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https://codeberg.org/forgejo/forgejo
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39 lines
1.6 KiB
Go
39 lines
1.6 KiB
Go
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// 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 block-based compression format.
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// It aims for very high speeds and reasonable compression.
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//
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// The C++ snappy implementation is at http://code.google.com/p/snappy/
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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 supported.
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*/
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const (
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tagLiteral = 0x00
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tagCopy1 = 0x01
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tagCopy2 = 0x02
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tagCopy4 = 0x03
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)
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