data_compression.move_to_front
==============================

.. py:module:: data_compression.move_to_front

.. autoapi-nested-parse::

   Move-to-front transform.

   The move-to-front transform encodes each symbol as its current index in an
   ordered alphabet, then moves that symbol to the front of the alphabet.
   It is commonly used after the Burrows-Wheeler transform in lossless
   compression pipelines.

   Reference: https://en.wikipedia.org/wiki/Move-to-front_transform



Functions
---------

.. autoapisummary::

   data_compression.move_to_front._validated_alphabet
   data_compression.move_to_front.move_to_front_decode
   data_compression.move_to_front.move_to_front_encode


Module Contents
---------------

.. py:function:: _validated_alphabet(alphabet: str) -> list[str]

   Return a mutable alphabet list after validating uniqueness.

   >>> _validated_alphabet("abc")
   ['a', 'b', 'c']
   >>> _validated_alphabet("aba")
   Traceback (most recent call last):
       ...
   ValueError: alphabet must contain unique characters


.. py:function:: move_to_front_decode(encoded_text: list[int], alphabet: str) -> str

   Decode a move-to-front encoded list of indexes.

   >>> move_to_front_decode([1, 1, 13, 1, 1, 1], "abcdefghijklmnopqrstuvwxyz")
   'banana'
   >>> move_to_front_decode([1, 1, 2, 1, 1, 1], "abn")
   'banana'
   >>> move_to_front_decode([], "abc")
   ''
   >>> move_to_front_decode([3], "abc")
   Traceback (most recent call last):
       ...
   ValueError: index 3 is not valid for alphabet size 3
   >>> move_to_front_decode([-1], "abc")
   Traceback (most recent call last):
       ...
   ValueError: index -1 is not valid for alphabet size 3


.. py:function:: move_to_front_encode(text: str, alphabet: str) -> list[int]

   Encode text using the move-to-front transform.

   >>> move_to_front_encode("banana", "abcdefghijklmnopqrstuvwxyz")
   [1, 1, 13, 1, 1, 1]
   >>> move_to_front_encode("banana", "abn")
   [1, 1, 2, 1, 1, 1]
   >>> move_to_front_encode("", "abc")
   []
   >>> move_to_front_encode("bad", "abc")
   Traceback (most recent call last):
       ...
   ValueError: character 'd' is not in the alphabet