How to simply compress a C++ string with LZMA

This blog post explains how to simply compress C++ with LZMA compression, using liblzma.

Use the following functions:

#include <stdlib.h>
#include "lzma.h"
#include <string>

// Level is between 0 (no compression), 9 (slow compression, small output).
std::string CompressWithLzma(const std::string& in, int level) {
  std::string result;
  result.resize(in.size() + (in.size() >> 2) + 128);
  size_t out_pos = 0;
  if (LZMA_OK != lzma_easy_buffer_encode(
      level, LZMA_CHECK_CRC32, NULL,
      reinterpret_cast<uint8_t*>(const_cast<char*>(in.data())), in.size(),
      reinterpret_cast<uint8_t*>(&result[0]), &out_pos, result.size()))
    abort();
  result.resize(out_pos);
  return result;
}

std::string DecompressWithLzma(const std::string& in) {
  static const size_t kMemLimit = 1 << 30;  // 1 GB.
  lzma_stream strm = LZMA_STREAM_INIT;
  std::string result;
  result.resize(8192);
  size_t result_used = 0;
  lzma_ret ret;
  ret = lzma_stream_decoder(&strm, kMemLimit, LZMA_CONCATENATED);
  if (ret != LZMA_OK)
    abort();
  size_t avail0 = result.size();
  strm.next_in = reinterpret_cast<const uint8_t*>(in.data());
  strm.avail_in = in.size();
  strm.next_out = reinterpret_cast<uint8_t*>(&result[0]);
  strm.avail_out = avail0;
  while (true) {
    ret = lzma_code(&strm, strm.avail_in == 0 ? LZMA_FINISH : LZMA_RUN);
    if (ret == LZMA_STREAM_END) {
      result_used += avail0 - strm.avail_out;
      if (0 != strm.avail_in)  // Guaranteed by lzma_stream_decoder().
        abort();
      result.resize(result_used);
      lzma_end(&strm);
      return result;
    }
    if (ret != LZMA_OK)
      abort();
    if (strm.avail_out == 0) {
      result_used += avail0 - strm.avail_out;
      result.resize(result.size() << 1);
      strm.next_out = reinterpret_cast<uint8_t*>(&result[0] + result_used);
      strm.avail_out = avail0 = result.size() - result_used;
    }
  }
}

Please note that in some use cases there may exist a solution which uses less memory.

See the liblzma/container. for documentation of lzma_easy_buffer_encode().

The decompression code was based on xzdec.c in xz-utils.