Faseeh Irfan Shopify Playfair Cipher Solver

python/solution.py

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+# Shopify Engineering Internship Challenge
+# Playfair Cipher Solver
+# Faseeh Irfan
+# University of Waterloo Software Engineering
+
+class PlayFairCipher:
+    # Constructor, I wanted to generalize this so it can work for different keys.
+    # Hence why it takes in the key and message. 
+    def __init__(self, key, message):
+        self.key = key
+        self.message = message
+        self.grid = self.gridGen(key)
+
+    # Generated the grid based on the key
+    def gridGen(self, key):
+        # Excluding J so we can have 5x5 grid,
+        # We can technically pick any letter as long as both parties know,
+        # but J is most common
+        letters = 'ABCDEFGHIKLMNOPQRSTUVWXYZ' 
+
+        # orignally thought about using 2D Lists, but using a single list
+        # and using modulo 5 to get the row and col is much simpler
+        grid = []
+        seen = set()
+        # first append the key to beginning of the grid 
+        for char in key:
+            if char not in seen:
+                grid.append(char)
+                seen.add(char)
+        # then append the rest of the letters
+        for char in letters:
+            if char not in seen:
+                grid.append(char)
+                seen.add(char)
+        return grid
+
+    def findCharPos(self, char):
+        # Returns the Row and Col of the character in the grid
+        # Using division and modulo on the index of the character, 
+        # since I've implemented the grid as a 1D array
+        ind = self.grid.index(char)
+        return ind // 5, ind % 5
+
+    def findChar(self, r, c):
+        # Returns the character at position (r, c) in the grid
+        return self.grid[r*5 + c]
+
+    def decryptPair(self, pair):
+        # Find the positions of the characters in the grid
+        r1, c1 = self.findCharPos(pair[0])
+        r2, c2 = self.findCharPos(pair[1])
+
+        # check if the pair is in same row or same column
+        # using modulo to wrap around the grid
+        if r1 == r2: #shift left
+            c1 = (c1 - 1) % 5
+            c2 = (c2 - 1) % 5
+        elif c1 == c2: #shift up
+            r1 = (r1 - 1) % 5
+            r2 = (r2 - 1) % 5
+        else: # if they form a diagonal, swap the columns
+            c1, c2 = c2, c1
+
+        decryptedPair = self.findChar(r1, c1) + self.findChar(r2, c2)
+        # getting rid of X's in the new pair and return
+        return decryptedPair.replace('X', '')
+
+    def decryption(self):    
+        result = ''
+
+        # loop through the message and decrypt pair by pair, appending to result as we go
+        for i in range(0, len(self.message), 2):
+            pair = self.message[i:i+2]
+            result += self.decryptPair(pair)
+
+        return result
+
+if __name__ == "__main__":
+    message = "IKEWENENXLNQLPZSLERUMRHEERYBOFNEINCHCV"
+    key = "SUPERSPY"
+    cipher = PlayFairCipher(key, message)
+    result = cipher.decryption()
+    print(result)
+