stupid-riddle.c

#include <stdio.h>

static inline void decompose(int i, char *buf) {
	buf[0] = i / 100;
	buf[1] = (i / 10) % 10;
	buf[2] = i % 10;
}

static inline int rule1(char *buf) {
	return buf[0] == 6 || buf[1] == 8 || buf[2] == 2;
}

static inline int rule2(char *buf) {
	return
		   buf[1] == 6 || buf[2] == 6
		|| buf[0] == 1 || buf[2] == 1
		|| buf[0] == 4 || buf[1] == 4;
}

static inline int rule3(char *buf) {
	int wrong = 0;
	wrong += (buf[1] == 2) ^ (buf[2] == 2);
	wrong += (buf[0] == 0) ^ (buf[2] == 0);
	wrong += (buf[0] == 6) ^ (buf[1] == 6);

	return wrong == 2;
}

static inline int rule4(char *buf) {
	for (int i = 0; i < 3; i++) {
		if (buf[i] == 7 || buf[i] == 3 || buf[i] == 8) {
			return 0;
		}
	}
	return 1;
}

static inline int rule5(char *buf) {
	return
		   buf[1] == 7 || buf[2] == 7
		|| buf[0] == 8 || buf[2] == 8
		|| buf[0] == 0 || buf[1] == 0;
}

int main(void) {
	char buf[3];

	for (int i = 0; i <= 999; i++) {
		decompose(i, buf);
		if (
			rule1(buf)
			&& rule2(buf)
			&& rule3(buf)
			&& rule4(buf)
			&& rule5(buf)
		) {
			printf("%d\n", i);
		}
	}

	return 0;
}