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47 lines
2.3 KiB
Go
47 lines
2.3 KiB
Go
// Copyright 2011 The Snappy-Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package snappy implements the Snappy compression format. It aims for very
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// high speeds and reasonable compression.
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//
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// There are actually two Snappy formats: block and stream. They are related,
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// but different: trying to decompress block-compressed data as a Snappy stream
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// will fail, and vice versa. The block format is the Decode and Encode
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// functions and the stream format is the Reader and Writer types.
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//
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// The block format, the more common case, is used when the complete size (the
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// number of bytes) of the original data is known upfront, at the time
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// compression starts. The stream format, also known as the framing format, is
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// for when that isn't always true.
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//
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// The canonical, C++ implementation is at https://github.com/google/snappy and
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// it only implements the block format.
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package snappy
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/*
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Each encoded block begins with the varint-encoded length of the decoded data,
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followed by a sequence of chunks. Chunks begin and end on byte boundaries. The
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first byte of each chunk is broken into its 2 least and 6 most significant bits
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called l and m: l ranges in [0, 4) and m ranges in [0, 64). l is the chunk tag.
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Zero means a literal tag. All other values mean a copy tag.
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For literal tags:
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- If m < 60, the next 1 + m bytes are literal bytes.
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- Otherwise, let n be the little-endian unsigned integer denoted by the next
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m - 59 bytes. The next 1 + n bytes after that are literal bytes.
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For copy tags, length bytes are copied from offset bytes ago, in the style of
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Lempel-Ziv compression algorithms. In particular:
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- For l == 1, the offset ranges in [0, 1<<11) and the length in [4, 12).
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The length is 4 + the low 3 bits of m. The high 3 bits of m form bits 8-10
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of the offset. The next byte is bits 0-7 of the offset.
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- For l == 2, the offset ranges in [0, 1<<16) and the length in [1, 65).
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The length is 1 + m. The offset is the little-endian unsigned integer
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denoted by the next 2 bytes.
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- For l == 3, this tag is a legacy format that is no longer issued by most
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encoders. Nonetheless, the offset ranges in [0, 1<<32) and the length in
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[1, 65). The length is 1 + m. The offset is the little-endian unsigned
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integer denoted by the next 4 bytes.
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*/
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