Add Bifid cipher (#5493)

* Add Bifid cipher

* Add missing type hint

* Fix variable names

ciphers/bifid.py

@@ -0,0 +1,110 @@
+#!/usr/bin/env python3
+
+"""
+The Bifid Cipher uses a Polybius Square to encipher a message in a way that
+makes it fairly difficult to decipher without knowing the secret.
+
+https://www.braingle.com/brainteasers/codes/bifid.php
+"""
+
+import numpy as np
+
+
+class BifidCipher:
+    def __init__(self) -> None:
+        SQUARE = [
+            ["a", "b", "c", "d", "e"],
+            ["f", "g", "h", "i", "k"],
+            ["l", "m", "n", "o", "p"],
+            ["q", "r", "s", "t", "u"],
+            ["v", "w", "x", "y", "z"],
+        ]
+        self.SQUARE = np.array(SQUARE)
+
+    def letter_to_numbers(self, letter: str) -> np.ndarray:
+        """
+        Return the pair of numbers that represents the given letter in the
+        polybius square
+
+        >>> np.array_equal(BifidCipher().letter_to_numbers('a'), [1,1])
+        True
+
+        >>> np.array_equal(BifidCipher().letter_to_numbers('u'), [4,5])
+        True
+        """
+        index1, index2 = np.where(self.SQUARE == letter)
+        indexes = np.concatenate([index1 + 1, index2 + 1])
+        return indexes
+
+    def numbers_to_letter(self, index1: int, index2: int) -> str:
+        """
+        Return the letter corresponding to the position [index1, index2] in
+        the polybius square
+
+        >>> BifidCipher().numbers_to_letter(4, 5) == "u"
+        True
+
+        >>> BifidCipher().numbers_to_letter(1, 1) == "a"
+        True
+        """
+        letter = self.SQUARE[index1 - 1, index2 - 1]
+        return letter
+
+    def encode(self, message: str) -> str:
+        """
+        Return the encoded version of message according to the polybius cipher
+
+        >>> BifidCipher().encode('testmessage') == 'qtltbdxrxlk'
+        True
+
+        >>> BifidCipher().encode('Test Message') == 'qtltbdxrxlk'
+        True
+
+        >>> BifidCipher().encode('test j') == BifidCipher().encode('test i')
+        True
+        """
+        message = message.lower()
+        message = message.replace(" ", "")
+        message = message.replace("j", "i")
+
+        first_step = np.empty((2, len(message)))
+        for letter_index in range(len(message)):
+            numbers = self.letter_to_numbers(message[letter_index])
+
+            first_step[0, letter_index] = numbers[0]
+            first_step[1, letter_index] = numbers[1]
+
+        second_step = first_step.reshape(2 * len(message))
+        encoded_message = ""
+        for numbers_index in range(len(message)):
+            index1 = int(second_step[numbers_index * 2])
+            index2 = int(second_step[(numbers_index * 2) + 1])
+            letter = self.numbers_to_letter(index1, index2)
+            encoded_message = encoded_message + letter
+
+        return encoded_message
+
+    def decode(self, message: str) -> str:
+        """
+        Return the decoded version of message according to the polybius cipher
+
+        >>> BifidCipher().decode('qtltbdxrxlk') == 'testmessage'
+        True
+        """
+        message = message.lower()
+        message.replace(" ", "")
+        first_step = np.empty(2 * len(message))
+        for letter_index in range(len(message)):
+            numbers = self.letter_to_numbers(message[letter_index])
+            first_step[letter_index * 2] = numbers[0]
+            first_step[letter_index * 2 + 1] = numbers[1]
+
+        second_step = first_step.reshape((2, len(message)))
+        decoded_message = ""
+        for numbers_index in range(len(message)):
+            index1 = int(second_step[0, numbers_index])
+            index2 = int(second_step[1, numbers_index])
+            letter = self.numbers_to_letter(index1, index2)
+            decoded_message = decoded_message + letter
+
+        return decoded_message