
+| 10 | 14 | 6 | 0 |
+| 11 | 16 | 5 | 0 |
+| 12 | 18 | 5 | 0 |
+| 13 | 19 | 5 | 0 |
+| 14 | 21 | 5 | 0 |
+| 15 | 22 | 5 | 0 |
+| 16 | 24 | 5 | 0 |
+| 17 | 25 | 5 | 32 |
+| 18 | 26 | 5 | 0 |
+| 19 | 27 | 6 | 0 |
+| 20 | 29 | 6 | 0 |
+| 21 | 31 | 6 | 0 |
+| 22 | 0 | 4 | 32 |
+| 23 | 1 | 4 | 0 |
+| 24 | 2 | 5 | 0 |
+| 25 | 4 | 5 | 32 |
+| 26 | 5 | 5 | 0 |
+| 27 | 7 | 5 | 32 |
+| 28 | 8 | 5 | 0 |
+| 29 | 10 | 5 | 32 |
+| 30 | 11 | 5 | 0 |
+| 31 | 13 | 6 | 0 |
+| 32 | 16 | 5 | 32 |
+| 33 | 17 | 5 | 0 |
+| 34 | 19 | 5 | 32 |
+| 35 | 20 | 5 | 0 |
+| 36 | 22 | 5 | 32 |
+| 37 | 23 | 5 | 0 |
+| 38 | 25 | 4 | 0 |
+| 39 | 25 | 4 | 16 |
+| 40 | 26 | 5 | 32 |
+| 41 | 28 | 6 | 0 |
+| 42 | 30 | 6 | 0 |
+| 43 | 0 | 4 | 48 |
+| 44 | 1 | 4 | 16 |
+| 45 | 2 | 5 | 32 |
+| 46 | 3 | 5 | 32 |
+| 47 | 5 | 5 | 32 |
+| 48 | 6 | 5 | 32 |
+| 49 | 8 | 5 | 32 |
+| 50 | 9 | 5 | 32 |
+| 51 | 11 | 5 | 32 |
+| 52 | 12 | 5 | 32 |
+| 53 | 15 | 6 | 0 |
+| 54 | 17 | 5 | 32 |
+| 55 | 18 | 5 | 32 |
+| 56 | 20 | 5 | 32 |
+| 57 | 21 | 5 | 32 |
+| 58 | 23 | 5 | 32 |
+| 59 | 24 | 5 | 32 |
+| 60 | 35 | 6 | 0 |
+| 61 | 34 | 6 | 0 |
+| 62 | 33 | 6 | 0 |
+| 63 | 32 | 6 | 0 |
+
+#### Match Length Code:
+
+| State | Symbol | Number_Of_Bits | Base |
+| ----- | ------ | -------------- | ---- |
+| 0 | 0 | 6 | 0 |
+| 1 | 1 | 4 | 0 |
+| 2 | 2 | 5 | 32 |
+| 3 | 3 | 5 | 0 |
+| 4 | 5 | 5 | 0 |
+| 5 | 6 | 5 | 0 |
+| 6 | 8 | 5 | 0 |
+| 7 | 10 | 6 | 0 |
+| 8 | 13 | 6 | 0 |
+| 9 | 16 | 6 | 0 |
+| 10 | 19 | 6 | 0 |
+| 11 | 22 | 6 | 0 |
+| 12 | 25 | 6 | 0 |
+| 13 | 28 | 6 | 0 |
+| 14 | 31 | 6 | 0 |
+| 15 | 33 | 6 | 0 |
+| 16 | 35 | 6 | 0 |
+| 17 | 37 | 6 | 0 |
+| 18 | 39 | 6 | 0 |
+| 19 | 41 | 6 | 0 |
+| 20 | 43 | 6 | 0 |
+| 21 | 45 | 6 | 0 |
+| 22 | 1 | 4 | 16 |
+| 23 | 2 | 4 | 0 |
+| 24 | 3 | 5 | 32 |
+| 25 | 4 | 5 | 0 |
+| 26 | 6 | 5 | 32 |
+| 27 | 7 | 5 | 0 |
+| 28 | 9 | 6 | 0 |
+| 29 | 12 | 6 | 0 |
+| 30 | 15 | 6 | 0 |
+| 31 | 18 | 6 | 0 |
+| 32 | 21 | 6 | 0 |
+| 33 | 24 | 6 | 0 |
+| 34 | 27 | 6 | 0 |
+| 35 | 30 | 6 | 0 |
+| 36 | 32 | 6 | 0 |
+| 37 | 34 | 6 | 0 |
+| 38 | 36 | 6 | 0 |
+| 39 | 38 | 6 | 0 |
+| 40 | 40 | 6 | 0 |
+| 41 | 42 | 6 | 0 |
+| 42 | 44 | 6 | 0 |
+| 43 | 1 | 4 | 32 |
+| 44 | 1 | 4 | 48 |
+| 45 | 2 | 4 | 16 |
+| 46 | 4 | 5 | 32 |
+| 47 | 5 | 5 | 32 |
+| 48 | 7 | 5 | 32 |
+| 49 | 8 | 5 | 32 |
+| 50 | 11 | 6 | 0 |
+| 51 | 14 | 6 | 0 |
+| 52 | 17 | 6 | 0 |
+| 53 | 20 | 6 | 0 |
+| 54 | 23 | 6 | 0 |
+| 55 | 26 | 6 | 0 |
+| 56 | 29 | 6 | 0 |
+| 57 | 52 | 6 | 0 |
+| 58 | 51 | 6 | 0 |
+| 59 | 50 | 6 | 0 |
+| 60 | 49 | 6 | 0 |
+| 61 | 48 | 6 | 0 |
+| 62 | 47 | 6 | 0 |
+| 63 | 46 | 6 | 0 |
+
+#### Offset Code:
+
+| State | Symbol | Number_Of_Bits | Base |
+| ----- | ------ | -------------- | ---- |
+| 0 | 0 | 5 | 0 |
+| 1 | 6 | 4 | 0 |
+| 2 | 9 | 5 | 0 |
+| 3 | 15 | 5 | 0 |
+| 4 | 21 | 5 | 0 |
+| 5 | 3 | 5 | 0 |
+| 6 | 7 | 4 | 0 |
+| 7 | 12 | 5 | 0 |
+| 8 | 18 | 5 | 0 |
+| 9 | 23 | 5 | 0 |
+| 10 | 5 | 5 | 0 |
+| 11 | 8 | 4 | 0 |
+| 12 | 14 | 5 | 0 |
+| 13 | 20 | 5 | 0 |
+| 14 | 2 | 5 | 0 |
+| 15 | 7 | 4 | 16 |
+| 16 | 11 | 5 | 0 |
+| 17 | 17 | 5 | 0 |
+| 18 | 22 | 5 | 0 |
+| 19 | 4 | 5 | 0 |
+| 20 | 8 | 4 | 16 |
+| 21 | 13 | 5 | 0 |
+| 22 | 19 | 5 | 0 |
+| 23 | 1 | 5 | 0 |
+| 24 | 6 | 4 | 16 |
+| 25 | 10 | 5 | 0 |
+| 26 | 16 | 5 | 0 |
+| 27 | 28 | 5 | 0 |
+| 28 | 27 | 5 | 0 |
+| 29 | 26 | 5 | 0 |
+| 30 | 25 | 5 | 0 |
+| 31 | 24 | 5 | 0 |
+
+
+
+Appendix B - Resources for implementers
+-------------------------------------------------
+
+An open source reference implementation is available on :
+https://github.com/facebook/zstd
+
+The project contains a frame generator, called [decodeCorpus],
+which can be used by any 3rd-party implementation
+to verify that a tested decoder is compliant with the specification.
+
+[decodeCorpus]: https://github.com/facebook/zstd/tree/v1.3.4/tests#decodecorpus---tool-to-generate-zstandard-frames-for-decoder-testing
+
+`decodeCorpus` generates random valid frames.
+A compliant decoder should be able to decode them all,
+or at least provide a meaningful error code explaining for which reason it cannot
+(memory limit restrictions for example).
+
+
+Version changes
+---------------
+- 0.3.9 : clarifications for Huffman-compressed literal sizes.
+- 0.3.8 : clarifications for Huffman Blocks and Huffman Tree descriptions.
+- 0.3.7 : clarifications for Repeat_Offsets, matching RFC8878
+- 0.3.6 : clarifications for Dictionary_ID
+- 0.3.5 : clarifications for Block_Maximum_Size
+- 0.3.4 : clarifications for FSE decoding table
+- 0.3.3 : clarifications for field Block_Size
+- 0.3.2 : remove additional block size restriction on compressed blocks
+- 0.3.1 : minor clarification regarding offset history update rules
+- 0.3.0 : minor edits to match RFC8478
+- 0.2.9 : clarifications for huffman weights direct representation, by Ulrich Kunitz
+- 0.2.8 : clarifications for IETF RFC discuss
+- 0.2.7 : clarifications from IETF RFC review, by Vijay Gurbani and Nick Terrell
+- 0.2.6 : fixed an error in huffman example, by Ulrich Kunitz
+- 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
notaz.gp2x.de / pcsx_rearmed.git / blobdiff