Added move to front algorithm

data_compression/move_to_front.py

@@ -0,0 +1,118 @@
+"""
+Move to Front (MTF) is a data compression technique that reorders
+symbols based on their frequency of occurrence.
+    - type of context based methods
+    - it lays somewhere between coding and transforming data
+    - it is core of the Burrows-Wheeler Transform (BWT)
+    - for simplicity we do not distinguish between upper and lower case
+
+
+Sources:
+    - https://en.wikipedia.org/wiki/Move-to-front_transform
+    - https://www.mbit.edu.in/wp-content/uploads/2020/05/data_compression.pdf
+        (chapter 6.4.1)
+"""
+
+english_alphabet = [
+    "a",
+    "b",
+    "c",
+    "d",
+    "e",
+    "f",
+    "g",
+    "h",
+    "i",
+    "j",
+    "k",
+    "l",
+    "m",
+    "n",
+    "o",
+    "p",
+    "q",
+    "r",
+    "s",
+    "t",
+    "u",
+    "v",
+    "w",
+    "x",
+    "y",
+    "z",
+]
+
+
+class MoveToFront:
+    """
+    Core class doing encoding and also decoding.
+
+    >>> mtf = MoveToFront()
+    >>> list(mtf.encode_text("algorithm"))
+    [0, 11, 7, 14, 17, 11, 19, 12, 15]
+    >>> mtf.decode_text([0, 11, 7, 14, 17, 11, 19, 12, 15])
+    'algorithm'
+    """
+
+    def __init__(self, source_alphabet: list[str] = english_alphabet) -> None:
+        """
+        If the source alphabet is not provided, it will be set to the english.
+        The class does not distinguish between UPPER and lower case.
+        """
+        self.source_alphabet = list(source_alphabet)
+
+    def encode_text(self, plain_text: str) -> list[int]:
+        """
+        Encodes given text. The output is list of char positions in the alphabet.
+
+        >>> mtf = MoveToFront()
+        >>> list(mtf.encode_text("algorithm"))
+        [0, 11, 7, 14, 17, 11, 19, 12, 15]
+        """
+
+        if not isinstance(plain_text, str):
+            raise TypeError("The parameter plain_text type must be str.")
+
+        # making a copy so we do not rotate the original alphabet
+        alphabet = list(self.source_alphabet)
+        encoded_text = []
+
+        for char in plain_text.lower():
+            # find the position of the char in the alphabet and add it to the result
+            char_position_in_alphabet = alphabet.index(char)
+            encoded_text.append(char_position_in_alphabet)
+
+            # move our char to the front of the alphabet
+            alphabet.pop(char_position_in_alphabet)
+            alphabet.insert(0, char)
+
+        return encoded_text
+
+    def decode_text(self, compressed_text: list[int]) -> str:
+        """
+        Decodes given text. The input is list of char positions in the alphabet.
+
+        >>> mtf = MoveToFront()
+        >>> mtf.decode_text([0, 11, 7, 14, 17, 11, 19, 12, 15])
+        'algorithm'
+        """
+
+        alphabet = list(self.source_alphabet)
+        decoded_text = []
+
+        for idx in compressed_text:
+            # find corresponding chart to given index
+            char = alphabet[idx]
+            decoded_text.append(char)
+
+            # move found char to the front of the alphabet
+            alphabet.pop(idx)
+            alphabet.insert(0, char)
+
+        return "".join(decoded_text)
+
+
+if __name__ == "__main__":
+    mtf = MoveToFront()
+    print(mtf.encode_text("algorithm"))
+    print(mtf.decode_text([0, 11, 7, 14, 17, 11, 19, 12, 15]))