Create lz78.py

added lz78 compression algorithm

Author: Ahmed99125

compression/lz78.py

@@ -0,0 +1,117 @@
+"""
+Sources:
+https://en.wikipedia.org/wiki/LZ77_and_LZ78#LZ78
+"""
+
+from dataclasses import dataclass
+
+__version__ = "1.0"
+__author__ = "Ahmed Tamer"
+
+@dataclass
+class Token:
+  """
+    Dataclass representing pair called token consisting of the dictionary index and a single character that follows the phrase in the dictionary.
+  """
+  
+  index: int
+  char: str
+  
+  def __repr__(self) -> str:
+    """
+      >>> token = Token(1, "c")
+      >>> repr(token)
+      '(1, c)'
+      >>> str(token)
+      '(1, c)'
+    """
+    return f"({self.index}, {self.char})"
+  
+
+class LZ78Compressor:
+  """
+    Class containing compress and decompress methods using LZ78 compression algorithm.
+  """
+  
+  def compress(self, text: str) -> list[Token]:
+    """
+      Compress the given string text using LZ78 compression algorithm.
+
+      Args:
+        text: string to be compressed
+
+      Returns:
+        output: the compressed text as a list of Tokens
+      
+      Tests:
+        >>> lz78_compressor = LZ78Compressor()
+        >>> str(lz78_compressor.compress("ababcbababaa"))
+        '[(0, a), (0, b), (1, b), (0, c), (2, a), (5, b), (1, a)]'
+        >>> str(lz78_compressor.compress("aacaacabcabaaac"))
+        '[(0, a), (1, c), (1, a), (0, c), (1, b), (4, a), (0, b), (3, a)]'
+    """
+
+    phrase_dict = {}
+    tokens = []
+    code = 1
+    phrase = ''
+    for char in text:
+      phrase += char
+      if phrase not in phrase_dict:
+        phrase_dict[phrase] = str(code)
+        if len(phrase) == 1:
+          tokens.append(Token('0', phrase))
+        else:
+          tokens.append(Token(phrase_dict[phrase[:-1]], phrase[-1]))
+        code += 1
+        phrase = ''
+    return tokens
+  
+  
+  def decompress(self, tokens: list[Token]) -> str:
+    """
+        Convert the list of tokens into an output string.
+
+        Args:
+          tokens: list containing pairs (index, char)
+
+        Returns:
+          output: decompressed text
+          
+        Tests:
+          >>> lz78_compressor = LZ78Compressor()
+          >>> lz78_compressor.decompress([Token(0, 'c'), Token(0, 'a'), Token(0, 'b'), Token(0, 'r'), Token(2, 'c'), 
+          ... Token(2, 'd'), Token(2, 'b'), Token(4, 'a'), Token(4, 'r'), Token(2, 'r'), Token(8, 'd')])
+          'cabracadabrarrarrad'
+          >>> lz78_compressor.decompress([Token(0, 'a'), Token(0, 'b'), Token(1, 'b'), Token(0, 'c'), 
+          ... Token(2, 'a'), Token(5, 'b'), Token(1, 'a')])
+          'ababcbababaa'
+          >>> lz78_compressor.decompress([Token(0, 'a'), Token(1, 'c'), Token(1, 'a'), Token(0, 'c'), 
+          ... Token(1, 'b'), Token(4, 'a'), Token(0, 'b'), Token(3, 'a')])
+          'aacaacabcabaaa'
+    """
+
+    text = ''
+    phrase_dict = {'0': ''}
+    code = 1
+    for token in tokens:
+      phrase = phrase_dict[str(token.index)] + token.char
+      phrase_dict[str(code)] = phrase
+      code += 1
+      text += phrase
+    return text
+  
+  
+if __name__ == '__main__':
+  from doctest import testmod
+  
+  testmod()
+  
+  
+  lz78_compressor = LZ78Compressor()
+  
+  # Example
+  text = 'aacaacabcabaaa'
+  tokens = lz78_compressor.compress(text)
+  decompressedText = lz78_compressor.decompress(tokens)
+  assert decompressedText == text, 'Invalid result.'