Roman numeral parser

roman.c

#include <stdio.h>

enum { I, V, X, L, C, D, M, StackSize };

const int values[] = { 1, 5, 10, 50, 100, 500, 1000 };

int
unroman(const char *s)
{
	int stack[StackSize];
	int i, n, sum;

	for(i = 0; i < StackSize; i++)
		stack[i] = 0;

	for(; *s != '\0'; s++) {
		switch(*s) {
		case 'I': n = I; break;
		case 'V': n = V; break;
		case 'X': n = X; break;
		case 'L': n = L; break;
		case 'C': n = C; break;
		case 'D': n = D; break;
		case 'M': n = M; break;
		default:
			return -1;
		}
		sum = values[n];
		for(i = 0; i < n; i++) {
			sum -= stack[i];
			stack[i] = 0;
		}
		stack[n] += sum;
	}
	sum = 0;
	for(i = 0; i < StackSize; i++)
		sum += stack[i];
	return sum;
}

int
main(int argc, char **argv)
{
	int i, n;

	for(i = 1; i < argc; i++) {
		n = unroman(argv[i]);
		if(n < 0) {
			fprintf(stderr, "%s: invalid numeral: %s\n", argv[0], argv[i]);
			continue;
		}
		printf("%s = %d\n", argv[i], n);
	}
	return 0;
}