mirror of https://github.com/ethereum/go-ethereum
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// Copyright 2017 The go-ethereum Authors
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// This file is part of the go-ethereum library.
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//
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// The go-ethereum library is free software: you can redistribute it and/or modify
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// it under the terms of the GNU Lesser General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// The go-ethereum library is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU Lesser General Public License for more details.
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//
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// You should have received a copy of the GNU Lesser General Public License
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// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
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package bitutil |
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/* |
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The compression algorithm implemented by CompressBytes and DecompressBytes is |
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optimized for "sparse" input data which contains a lot of zero bytes. Decompression |
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requires knowledge of the decompressed data length. Compression works as follows: |
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if data only contains zeroes, |
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CompressBytes(data) == nil |
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otherwise if len(data) <= 1, |
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CompressBytes(data) == data |
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otherwise: |
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CompressBytes(data) == append(CompressBytes(nonZeroBits(data)), nonZeroBytes(data)...) |
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where |
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nonZeroBits(data) is a bit vector with len(data) bits (MSB first): |
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nonZeroBits(data)[i/8] && (1 << (7-i%8)) != 0 if data[i] != 0 |
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len(nonZeroBits(data)) == (len(data)+7)/8 |
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nonZeroBytes(data) contains the non-zero bytes of data in the same order |
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*/ |
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// CompressBytes compresses the input byte slice
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func CompressBytes(data []byte) []byte { |
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if len(data) == 0 { |
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return nil |
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} |
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if len(data) == 1 { |
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if data[0] == 0 { |
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return nil |
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} else { |
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return data |
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} |
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} |
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bitsLen := (len(data) + 7) / 8 |
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nonZeroBits := make([]byte, bitsLen) |
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nonZeroBytes := make([]byte, 0, len(data)) |
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for i, b := range data { |
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if b != 0 { |
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nonZeroBytes = append(nonZeroBytes, b) |
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nonZeroBits[i/8] |= 1 << byte(7-i%8) |
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} |
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} |
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if len(nonZeroBytes) == 0 { |
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return nil |
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} |
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return append(CompressBytes(nonZeroBits), nonZeroBytes...) |
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} |
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// DecompressBytes decompresses data with a known target size.
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// In addition to the decompressed output, the function returns the length of
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// compressed input data corresponding to the output. The input slice may be longer.
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// If the input slice is too short, (nil, -1) is returned.
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func DecompressBytes(data []byte, targetLen int) ([]byte, int) { |
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decomp := make([]byte, targetLen) |
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if len(data) == 0 { |
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return decomp, 0 |
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} |
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if targetLen == 1 { |
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return data[0:1], 1 |
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} |
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bitsLen := (targetLen + 7) / 8 |
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nonZeroBits, ptr := DecompressBytes(data, bitsLen) |
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if ptr < 0 { |
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return nil, -1 |
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} |
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for i, _ := range decomp { |
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if nonZeroBits[i/8]&(1<<byte(7-i%8)) != 0 { |
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if ptr == len(data) { |
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return nil, -1 |
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} |
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decomp[i] = data[ptr] |
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ptr++ |
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} |
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} |
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return decomp, ptr |
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} |
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