import argparse 

def encode_shift_cipher(text, key):
    result = ""
    alphabet = 'abcdefghijklmnopqrstuvwxyz'
    shift_direction = 1
    shift_count = 0
    
    for char in text:
        if char.isalpha():
            shift = int(key[shift_count]) * shift_direction
            index = alphabet.index(char.lower())
            
            # Calculate the new index and wrap around if it's less than 0
            new_index = (index + shift) % 26
            if new_index < 0:
                new_index += 26  # Wrap around to 'z'
            
            result += alphabet[new_index]
            
            shift_direction *= -1
            shift_count = (shift_count + 1) % len(key)
        else:
            result += char
    
    return result

def decode_shift_cipher(text, key):
    result = ""
    alphabet = 'abcdefghijklmnopqrstuvwxyz'
    shift_direction = 1
    shift_count = 0
    
    for char in text:
        if char.isalpha():
            shift = int(key[shift_count]) * shift_direction
            index = alphabet.index(char.lower())
            
            # Calculate the new index and wrap around if it's less than 0
            new_index = (index - shift) % 26
            if new_index < 0:
                new_index += 26  # Wrap around to 'z'
            
            result += alphabet[new_index]
            
            shift_direction *= -1
            shift_count = (shift_count + 1) % len(key)
        else:
            result += char
    
    return result


def main():
    parser = argparse.ArgumentParser(description="Shift Cipher Encoder/Decoder")
    parser.add_argument("-e", "--encode", action="store_true", help="Encode the input")
    parser.add_argument("-d", "--decode", action="store_true", help="Decode the input")
    parser.add_argument("-k", "--key", required=True, help="Encryption/Decryption key (e.g., '1234')")
    parser.add_argument("-i", "--input", help="Input file to process")
    parser.add_argument("-o", "--output", help="Output file (optional)")
    parser.add_argument("-s", "--string", help="Input string to process")

    args = parser.parse_args()

    if not args.encode and not args.decode:
        print("Please specify whether to encode or decode using -e or -d.")
        return

    if args.encode and args.decode:
        print("Please choose either -e (encode) or -d (decode), not both.")
        return

    if args.input and args.string:
        print("Please choose either -i (input file) or -s (input string), not both.")
        return

    if args.input:
        # Read input from a file
        try:
            with open(args.input, "r") as file:
                input_text = file.read()
        except FileNotFoundError:
            print(f"Error: Input file '{args.input}' not found.")
            return
    elif args.string:
        # Use the input string
        input_text = args.string
    else:
        # Read input from the command line
        input_text = input("Enter the text to encode/decode: ")

    key = args.key

    if args.encode:
        output_text = encode_shift_cipher(input_text, key)
    else:
        output_text = decode_shift_cipher(input_text, key)

    if args.output:
        # Write the result to an output file
        with open(args.output, "w") as file:
            file.write(output_text)
        print(f"Output written to '{args.output}'")
    else:
        # Print the result to the console
        print("Result:")
        print(output_text)

if __name__ == "__main__":
    main()