State machine example in C

dieroll.c

/* Knuth-Yao state machine - model a fair die using only a fair coin
 *
 * See <https://www.prismmodelchecker.org/casestudies/dice.php>.
 *
 */

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int counter = 0;
int counts[6] = {0};

bool coin_flip(void)
{
   return (rand() % 2) == 0;
}

void report(void)
{
    for (int i = 0; i < 6; i++) {
        printf("%.3f ", (double) counts[i] / counter);
    }
    printf("\n");
}

int main(int argc, char *argv[])
{
    if (argc < 2) {
        fprintf(stderr, "Usage: %s N\n", argv[0]);
        return EXIT_FAILURE;
    }
    int n = atoi(argv[1]);

    srand(time(NULL));
s0:
    if (++counter > n) { report(); goto end; }
    if (coin_flip()) { goto s1; } else { goto s2; }
s1:
    if (coin_flip()) { goto s3; } else { goto s4; }
s2:
    if (coin_flip()) { goto s5; } else { goto s6; }
s3:
    if (coin_flip()) { goto s1; } else { counts[0]++; goto s0; }
s4:
    if (coin_flip()) { counts[1]++; goto s0; } else { counts[2]++; goto s0; }
s5:
    if (coin_flip()) { counts[3]++; goto s0; } else { counts[4]++; goto s0; }
s6:
    if (coin_flip()) { goto s2; } else { counts[5]++; goto s0; }
end:
    return EXIT_SUCCESS;
}

Makefile

CFLAGS= -g -std=c11 -Wall -Wextra -Wpedantic
TARGET=dieroll

$(TARGET): $(TARGET).c
	$(CC) $(CFLAGS) -o $@ $^ $(LDLIBS)

test: $(TARGET)
	./$< 100000

clean:
	rm -f $(TARGET)