#include <stddef.h> /* stdin, stdout */

/* When rot13(1) is a part of ~trinity/src, which doesn't assume the system has
 * <sysexits.h> and generates it out of a table. */
#ifdef DONT_USE_SYSTEM_SYSEXITS
#	include "../include/sysexits.h" /* EX_OK, EX_USAGE */
#else
#	include <sysexits.h> /* EX_OK, EX_USAGE */
#endif

#ifdef U_UNICODE_VERSION /* libicu Unicode support */
#	define CHARACTER UChar32

#	include <uchar.h> /* u_isupper(3) */
#	define ISUPPER u_isupper

#	include <ustdio.h> /* U_EOF */
#	define ENDOFFILE U_EOF
#else /* ifdef U_UNICODE_VERSION */
#	define CHARACTER int

#	include <ctype.h> /* isalpha(3), isupper(3) */
#	define ISALPHA isalpha
#	define ISUPPER isupper

#	include <stdio.h> /* getc(3), putc(3) */
#	define ENDOFFILE EOF
#	define GETC getc
#	define PUTC putc
#endif /* ifdef U_UNICODE_VERSION */

static char *program_name = "rot13";

// /* Assumes this is a character in the provided glyph system.
//  * Probably not Unicode-capable but doesn't assume Alphabet.
//  * Returns `c` rotated around `a` by `r` characters. */
// #include <string.h> /* strchr(3), strlen(3) */
// static int
// caesar(char c, int r, const char *a){
// 	return a[(strchr(a, c) - a + r) % strlen(a)];
// }

/* Or... make some static arrays of the alphabet * 1.5, so c + '\0' + 13 is
 * always a valid index and always reflects the correct rotation (13 places).
 */
static char alpha_upper[] = "ABCDEFGHIJKLMNOPQRSTUVWXYZABCDEFGHIJKLM";
static char alpha_lower[] = "abcdefghijklmnopqrstuvwxyzabcdefghijklm";

int main(int argc, char *argv[]){

	if(argc > 1){
		fprintf(stderr, "Usage: %s\n",
			argv[0] == NULL ? program_name : argv[0]
		);
		return EX_USAGE;
	}

	{ /* rot13 */
		char *a; /* easier than doing freaky math */
		CHARACTER c;

		while((c = GETC(stdin)) != ENDOFFILE)
			if(isalpha(c)){
				a = isupper(c) ? alpha_upper : alpha_lower;
				PUTC(a[c - a[0] + 13], stdout);
			}else
				PUTC(c, stdout);
	}

	return EX_OK;
}