Add Polybius cipher (#5409)

* Add polybius cipher

* Fix polybius.py build issues and add test

ciphers/polybius.py

@@ -0,0 +1,96 @@
+#!/usr/bin/env python3
+
+"""
+A Polybius Square is a table that allows someone to translate letters into numbers.
+
+https://www.braingle.com/brainteasers/codes/polybius.php
+"""
+
+import numpy as np
+
+
+class PolybiusCipher:
+    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(PolybiusCipher().letter_to_numbers('a'), [1,1])
+        True
+
+        >>> np.array_equal(PolybiusCipher().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
+
+        >>> PolybiusCipher().numbers_to_letter(4, 5) == "u"
+        True
+
+        >>> PolybiusCipher().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
+
+        >>> PolybiusCipher().encode("test message") == "44154344 32154343112215"
+        True
+
+        >>> PolybiusCipher().encode("Test Message") == "44154344 32154343112215"
+        True
+        """
+        message = message.lower()
+        message = message.replace("j", "i")
+
+        encoded_message = ""
+        for letter_index in range(len(message)):
+            if message[letter_index] != " ":
+                numbers = self.letter_to_numbers(message[letter_index])
+                encoded_message = encoded_message + str(numbers[0]) + str(numbers[1])
+            elif message[letter_index] == " ":
+                encoded_message = encoded_message + " "
+
+        return encoded_message
+
+    def decode(self, message: str) -> str:
+        """
+        Return the decoded version of message according to the polybius cipher
+
+        >>> PolybiusCipher().decode("44154344 32154343112215") == "test message"
+        True
+
+        >>> PolybiusCipher().decode("4415434432154343112215") == "testmessage"
+        True
+        """
+        message = message.replace(" ", "  ")
+        decoded_message = ""
+        for numbers_index in range(int(len(message) / 2)):
+            if message[numbers_index * 2] != " ":
+                index1 = message[numbers_index * 2]
+                index2 = message[numbers_index * 2 + 1]
+
+                letter = self.numbers_to_letter(int(index1), int(index2))
+                decoded_message = decoded_message + letter
+            elif message[numbers_index * 2] == " ":
+                decoded_message = decoded_message + " "
+
+        return decoded_message