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How to simply compress a C string with LZMA

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.

Posted by Unknown at 12:45 AM

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