mirror of https://github.com/facebook/rocksdb.git
111 lines
4.3 KiB
Plaintext
111 lines
4.3 KiB
Plaintext
Snappy compressed format description
|
|
Last revised: 2011-10-05
|
|
|
|
|
|
This is not a formal specification, but should suffice to explain most
|
|
relevant parts of how the Snappy format works. It is originally based on
|
|
text by Zeev Tarantov.
|
|
|
|
Snappy is a LZ77-type compressor with a fixed, byte-oriented encoding.
|
|
There is no entropy encoder backend nor framing layer -- the latter is
|
|
assumed to be handled by other parts of the system.
|
|
|
|
This document only describes the format, not how the Snappy compressor nor
|
|
decompressor actually works. The correctness of the decompressor should not
|
|
depend on implementation details of the compressor, and vice versa.
|
|
|
|
|
|
1. Preamble
|
|
|
|
The stream starts with the uncompressed length (up to a maximum of 2^32 - 1),
|
|
stored as a little-endian varint. Varints consist of a series of bytes,
|
|
where the lower 7 bits are data and the upper bit is set iff there are
|
|
more bytes to be read. In other words, an uncompressed length of 64 would
|
|
be stored as 0x40, and an uncompressed length of 2097150 (0x1FFFFE)
|
|
would be stored as 0xFE 0xFF 0x7F.
|
|
|
|
|
|
2. The compressed stream itself
|
|
|
|
There are two types of elements in a Snappy stream: Literals and
|
|
copies (backreferences). There is no restriction on the order of elements,
|
|
except that the stream naturally cannot start with a copy. (Having
|
|
two literals in a row is never optimal from a compression point of
|
|
view, but nevertheless fully permitted.) Each element starts with a tag byte,
|
|
and the lower two bits of this tag byte signal what type of element will
|
|
follow:
|
|
|
|
00: Literal
|
|
01: Copy with 1-byte offset
|
|
10: Copy with 2-byte offset
|
|
11: Copy with 4-byte offset
|
|
|
|
The interpretation of the upper six bits are element-dependent.
|
|
|
|
|
|
2.1. Literals (00)
|
|
|
|
Literals are uncompressed data stored directly in the byte stream.
|
|
The literal length is stored differently depending on the length
|
|
of the literal:
|
|
|
|
- For literals up to and including 60 bytes in length, the upper
|
|
six bits of the tag byte contain (len-1). The literal follows
|
|
immediately thereafter in the bytestream.
|
|
- For longer literals, the (len-1) value is stored after the tag byte,
|
|
little-endian. The upper six bits of the tag byte describe how
|
|
many bytes are used for the length; 60, 61, 62 or 63 for
|
|
1-4 bytes, respectively. The literal itself follows after the
|
|
length.
|
|
|
|
|
|
2.2. Copies
|
|
|
|
Copies are references back into previous decompressed data, telling
|
|
the decompressor to reuse data it has previously decoded.
|
|
They encode two values: The _offset_, saying how many bytes back
|
|
from the current position to read, and the _length_, how many bytes
|
|
to copy. Offsets of zero can be encoded, but are not legal;
|
|
similarly, it is possible to encode backreferences that would
|
|
go past the end of the block (offset > current decompressed position),
|
|
which is also nonsensical and thus not allowed.
|
|
|
|
As in most LZ77-based compressors, the length can be larger than the offset,
|
|
yielding a form of run-length encoding (RLE). For instance,
|
|
"xababab" could be encoded as
|
|
|
|
<literal: "xab"> <copy: offset=2 length=4>
|
|
|
|
Note that since the current Snappy compressor works in 32 kB
|
|
blocks and does not do matching across blocks, it will never produce
|
|
a bitstream with offsets larger than about 32768. However, the
|
|
decompressor should not rely on this, as it may change in the future.
|
|
|
|
There are several different kinds of copy elements, depending on
|
|
the amount of bytes to be copied (length), and how far back the
|
|
data to be copied is (offset).
|
|
|
|
|
|
2.2.1. Copy with 1-byte offset (01)
|
|
|
|
These elements can encode lengths between [4..11] bytes and offsets
|
|
between [0..2047] bytes. (len-4) occupies three bits and is stored
|
|
in bits [2..4] of the tag byte. The offset occupies 11 bits, of which the
|
|
upper three are stored in the upper three bits ([5..7]) of the tag byte,
|
|
and the lower eight are stored in a byte following the tag byte.
|
|
|
|
|
|
2.2.2. Copy with 2-byte offset (10)
|
|
|
|
These elements can encode lengths between [1..64] and offsets from
|
|
[0..65535]. (len-1) occupies six bits and is stored in the upper
|
|
six bits ([2..7]) of the tag byte. The offset is stored as a
|
|
little-endian 16-bit integer in the two bytes following the tag byte.
|
|
|
|
|
|
2.2.3. Copy with 4-byte offset (11)
|
|
|
|
These are like the copies with 2-byte offsets (see previous subsection),
|
|
except that the offset is stored as a 32-bit integer instead of a
|
|
16-bit integer (and thus will occupy four bytes).
|