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// 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
}
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