/* As much of a calculator as I could
 * write in 20 minutes on my cellphone.
 * Evaluates postfix integer arithmetic.
 * Example, print((3 + 4) * (5 - 6)):
 *
 * ./postfix 3 4 + 5 6 - . ,
 *
 * Some limitations/bugs:
 * - No negative numbers
 * - No decimals/fractions
 * - Uses . for multiplication (* is
 *   inconvenient in the Unix shell)
 * - Only prints out numbers when
 *   explicitly requested with ,
 * - Ignores invalid input instead
 *   of barfing
 * - No vectors
 * - No macros
 * - Fails to detect stack underflow
 *   and overflow
 * - has an unused variable y and a
 *   copy of argv in s (intended for
 *   macros)
 * - I think C99 or something added
 *   switch case range values?  No,
 *   '0'...'9' is a GCC extension still:
 * https://gcc.gnu.org/onlinedocs/gcc/Case-Ranges.html
 *
 * On looking at the generated
 * assembly, I have additional
 * regrets:
 * - making sp a global prevents GCC
 *   from allocating it to a
 *   register
 * - using int rather than size_t or
 *   long generates additional
 *   instructions for sign extension
 * - relying on GCC’s optimization
 *   of the switch() makes the inner
 *   interpreter loop 14
 *   instructions long, which is
 *   large; a table of &&labels
 *   would be significantly better
 */
#include <stdio.h>
#include <stdlib.h>

void say(int n)
{
	printf("• %d\n", n);
}

int stack[10];
int sp;
#define IF break; case
#define push(x) stack[sp++] = (x)
#define pop(x) stack[--sp]
int main(int argc, char **argv)
{
	char **s = argv;
	int tos = 0, y;
	while (--argc) {
		switch(**++s) {
		IF '+': tos += pop(); 
		IF '.': tos *= pop(); 
		IF '/': tos = pop() / tos; 
		IF '-': tos = pop() - tos; 
		IF ',': say(tos); tos = pop();
		IF '0': case '1': case '2':
		case '3': case '4': case '5':
		case '6': case '7': case '8':
		case '9':
			push(tos); tos = atoi(*s);
		}
	}
	return 0;
}