1 files changed, 170 insertions, 0 deletions
diff --git a/common/bitutil/compress.go b/common/bitutil/compress.go
new file mode 100644
index 000000000..c057cee4a
--- /dev/null
+++ b/common/bitutil/compress.go
@@ -0,0 +1,170 @@
+// 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
+
+import "errors"
+
+var (
+	// errMissingData is returned from decompression if the byte referenced by
+	// the bitset header overflows the input data.
+	errMissingData = errors.New("missing bytes on input")
+
+	// errUnreferencedData is returned from decompression if not all bytes were used
+	// up from the input data after decompressing it.
+	errUnreferencedData = errors.New("extra bytes on input")
+
+	// errExceededTarget is returned from decompression if the bitset header has
+	// more bits defined than the number of target buffer space available.
+	errExceededTarget = errors.New("target data size exceeded")
+
+	// errZeroContent is returned from decompression if a data byte referenced in
+	// the bitset header is actually a zero byte.
+	errZeroContent = errors.New("zero byte in input content")
+)
+
+// 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(nonZeroBitset(data)), nonZeroBytes(data)...)
+//       where
+//         nonZeroBitset(data) is a bit vector with len(data) bits (MSB first):
+//             nonZeroBitset(data)[i/8] && (1 << (7-i%8)) != 0  if data[i] != 0
+//             len(nonZeroBitset(data)) == (len(data)+7)/8
+//         nonZeroBytes(data) contains the non-zero bytes of data in the same order
+
+// CompressBytes compresses the input byte slice according to the sparse bitset
+// representation algorithm. If the result is bigger than the original input, no
+// compression is done.
+func CompressBytes(data []byte) []byte {
+	if out := bitsetEncodeBytes(data); len(out) < len(data) {
+		return out
+	}
+	cpy := make([]byte, len(data))
+	copy(cpy, data)
+	return cpy
+}
+
+// bitsetEncodeBytes compresses the input byte slice according to the sparse
+// bitset representation algorithm.
+func bitsetEncodeBytes(data []byte) []byte {
+	// Empty slices get compressed to nil
+	if len(data) == 0 {
+		return nil
+	}
+	// One byte slices compress to nil or retain the single byte
+	if len(data) == 1 {
+		if data[0] == 0 {
+			return nil
+		}
+		return data
+	}
+	// Calculate the bitset of set bytes, and gather the non-zero bytes
+	nonZeroBitset := make([]byte, (len(data)+7)/8)
+	nonZeroBytes := make([]byte, 0, len(data))
+
+	for i, b := range data {
+		if b != 0 {
+			nonZeroBytes = append(nonZeroBytes, b)
+			nonZeroBitset[i/8] |= 1 << byte(7-i%8)
+		}
+	}
+	if len(nonZeroBytes) == 0 {
+		return nil
+	}
+	return append(bitsetEncodeBytes(nonZeroBitset), nonZeroBytes...)
+}
+
+// DecompressBytes decompresses data with a known target size. If the input data
+// matches the size of the target, it means no compression was done in the first
+// place.
+func DecompressBytes(data []byte, target int) ([]byte, error) {
+	if len(data) > target {
+		return nil, errExceededTarget
+	}
+	if len(data) == target {
+		cpy := make([]byte, len(data))
+		copy(cpy, data)
+		return cpy, nil
+	}
+	return bitsetDecodeBytes(data, target)
+}
+
+// bitsetDecodeBytes decompresses data with a known target size.
+func bitsetDecodeBytes(data []byte, target int) ([]byte, error) {
+	out, size, err := bitsetDecodePartialBytes(data, target)
+	if err != nil {
+		return nil, err
+	}
+	if size != len(data) {
+		return nil, errUnreferencedData
+	}
+	return out, nil
+}
+
+// bitsetDecodePartialBytes decompresses data with a known target size, but does
+// not enforce consuming all the input bytes. In addition to the decompressed
+// output, the function returns the length of compressed input data corresponding
+// to the output as the input slice may be longer.
+func bitsetDecodePartialBytes(data []byte, target int) ([]byte, int, error) {
+	// Sanity check 0 targets to avoid infinite recursion
+	if target == 0 {
+		return nil, 0, nil
+	}
+	// Handle the zero and single byte corner cases
+	decomp := make([]byte, target)
+	if len(data) == 0 {
+		return decomp, 0, nil
+	}
+	if target == 1 {
+		decomp[0] = data[0] // copy to avoid referencing the input slice
+		if data[0] != 0 {
+			return decomp, 1, nil
+		}
+		return decomp, 0, nil
+	}
+	// Decompress the bitset of set bytes and distribute the non zero bytes
+	nonZeroBitset, ptr, err := bitsetDecodePartialBytes(data, (target+7)/8)
+	if err != nil {
+		return nil, ptr, err
+	}
+	for i := 0; i < 8*len(nonZeroBitset); i++ {
+		if nonZeroBitset[i/8]&(1<<byte(7-i%8)) != 0 {
+			// Make sure we have enough data to push into the correct slot
+			if ptr >= len(data) {
+				return nil, 0, errMissingData
+			}
+			if i >= len(decomp) {
+				return nil, 0, errExceededTarget
+			}
+			// Make sure the data is valid and push into the slot
+			if data[ptr] == 0 {
+				return nil, 0, errZeroContent
+			}
+			decomp[i] = data[ptr]
+			ptr++
+		}
+	}
+	return decomp, ptr, nil
+}