added doctest to playfair_cipher.py (#10823)

* added doctest to playfair_cipher.py

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Co-authored-by: Keyboard-1 <142900182+Keyboard-1@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Tianyi Zheng <tianyizheng02@gmail.com>

ciphers/playfair_cipher.py

@@ -1,3 +1,24 @@
+"""
+https://en.wikipedia.org/wiki/Playfair_cipher#Description
+
+The Playfair cipher was developed by Charles Wheatstone in 1854
+It's use was heavily promotedby Lord Playfair, hence its name
+
+Some features of the Playfair cipher are:
+
+1) It was the first literal diagram substitution cipher
+2) It is a manual symmetric encryption technique
+3) It is a multiple letter encryption cipher
+
+The implementation in the code below encodes alphabets only.
+It removes spaces, special characters and numbers from the
+code.
+
+Playfair is no longer used by military forces because of known
+insecurities and of the advent of automated encryption devices.
+This cipher is regarded as insecure since before World War I.
+"""
+
 import itertools
 import string
 from collections.abc import Generator, Iterable
@@ -60,11 +81,26 @@ def generate_table(key: str) -> list[str]:
 
 
 def encode(plaintext: str, key: str) -> str:
+    """
+    Encode the given plaintext using the Playfair cipher.
+    Takes the plaintext and the key as input and returns the encoded string.
+
+    >>> encode("Hello", "MONARCHY")
+    'CFSUPM'
+    >>> encode("attack on the left flank", "EMERGENCY")
+    'DQZSBYFSDZFMFNLOHFDRSG'
+    >>> encode("Sorry!", "SPECIAL")
+    'AVXETX'
+    >>> encode("Number 1", "NUMBER")
+    'UMBENF'
+    >>> encode("Photosynthesis!", "THE SUN")
+    'OEMHQHVCHESUKE'
+    """
+
     table = generate_table(key)
     plaintext = prepare_input(plaintext)
     ciphertext = ""
 
-    # https://en.wikipedia.org/wiki/Playfair_cipher#Description
     for char1, char2 in chunker(plaintext, 2):
         row1, col1 = divmod(table.index(char1), 5)
         row2, col2 = divmod(table.index(char2), 5)
@@ -83,10 +119,20 @@ def encode(plaintext: str, key: str) -> str:
 
 
 def decode(ciphertext: str, key: str) -> str:
+    """
+    Decode the input string using the provided key.
+
+    >>> decode("BMZFAZRZDH", "HAZARD")
+    'FIREHAZARD'
+    >>> decode("HNBWBPQT", "AUTOMOBILE")
+    'DRIVINGX'
+    >>> decode("SLYSSAQS", "CASTLE")
+    'ATXTACKX'
+    """
+
     table = generate_table(key)
     plaintext = ""
 
-    # https://en.wikipedia.org/wiki/Playfair_cipher#Description
     for char1, char2 in chunker(ciphertext, 2):
         row1, col1 = divmod(table.index(char1), 5)
         row2, col2 = divmod(table.index(char2), 5)
@@ -102,3 +148,12 @@ def decode(ciphertext: str, key: str) -> str:
             plaintext += table[row2 * 5 + col1]
 
     return plaintext
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()
+
+    print("Encoded:", encode("BYE AND THANKS", "GREETING"))
+    print("Decoded:", decode("CXRBANRLBALQ", "GREETING"))