Task: LZW compression


C++: #include <string>
#include <map>




template <typename Iterator>
Iterator compress(const std::string &uncompressed, Iterator result) {
  
  int dictSize = 256;
  std::map<std::string,int> dictionary;
  for (int i = 0; i < 256; i++)
    dictionary[std::string(1, i)] = i;
  
  std::string w;
  for (std::string::const_iterator it = uncompressed.begin();
       it != uncompressed.end(); ++it) {
    char c = *it;
    std::string wc = w + c;
    if (dictionary.count(wc))
      w = wc;
    else {
      *result++ = dictionary[w];
      
      dictionary[wc] = dictSize++;
      w = std::string(1, c);
    }
  }
  
  
  if (!w.empty())
    *result++ = dictionary[w];
  return result;
}



template <typename Iterator>
std::string decompress(Iterator begin, Iterator end) {
  
  int dictSize = 256;
  std::map<int,std::string> dictionary;
  for (int i = 0; i < 256; i++)
    dictionary[i] = std::string(1, i);
  
  std::string w(1, *begin++);
  std::string result = w;
  std::string entry;
  for ( ; begin != end; begin++) {
    int k = *begin;
    if (dictionary.count(k))
      entry = dictionary[k];
    else if (k == dictSize)
      entry = w + w[0];
    else
      throw "Bad compressed k";
    
    result += entry;
    
    
    dictionary[dictSize++] = w + entry[0];
    
    w = entry;
  }
  return result;
}

#include <iostream>
#include <iterator>
#include <vector>

int main() {
  std::vector<int> compressed;
  compress("TOBEORNOTTOBEORTOBEORNOT", std::back_inserter(compressed));
  copy(compressed.begin(), compressed.end(), std::ostream_iterator<int>(std::cout, ", "));
  std::cout << std::endl;
  std::string decompressed = decompress(compressed.begin(), compressed.end());
  std::cout << decompressed << std::endl;
  
  return 0;
}




Java: import java.util.*;

public class LZW {
    
    public static List<Integer> compress(String uncompressed) {
        
        int dictSize = 256;
        Map<String,Integer> dictionary = new HashMap<String,Integer>();
        for (int i = 0; i < 256; i++)
            dictionary.put("" + (char)i, i);
        
        String w = "";
        List<Integer> result = new ArrayList<Integer>();
        for (char c : uncompressed.toCharArray()) {
            String wc = w + c;
            if (dictionary.containsKey(wc))
                w = wc;
            else {
                result.add(dictionary.get(w));
                
                dictionary.put(wc, dictSize++);
                w = "" + c;
            }
        }
 
        
        if (!w.equals(""))
            result.add(dictionary.get(w));
        return result;
    }
    
    
    public static String decompress(List<Integer> compressed) {
        
        int dictSize = 256;
        Map<Integer,String> dictionary = new HashMap<Integer,String>();
        for (int i = 0; i < 256; i++)
            dictionary.put(i, "" + (char)i);
        
        String w = "" + (char)(int)compressed.remove(0);
        StringBuffer result = new StringBuffer(w);
        for (int k : compressed) {
            String entry;
            if (dictionary.containsKey(k))
                entry = dictionary.get(k);
            else if (k == dictSize)
                entry = w + w.charAt(0);
            else
                throw new IllegalArgumentException("Bad compressed k: " + k);
            
            result.append(entry);
            
            
            dictionary.put(dictSize++, w + entry.charAt(0));
            
            w = entry;
        }
        return result.toString();
    }

    public static void main(String[] args) {
        List<Integer> compressed = compress("TOBEORNOTTOBEORTOBEORNOT");
        System.out.println(compressed);
        String decompressed = decompress(compressed);
        System.out.println(decompressed);
    }
}