caesar.c

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

static char *ALPHABET = "abcdefghijklmnopqrstuvwxyz";

static void rotate(char **string, int r)
{
	char *s = *string;
	int i, c;
	r %= 26;
	for(i = 0; i < strlen(s); i++) {
		c = s[i] | 0x20;
		if('a' <= c && c <= 'z') {
			s[i] = ALPHABET[((int)(('z' - 'a' + 1) + (c - 'a')) + r) % 26];
		}
	}
}

int main(int argc, char **argv)
{
	int c;
	char *_string = NULL;
	char *_rot = NULL;

	while((c = getopt(argc, argv, "s:r:")) != -1) {
		switch(c) {
		case 's':
			_string = optarg;
			break;
		case 'r':
			_rot = optarg;
			break;
		default:
			printf("Usage: caesar -s string -r rotation\n");
			return EXIT_FAILURE;
		}
	}
	if(_string == NULL || _rot == NULL) {
		printf("Usage: caesar -s string -r rotation\n");
		return EXIT_FAILURE;
	}
	rotate(&_string, atoi(_rot));
	printf("%s\n", _string);

	return EXIT_SUCCESS;
}