# Python Caeser Cipher Helper v1.1.0
# Usage (encrypt): python3 caeser-cipher.py encrypt secretmessage
# Usage (decrypt): python3 caeser-cipher.py decrypt secretmessage

import re # Import Regex tools
import string # Import string tools
import collections # Import tools to sort letters.
from sys import argv # Import tool to process command line arguments.

#if action == "help": # If error, print help https://stackoverflow.com/questions/4042452/display-help-message-with-python-argparse-when-script-is-called-without-any-argu
    #print("\nUsage (encrypt): python3 caeser-cipher.py encrypt secretmessage")
    #print("Usage (decrypt): python3 caeser-cipher.py decrypt secretmessage\n")

script, action, ciphertext = argv # Take command line input.

#ciphertext = input("Please type in your cipher text:") # Set variable cipertext to the input from user.
shiftNum = input("Please enter the shift you would like to use (Just hit enter if you don't know the shift used):") # Get input
#action = input("Please enter encrypt or decrypt):") # Get input
ciphertext = re.sub (r'([^a-zA-Z]+?)', '', ciphertext) # Remove all non-letters.
ciphertext = ciphertext.lower() # Make it all lowercase. https://www.geeksforgeeks.org/isupper-islower-lower-upper-python-applications/
letters = list(string.ascii_lowercase) # Use a list of lowercase letters. https://stackoverflow.com/questions/43918437/how-to-iterate-through-the-alphabet

def encrypt(ciphertext, shiftNum, letters): # Grab parameters

    # Create local variables for this function.

    letterCount = 0
    letterPos = 0
    answer = ""

    for c in ciphertext: # For every letter in the ciphertext
        while letterPos < 26: # While the letter's position in the alphabet is less than 26 (and thus valid), continue.

            if ciphertext[letterCount] == letters[letterPos]: # Match the letter in the ciphertext to a letter in the alphabet and once they match, continue.
                letter = int(letterPos) + int(shiftNum) # Take the position of the letter and the shift number and add them for the enciphered letter.

                if letter > 25: # If the enciphered letter's position is not valid because it is too high, fix it by rotating around the alphabet.
                    letter = letter - 26
                if letter < 0:
                    letter = letter + 26 # If the enciphered letter's position is not valid because it is too low, fix it by rotating around the alphabet.

                answer = answer + letters[letter] # Add letters together to get the enciphered text.

            letterPos = letterPos + 1 # Iterate through letter positions in the alphabet. (neccessary to find one that matches.)

        if letterPos > 25: # If the letter position is greater than 25, cycle back through the alphabet.
            letterPos = letterPos - 26

        letterCount = letterCount + 1 # Keep track of how many times we're doing this.

    print("\nYour enciphered text is: " + answer)


def decrypt(ciphertext, shiftNum, letters, commonletter): # Grab parameters

    # Create local variables for this function.

    letterCount = 0
    letterPos = 0
    answer = ""

    for c in ciphertext: # For every letter in the ciphertext
        while letterPos < 26: # While the letter's position in the alphabet is less than 26 (and thus valid), continue.

            if ciphertext[letterCount] == letters[letterPos]: # Match the letter in the ciphertext to a letter in the alphabet and once they match, continue.
                letter = int(letterPos) - int(shiftNum) # Take the position of the letter and the shift number and add them for the enciphered letter.

                if letter > 25: # If the enciphered letter's position is not valid because it is too high, fix it by rotating around the alphabet.
                    letter = letter - 26
                if letter < 0:
                    letter = letter + 26 # If the enciphered letter's position is not valid because it is too low, fix it by rotating around the alphabet.

                answer = answer + letters[letter] # Add letters together to get the enciphered text.

            letterPos = letterPos + 1 # Iterate through letter positions in the alphabet. (neccessary to find one that matches.)

        if letterPos > 25: # If the letter position is greater than 25, cycle back through the alphabet.
            letterPos = letterPos - 26

        letterCount = letterCount + 1 # Keep track of how many times we're doing this.

    print("\nYour decrypted text is: " + answer)


def freq(ciphertext):
    # Create local variables for this function.

    commonletter = 0
    letterCount = 0
    letterPos = 0
    answer = ""

    commonletter = collections.Counter(ciphertext).most_common()[0][0] # Find most common letter and thus the shift https://stackoverflow.com/questions/47251934/how-to-count-the-most-frequent-letter-in-a-string

    for c in ciphertext:
        while letterPos < 26:

            if not commonletter == 0:
                if commonletter == letters[letterPos]: # If the most frequent letter matches the current letter, set the shift to that letter because that is the shift for the cipher. Forgot the double equals for comparison.

                    shiftNum = letterPos - 4

            if letterPos > 25: # If the letter position is greater than 25, cycle back through the alphabet.
                letterPos = letterPos - 26

            letterPos = letterPos + 1 # Iterate through letter positions in the alphabet. (neccessary to find one that matches.)

    decrypt(ciphertext, shiftNum, letters, commonletter) # Decrypt using the frequency found in here.


if action == "encrypt":
    encrypt(ciphertext, shiftNum, letters) # Start the encrypt() function while passing 3 variables as parameters.
if action == "decrypt":
    if shiftNum == "":
        freq(ciphertext)
    else:
        decrypt(ciphertext, shiftNum, letters, commonletter)