Data commit

Task/LZW-compression/Python/lzw-compression.py

@@ -0,0 +1,63 @@
+def compress(uncompressed):
+    """Compress a string to a list of output symbols."""
+
+    # Build the dictionary.
+    dict_size = 256
+    dictionary = dict((chr(i), i) for i in range(dict_size))
+    # in Python 3: dictionary = {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])
+            # Add wc to the dictionary.
+            dictionary[wc] = dict_size
+            dict_size += 1
+            w = c
+
+    # Output the code for w.
+    if w:
+        result.append(dictionary[w])
+    return result
+
+
+def decompress(compressed):
+    """Decompress a list of output ks to a string."""
+    from io import StringIO
+
+    # Build the dictionary.
+    dict_size = 256
+    dictionary = dict((i, chr(i)) for i in range(dict_size))
+    # in Python 3: dictionary = {i: chr(i) for i in range(dict_size)}
+
+    # use StringIO, otherwise this becomes O(N^2)
+    # due to string concatenation in a loop
+    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)
+
+        # Add w+entry[0] to the dictionary.
+        dictionary[dict_size] = w + entry[0]
+        dict_size += 1
+
+        w = entry
+    return result.getvalue()
+
+
+# How to use:
+compressed = compress('TOBEORNOTTOBEORTOBEORNOT')
+print (compressed)
+decompressed = decompress(compressed)
+print (decompressed)