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0.1.0 (11/04/17)
This document defines a format for compressed data to be stored so that subranges of the data can be efficiently decompressed without requiring the entire document to be decompressed. This is done by splitting up the input data into frames, each of which are compressed independently, and so can be decompressed independently. Decompression then takes advantage of a provided ‘seek table’, which allows the decompressor to immediately jump to the desired data. This is done in a way that is compatible with the original Zstandard format by placing the seek table in a Zstandard skippable frame.
In this document:
[ and ] are used to indicate optional fields or parameters.Mixed_Case_With_UnderscoresThe format consists of a number of frames (Zstandard compressed frames and skippable frames), followed by a final skippable frame at the end containing the seek table.
The structure of the seek table frame is as follows:
Skippable_Magic_Number | Frame_Size | [Seek_Table_Entries] | Seek_Table_Footer |
|---|---|---|---|
| 4 bytes | 4 bytes | 8-12 bytes each | 9 bytes |
Skippable_Magic_Number
Value : 0x184D2A5E. This is for compatibility with Zstandard skippable frames. Since it is legal for other Zstandard skippable frames to use the same magic number, it is not recommended for a decoder to recognize frames solely on this.
Frame_Size
The total size of the skippable frame, not including the Skippable_Magic_Number or Frame_Size. This is for compatibility with Zstandard skippable frames.
Seek_Table_FooterThe seek table footer format is as follows:
Number_Of_Frames | Seek_Table_Descriptor | Seekable_Magic_Number |
|---|---|---|
| 4 bytes | 1 byte | 4 bytes |
Seekable_Magic_Number
Value : 0x8F92EAB1. This value must be the last bytes present in the compressed file so that decoders can efficiently find it and determine if there is an actual seek table present.
Number_Of_Frames
The number of stored frames in the data.
Seek_Table_Descriptor
A bitfield describing the format of the seek table.
| Bit number | Field name |
|---|---|
| 7 | Checksum_Flag |
| 6-2 | Reserved_Bits |
| 1-0 | Unused_Bits |
While only Checksum_Flag currently exists, there are 7 other bits in this field that can be used for future changes to the format, for example the addition of inline dictionaries.
Checksum_Flag
If the checksum flag is set, each of the seek table entries contains a 4 byte checksum of the uncompressed data contained in its frame.
Reserved_Bits are not currently used but may be used in the future for breaking changes, so a compliant decoder should ensure they are set to 0. Unused_Bits may be used in the future for non-breaking changes, so a compliant decoder should not interpret these bits.
Seek_Table_EntriesSeek_Table_Entries consists of Number_Of_Frames (one for each frame in the data, not including the seek table frame) entries of the following form, in sequence:
Compressed_Size | Decompressed_Size | [Checksum] |
|---|---|---|
| 4 bytes | 4 bytes | 4 bytes |
Compressed_Size
The compressed size of the frame. The cumulative sum of the Compressed_Size fields of frames 0 to i gives the offset in the compressed file of frame i+1.
Decompressed_Size
The size of the decompressed data contained in the frame. For skippable or otherwise empty frames, this value is 0.
Checksum
Only present if Checksum_Flag is set in the Seek_Table_Descriptor. Value : the least significant 32 bits of the XXH64 digest of the uncompressed data, stored in little-endian format.