adding decode.py. This file is used for retrieving hidden message

decode.py

+from PIL import Image
+
+
+def decodeLzw(compressed):
+    dict_size = 256
+    dictionary = {}
+    for i in range(dict_size):
+        dictionary[i] = chr(i)
+
+    if compressed:
+        w = chr(compressed.pop(0))
+
+    result = [w]
+
+    for k in compressed:
+        if k in dictionary:
+            entry = dictionary[k]
+        elif k == len(dictionary):
+            entry = w + w[0]
+        result.append(entry)
+
+        dictionary[dict_size] = w + entry[0]
+        dict_size += 1
+
+        w = entry
+
+    return ''.join(result)
+
+
+if __name__ == '__main__':
+    img = Image.open('output.png')
+    pixelMap = img.load()
+
+    # extracting the length of compressed message
+    compressed_len = 1  # there is extra character in encoded_version
+    for i in range(0, 8):
+        compressed_len = compressed_len + (pixelMap[0, i][0] % 4) * (4 ** (7 - i))
+
+    compressed = []
+
+    curr_code = 0
+    code_ctr = 0
+    loop_ctr = 0
+    for i in range(0, img.size[0]):
+        for j in range(0, img.size[1]):
+            if loop_ctr > compressed_len * 8:
+                break;
+
+            if code_ctr == 8:
+                compressed.append(curr_code)
+                curr_code = (pixelMap[i, j][0] % 4) * (4 ** 7)
+                code_ctr = 1
+            else:
+                curr_code = curr_code + (pixelMap[i, j][0] % 4) * (4 ** (7 - code_ctr))
+                code_ctr = code_ctr + 1
+            loop_ctr = loop_ctr + 1
+
+        if loop_ctr > compressed_len * 8:
+            break;
+
+    n = len(compressed) - 1
+    compressed[0], compressed[n] = compressed[n], compressed[0]
+    compressed.pop(n)
+    print(decodeLzw(compressed))
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