Subject:Data compression - LWZ algorithm.Source: http://www.cplusplus.com/articles/iL18T05o/#Version1 I tested the following source code for a compression and decompression of .txt files and a gif.  {| class="wikitable mw-collapsible mw-collapsed"! lwz.cpp( ... )|-|<syntaxhighlight lang="cpp">////// @file/// @author Julius Pettersson/// @copyright MIT/Expat License./// @brief LZW file compressor/// @version 1////// This is the C++11 implementation of a Lempel-Ziv-Welch single-file command-line compressor./// It uses the simpler fixed-width code compression method./// It was written with Doxygen comments.////// @see http://en.wikipedia.org/wiki/Lempel%E2%80%93Ziv%E2%80%93Welch/// @see http://marknelson.us/2011/11/08/lzw-revisited//// @see http://www.cs.duke.edu/csed/curious/compression/lzw.html/// @see http://warp.povusers.org/EfficientLZW/index.html/// @see http://en.cppreference.com//// @see http://www.doxygen.org//// #include <cstdint>#include <cstdlib>#include <exception>#include <fstream>#include <ios>#include <iostream>#include <istream>#include <limits>#include <map>#include <ostream>#include <stdexcept>#include <string>#include <vector> /// Type used to store and retrieve codes.using CodeType = std::uint16_t; namespace globals { /// Dictionary Maximum Size (when reached, the dictionary will be reset)const CodeType dms {std::numeric_limits<CodeType>::max()}; } // namespace globals ////// @brief Helper operator intended to simplify code./// @param vc original vector/// @param c element to be appended/// @returns vector resulting from appending `c` to `vc`///std::vector<char> operator + (std::vector<char> vc, char c){ vc.push_back(c); return vc;} ////// @brief Compresses the contents of `is` and writes the result to `os`./// @param [in] is input stream/// @param [out] os output stream///void compress(std::istream &is, std::ostream &os){ std::map<std::vector<char>, CodeType> dictionary;  // "named" lambda function, used to reset the dictionary to its initial contents const auto reset_dictionary = [&dictionary] { dictionary.clear();  const long int minc = std::numeric_limits<char>::min(); const long int maxc = std::numeric_limits<char>::max();  for (long int c = minc; c <= maxc; ++c) { // to prevent Undefined Behavior, resulting from reading and modifying // the dictionary object at the same time const CodeType dictionary_size = dictionary.size();  dictionary[{static_cast<char> (c)}] = dictionary_size; } };  reset_dictionary();  std::vector<char> s; // String char c;  while (is.get(c)) { // dictionary's maximum size was reached if (dictionary.size() == globals::dms) reset_dictionary();  s.push_back(c);  if (dictionary.count(s) == 0) { // to prevent Undefined Behavior, resulting from reading and modifying // the dictionary object at the same time const CodeType dictionary_size = dictionary.size();  dictionary[s] = dictionary_size; s.pop_back(); os.write(reinterpret_cast<const char *> (&dictionary.at(s)), sizeof (CodeType)); s = {c}; } }  if (!s.empty()) os.write(reinterpret_cast<const char *> (&dictionary.at(s)), sizeof (CodeType));} ////// @brief Decompresses the contents of `is` and writes the result to `os`./// @param [in] is input stream/// @param [out] os output stream///void decompress(std::istream &is, std::ostream &os){ std::vector<std::vector<char>> dictionary;  // "named" lambda function, used to reset the dictionary to its initial contents const auto reset_dictionary = [&dictionary] { dictionary.clear(); dictionary.reserve(globals::dms);  const long int minc = std::numeric_limits<char>::min(); const long int maxc = std::numeric_limits<char>::max();  for (long int c = minc; c <= maxc; ++c) dictionary.push_back({static_cast<char> (c)}); };  reset_dictionary();  std::vector<char> s; // String CodeType k; // Key  while (is.read(reinterpret_cast<char *> (&k), sizeof (CodeType))) { // dictionary's maximum size was reached if (dictionary.size() == globals::dms) reset_dictionary();  if (k > dictionary.size()) throw std::runtime_error("invalid compressed code");  if (k == dictionary.size()) dictionary.push_back(s + s.front()); else if (!s.empty()) dictionary.push_back(s + dictionary.at(k).front());  os.write(&dictionary.at(k).front(), dictionary.at(k).size()); s = dictionary.at(k); }  if (!is.eof() || is.gcount() != 0) throw std::runtime_error("corrupted compressed file");} ////// @brief Prints usage information and a custom error message./// @param s custom error message to be printed/// @param su Show Usage information///void print_usage(const std::string &s = "", bool su = true){ if (!s.empty()) std::cerr << "\nERROR: " << s << '\n';  if (su) { std::cerr << "\nUsage:\n"; std::cerr << "\tprogram -flag input_file output_file\n\n"; std::cerr << "Where `flag' is either `c' for compressing, or `d' for decompressing, and\n"; std::cerr << "`input_file' and `output_file' are distinct files.\n\n"; std::cerr << "Examples:\n"; std::cerr << "\tlzw_v1.exe -c license.txt license.lzw\n"; std::cerr << "\tlzw_v1.exe -d license.lzw new_license.txt\n"; }  std::cerr << std::endl;} ////// @brief Actual program entry point./// @param argc number of command line arguments/// @param [in] argv array of command line arguments/// @retval EXIT_FAILURE for failed operation/// @retval EXIT_SUCCESS for successful operation///int main(int argc, char *argv[]){ if (argc != 4) { print_usage("Wrong number of arguments."); return EXIT_FAILURE; }  enum class Mode { Compress, Decompress };  Mode m;  if (std::string(argv[1]) == "-c") m = Mode::Compress; else if (std::string(argv[1]) == "-d") m = Mode::Decompress; else { print_usage(std::string("flag `") + argv[1] + "' is not recognized."); return EXIT_FAILURE; }  std::ifstream input_file(argv[2], std::ios_base::binary);  if (!input_file.is_open()) { print_usage(std::string("input_file `") + argv[2] + "' could not be opened."); return EXIT_FAILURE; }  std::ofstream output_file(argv[3], std::ios_base::binary);  if (!output_file.is_open()) { print_usage(std::string("output_file `") + argv[3] + "' could not be opened."); return EXIT_FAILURE; }  try { input_file.exceptions(std::ios_base::badbit); output_file.exceptions(std::ios_base::badbit | std::ios_base::failbit);  if (m == Mode::Compress) compress(input_file, output_file); else if (m == Mode::Decompress) decompress(input_file, output_file); } catch (const std::ios_base::failure &f) { print_usage(std::string("File input/output failure: ") + f.what() + '.', false); return EXIT_FAILURE; } catch (const std::exception &e) { print_usage(std::string("Caught exception: ") + e.what() + '.', false); return EXIT_FAILURE; }  return EXIT_SUCCESS;} </syntaxhighlight>|}