encoding

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#include <ctype.h>
#include <stdio.h>
#include <string.h>

#define LETTERS_IN_ALPHABET (26)

char encode_character(char character_to_encode, int character_position)
{
    char temp = character_to_encode - 'A' + character_position;
    temp %= LETTERS_IN_ALPHABET;
    return (temp + LETTERS_IN_ALPHABET) % LETTERS_IN_ALPHABET + 'A';
}

char decode_character(char character_to_decode, int character_position)
{
    char temp = character_to_decode - 'A' - character_position;
    temp %= LETTERS_IN_ALPHABET;
    return (temp + LETTERS_IN_ALPHABET) % LETTERS_IN_ALPHABET + 'A';
}

int main(int argc, char *argv[])
{
    if (argc == 1) {
        printf("Please provide text to encode or decode.\n");
        return 0;
    }

    char (*function_to_use)(char, int);
    int argstart = 1;

    argv[1][1] = toupper(argv[1][1]);
    if (strcmp("-E", argv[1]) == 0) {
        function_to_use = encode_character;
        argstart++;
    } else if (strcmp("-D", argv[1]) == 0) {
        function_to_use = decode_character;
        argstart++;
    } else {
        function_to_use = encode_character;
    }

    for (int i = argstart, j = 0; i < argc; i++) {
        char *string = argv[i];
        while (*string) {
            if (isalpha(*string)) {
                printf("%c", function_to_use(toupper(*string), j));
                j++;
            } else if (*string == ' ') {
                printf(" ");
            }
            *string++;
        }
        printf(" ");
    }
    printf("\n");
    return 0;
}