c3 assignment

//
//  main.c
//  c3
//
//  Created by Betsalel Williamson on 9/15/11.
//
//  Chapter 3:
//  Write an encryption and decryption program.
//  Encrypt each digit by adding 7 and taking the remainder after division by 10.
//  After encrypting each digit, swap the first and third then swap the second and fourth.
//  Decryption should reverse the process.
//  Program must input a single integer.
//  Program must do encode and decode in one file.

#include <stdio.h>
#include <stdlib.h>


//below is how to get the
#define ONES_PLACE(A)      (A/1)    -    ((A/10) *10)
#define TENS_PLACE(A)      (A/10)   -   ((A/100) *10)
#define HUNDREDS_PLACE(A)  (A/100)  -  ((A/1000) *10)
#define THOUSANDS_PLACE(A) (A/1000) - ((A/10000) *10)

static int GetInputMessage (void) {
    int input;

    printf("Enter a four digit number: ");
    scanf("%d", &input);

    return input;
}

static int GetChoiceMessage (void) {
    int input;

    printf("Encode (1) Decode (2): ");
    scanf("%d", &input);

    return input;
}

static int GetContinueMessage (void) {
    int input;

    printf("Continue (1) Exit (0): ");
    scanf("%d", &input);

    if (input != 0) {
        input = GetChoiceMessage();
    }

    return input;
}

static int Encode (int notCode) {

    // get the 4 digits
    int fourthDigit = ONES_PLACE(notCode);
    int thirdDigit  = TENS_PLACE(notCode);
    int secondDigit = HUNDREDS_PLACE(notCode);
    int firstDigit  = THOUSANDS_PLACE(notCode);

    //  the below commented out code is psudeo-degugging tools
    //    printf("notcode %d\n", notCode);
    //    printf("%d, %d, %d, %d\n", firstDigit, secondDigit, thirdDigit, fourthDigit);

    fourthDigit += 7;
    thirdDigit  += 7;
    secondDigit += 7;
    firstDigit  += 7;

    //    printf("notcode %d\n", notCode);
    //    printf("%d, %d, %d, %d\n", firstDigit, secondDigit, thirdDigit, fourthDigit);

    fourthDigit = fourthDigit%10;
    thirdDigit  = thirdDigit%10;
    secondDigit = secondDigit%10;
    firstDigit  = firstDigit%10;

    //    printf("notcode %d\n", notCode);
    //    printf("%d, %d, %d, %d\n", firstDigit, secondDigit, thirdDigit, fourthDigit);

    return printf("Encoded Digits: %d%d%d%d\n", thirdDigit, fourthDigit, firstDigit, secondDigit);
}

static int Decode (int code) {

    //get the four digits
    int fourthDigit = ONES_PLACE(code);
    int thirdDigit  = TENS_PLACE(code);
    int secondDigit = HUNDREDS_PLACE(code);
    int firstDigit  = THOUSANDS_PLACE(code);

    //  the below commented out code is psudeo-degugging tools
    //    printf("code %d\n", code);
    //    printf("%d, %d, %d, %d\n", firstDigit, secondDigit, thirdDigit, fourthDigit);

    fourthDigit = fourthDigit+10;
    thirdDigit  = thirdDigit+10;
    secondDigit = secondDigit+10;
    firstDigit  = firstDigit+10;

    //    printf("code %d\n", code);
    //    printf("%d, %d, %d, %d\n", firstDigit, secondDigit, thirdDigit, fourthDigit);

    fourthDigit -= 7;
    thirdDigit  -= 7;
    secondDigit -= 7;
    firstDigit  -= 7;

    //    printf("code %d\n", code);
    //    printf("%d, %d, %d, %d\n", firstDigit, secondDigit, thirdDigit, fourthDigit);

    fourthDigit = fourthDigit%10;
    thirdDigit  = thirdDigit%10;
    secondDigit = secondDigit%10;
    firstDigit  = firstDigit%10;

    //    printf("code %d\n", code);
    //    printf("%d, %d, %d, %d\n", firstDigit, secondDigit, thirdDigit, fourthDigit);

    return printf("Decoded Digits: %d%d%d%d\n", thirdDigit, fourthDigit, firstDigit, secondDigit);
}


int main (int argc, const char * argv[])
{
    //  the input is declared and initialized before the while loop to make sure that it's not 0.
    int input = GetChoiceMessage();

    while (input != 0) {

        if (input == 1) {
            input = GetInputMessage();
            Encode(input);
        }

        else if (input == 2) {
            input = GetInputMessage();
            Decode(input);
        }

        input = GetContinueMessage();
    }

    return EXIT_SUCCESS;
}