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author | Zsolt Felfoldi <zsfelfoldi@gmail.com> | 2017-05-06 02:11:55 +0800 |
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committer | Zsolt Felfoldi <zsfelfoldi@gmail.com> | 2017-05-06 02:24:48 +0800 |
commit | fd5d51c9ae3256a1f24cf974dcd02433a259677e (patch) | |
tree | 6fb1136c056980d023ca2197b68cc81be24948f4 /common/bitutil/compress.go | |
parent | 2ec5cf1673e19da85471892875934fe0564a209d (diff) | |
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common/bitutil: added data compression algorithm
Diffstat (limited to 'common/bitutil/compress.go')
-rw-r--r-- | common/bitutil/compress.go | 93 |
1 files changed, 93 insertions, 0 deletions
diff --git a/common/bitutil/compress.go b/common/bitutil/compress.go new file mode 100644 index 000000000..c6c139ab9 --- /dev/null +++ b/common/bitutil/compress.go @@ -0,0 +1,93 @@ +// Copyright 2017 The go-ethereum Authors +// This file is part of the go-ethereum library. +// +// The go-ethereum library is free software: you can redistribute it and/or modify +// it under the terms of the GNU Lesser General Public License as published by +// the Free Software Foundation, either version 3 of the License, or +// (at your option) any later version. +// +// The go-ethereum library is distributed in the hope that it will be useful, +// but WITHOUT ANY WARRANTY; without even the implied warranty of +// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the +// GNU Lesser General Public License for more details. +// +// You should have received a copy of the GNU Lesser General Public License +// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>. + +package bitutil + +/* +The compression algorithm implemented by CompressBytes and DecompressBytes is +optimized for "sparse" input data which contains a lot of zero bytes. Decompression +requires knowledge of the decompressed data length. Compression works as follows: + +if data only contains zeroes, + CompressBytes(data) == nil +otherwise if len(data) <= 1, + CompressBytes(data) == data +otherwise: + CompressBytes(data) == append(CompressBytes(nonZeroBits(data)), nonZeroBytes(data)...) +where + nonZeroBits(data) is a bit vector with len(data) bits (MSB first): + nonZeroBits(data)[i/8] && (1 << (7-i%8)) != 0 if data[i] != 0 + len(nonZeroBits(data)) == (len(data)+7)/8 + nonZeroBytes(data) contains the non-zero bytes of data in the same order +*/ + +// CompressBytes compresses the input byte slice +func CompressBytes(data []byte) []byte { + if len(data) == 0 { + return nil + } + if len(data) == 1 { + if data[0] == 0 { + return nil + } else { + return data + } + } + + bitsLen := (len(data) + 7) / 8 + nonZeroBits := make([]byte, bitsLen) + nonZeroBytes := make([]byte, 0, len(data)) + for i, b := range data { + if b != 0 { + nonZeroBytes = append(nonZeroBytes, b) + nonZeroBits[i/8] |= 1 << byte(7-i%8) + } + } + if len(nonZeroBytes) == 0 { + return nil + } + return append(CompressBytes(nonZeroBits), nonZeroBytes...) +} + +// DecompressBytes decompresses data with a known target size. +// In addition to the decompressed output, the function returns the length of +// compressed input data corresponding to the output. The input slice may be longer. +// If the input slice is too short, (nil, -1) is returned. +func DecompressBytes(data []byte, targetLen int) ([]byte, int) { + decomp := make([]byte, targetLen) + if len(data) == 0 { + return decomp, 0 + } + if targetLen == 1 { + return data[0:1], 1 + } + + bitsLen := (targetLen + 7) / 8 + nonZeroBits, ptr := DecompressBytes(data, bitsLen) + if ptr < 0 { + return nil, -1 + } + for i, _ := range decomp { + if nonZeroBits[i/8]&(1<<byte(7-i%8)) != 0 { + if ptr == len(data) { + return nil, -1 + } + decomp[i] = data[ptr] + ptr++ + } + } + return decomp, ptr +} |