Task: LZW compression





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);
    }
}



Python: def compress(uncompressed):
    

    
    dict_size = 256
    dictionary = dict((chr(i), i) for i in range(dict_size))
    

    w = ""
    result = []
    for c in uncompressed:
        wc = w + c
        if wc in dictionary:
            w = wc
        else:
            result.append(dictionary[w])
            
            dictionary[wc] = dict_size
            dict_size += 1
            w = c

    
    if w:
        result.append(dictionary[w])
    return result


def decompress(compressed):
    
    from io import StringIO

    
    dict_size = 256
    dictionary = dict((i, chr(i)) for i in range(dict_size))
    

    
    
    result = StringIO()
    w = chr(compressed.pop(0))
    result.write(w)
    for k in compressed:
        if k in dictionary:
            entry = dictionary[k]
        elif k == dict_size:
            entry = w + w[0]
        else:
            raise ValueError('Bad compressed k: %s' % k)
        result.write(entry)

        
        dictionary[dict_size] = w + entry[0]
        dict_size += 1

        w = entry
    return result.getvalue()



compressed = compress('TOBEORNOTTOBEORTOBEORNOT')
print (compressed)
decompressed = decompress(compressed)
print (decompressed)