path: root/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

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.
func CompressBytes(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(CompressBytes(nonZeroBitset), 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 as the input slice may be longer.
func DecompressBytes(data []byte, target int) ([]byte, error) {
    out, size, err := decompressBytes(data, target)
    if err != nil {
        return nil, err
    }
    if size != len(data) {
        return nil, ErrUnreferencedData
    }
    return out, nil
}

// 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 as the input slice may be longer.
func decompressBytes(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 := decompressBytes(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
}