*  This program takes input from a stream.
*  Encodes or Decodes a Caesar Cipher given
*  an offset.
*  A typical command line would be:
*  "cc_code d 7 < cipher.dat"
*  "cc_code d 7 < cipher.dat  > results.txt"
*  "cc_code e 6 < results.txt > cipher.dat"
*  "d" decodes, "e" encodes.                 */

#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <stdlib.h>
/*---------------------*/
#define A 65
#define Z 90
/*----------------------*/
char alpha[26];
char buffer[16000];
/*-----------------------*/
void fill_array(char *array,int offset);      /* Prototypes */
void print_array(char *array);
int lookup_letter(int letter,char *array);
int test_letter(int letter);
/*-----------------------*/
int main(int argc, char *argv[])
{
        int offset,i;
        char ch;
        if((argc != 3))
                {
                        printf("Use: %s  d offset < input_file (To Decode)\n", argv[0]);
                        printf("     %s  e offset < input_file (To Encode)\n", argv[0]);
                        return 1;
                }
        if (strcmp(argv[1],"d")==0)
                offset=(26 - atoi(argv[2]));
        else offset=atoi(argv[2]);
        fill_array(alpha,offset);
        for( i = 0;((ch = getchar()) != EOF); i++ )
        buffer[i] = (char)test_letter(ch);
        buffer[i] = '\0';
        printf( "%s\n", buffer );
        return 0;
}
/*-------------------*/
void fill_array(char *array,int offset)
{
        int i,j,k=0;
        for(i=(A + offset);i <=(Z + offset); i++)
                {
                        j=i;
                        if (j>Z)
                                {
                                        j=(j-26);  /* If we went past Z, go back around. */
                                }
                        array[k]=j;
                        k++;
                }
}
/*------------------*/
/* Kept this procedure for troubleshooting */
void print_array(char *array)
{
        int i;
        for(i=0;i <= 25;i++)
                {
                        printf("%c",array[i]);
                }
        printf("\n");
}
/*------------------*/
int lookup_letter(int letter,char *array)
{
        int i=0;
        while (array[i] != letter)
                {
                        i++;
                }
        return (i + A);
}
/*------------------*/
int test_letter(int letter)
{
        int ch;
        ch=toupper(letter);
        if isalpha(ch)
                return lookup_letter(ch,alpha);
        else
                return ch;
}
/*------------------*/