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93 lines
2.8 KiB
93 lines
2.8 KiB
// 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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