Solving Susi's problem 934 in the Italian magazine "La settimana enigmistica"

susi934.c

/*
    A quiz with Susi 934 (in the Italian magazine
                          La settimana enigmistica)

    Alfio, Biagio, Carlo and Dino are four children,
    one of them broke the window of a building while
    all four were playing with slingshots.

    Questioned to know who did it,

    Alfio said Biagio is a liar,
    Biagio said Carlo is a liar,
    Carlo said Biagio is a liar,
    Dino said Alfio is a liar.

    How many youngsters are not liars?
 */
#include <stdio.h>

enum child
{
    ALFIO = 0,
    BIAGIO,
    CARLO,
    DINO
};

const char *names[] = {
    [ALFIO]  = "Alfio",
    [BIAGIO] = "Biagio",
    [CARLO]  = "Carlo",
    [DINO]   = "Dino"
};

// [] says that {} is a liar...
struct
{
    int who_lies;
} child_says[] = {
    [ALFIO]  = {BIAGIO},
    [BIAGIO] = {CARLO},
    [CARLO]  = {BIAGIO},
    [DINO]   = {ALFIO}
};

// this should be generated from child_says...
struct
{
    int a_liar_by[4];
} is_said_to_be[] = {
    [ALFIO]  = { {DINO, -1} },
    [BIAGIO] = { {ALFIO, CARLO, -1} },
    [CARLO]  = { {BIAGIO, -1} },
    [DINO]   = { {-1} }
};


void liars_to_map(int n, int *l)
{
    int m = 1, i;
    for (i = 0; i < 4; ++i, m <<= 1, ++l) {
        *l = (n & m) >> i;
    }
}

int main(void)
{
    /*
      Let us suppose the Truth is that:

       0 0 0 0  no one lies
       0 0 0 1  ALFIO lies,
       0 0 1 0  BIAGIO lies,
       0 0 1 1  BIAGIO and ALFIO lie...
       ...
     */
    int liars;
    int liars_map[4];
    int found;

    for (liars = 0; liars < 16; ++liars) {
        liars_to_map(liars, liars_map);
        found = 1;
        for (int i = 0; i < 4; ++i) {
            int h = !liars_map[i];
            if ((h  && !liars_map[child_says[i].who_lies]) ||
                (!h && liars_map[child_says[i].who_lies])) {
                found = 0;
                goto skip; // or break;
            }
            int who;
            for (int j = 0;
                 (who = is_said_to_be[i].a_liar_by[j]) >= 0;
                 ++j) {
                if ((h  && !liars_map[who]) ||
                    (!h && liars_map[who])) {
                    found = 0;
                    goto skip;
                }
            }            
        }
      skip:
        if (found) {
            for (int k = 0; k < 4; ++k) {
                printf("%s %s\n",
                       names[k], liars_map[k] ? "lies" : "is honest");
            }
            printf("\n");
        }
    }
    
    return 0;
}