Caesar cipher

diff --git a/cipher.py b/cipher.py
new file mode 100644 (file)
index 0000000..e3c183d
--- /dev/null
+++ b/cipher.py
@@ -0,0 +1,73 @@
+import string
+
+def caesar_encipher_letter(letter, shift):
+    """Encipher a letter, given a shift amount
+
+    >>> caesar_encipher_letter('a', 1)
+    'b'
+    >>> caesar_encipher_letter('a', 2)
+    'c'
+    >>> caesar_encipher_letter('b', 2)
+    'd'
+    >>> caesar_encipher_letter('x', 2)
+    'z'
+    >>> caesar_encipher_letter('y', 2)
+    'a'
+    >>> caesar_encipher_letter('z', 2)
+    'b'
+    >>> caesar_encipher_letter('z', -1)
+    'y'
+    >>> caesar_encipher_letter('a', -1)
+    'z'
+    """
+    if letter in string.ascii_letters:
+        if letter in string.ascii_uppercase:
+            alphabet_start = ord('A')
+        else:
+            alphabet_start = ord('a')
+        return chr(((ord(letter) - alphabet_start + shift) % 26) + 
+                   alphabet_start)
+    else:
+        return letter
+
+def caesar_decipher_letter(letter, shift):
+    """Decipher a letter, given a shift amount
+    
+    >>> caesar_decipher_letter('b', 1)
+    'a'
+    >>> caesar_decipher_letter('b', 2)
+    'z'
+    """
+    return caesar_encipher_letter(letter, -shift)
+
+def caesar_encipher(message, shift):
+    """Encipher a message with the Caesar cipher of given shift
+    
+    >>> caesar_encipher('abc', 1)
+    'bcd'
+    >>> caesar_encipher('abc', 2)
+    'cde'
+    >>> caesar_encipher('abcxyz', 2)
+    'cdezab'
+    >>> caesar_encipher('ab cx yz', 2)
+    'cd ez ab'
+    """
+    enciphered = [caesar_encipher_letter(l, shift) for l in message]
+    return ''.join(enciphered)
+
+def caesar_decipher(message, shift):
+    """Decipher a message with the Caesar cipher of given shift
+    
+    >>> caesar_decipher('bcd', 1)
+    'abc'
+    >>> caesar_decipher('cde', 2)
+    'abc'
+    >>> caesar_decipher('cd ez ab', 2)
+    'ab cx yz'
+    """
+    return caesar_encipher(message, -shift)
+
+
+if __name__ == "__main__":
+    import doctest
+    doctest.testmod()