common/bitutil: added data compression algorithm

common/bitutil/compress.go

+// 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
+}