#include <errno.h> /* errno */
#include <stdlib.h> /* strtol(3), size_t */
#if !defined EX_OK || !defined EX_USAGE
#	include <sysexits.h> /* EX_OK, EX_USAGE */
#endif

CHAR caesar(CHAR c, size_t r);

static char *program_name = "rot";

int main(int argc, char *argv[]){
	CHAR c;
	size_t r;

	if(argc > 2){
		fprintf(stderr, "Usage: %s (rotation)\n",
			argv[0] == NULL ? program_name : argv[0]
		);
		return EX_USAGE;
	}

	if(argc == 2){
		r = strtol(argv[1], &argv[1], 10);
		if(*argv[1] != '\0' || errno != 0){
			fprintf(stderr, "%s: Invalid rotation\n", argv[0]);
			return EX_USAGE;
		}
	}else
		r = 13; /* default to rot13 */


	while((c = GETC(stdin)) != ENDOFFILE)
		if(PUTC(caesar(c, r), stdout) == ENDOFFILE)
			break;
	
	return EX_OK;
}