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+# Zstandard Seekable 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
+0.1.0 (11/04/17)
+
+## Introduction
+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.
+
+### 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`
+- All numeric fields are little-endian unless specified otherwise
+
+## Format
+
+The 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.
+
+### Seek Table Format
+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].
+
+[Zstandard skippable frames]: https://github.com/facebook/zstd/blob/release/doc/zstd_compression_format.md#skippable-frames
+
+#### `Seek_Table_Footer`
+The 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_Entries`__
+
+`Seek_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.
+
+## Version Changes
+- 0.1.0: initial version