N Queens Solver

n_queens.c

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

#define PRINT_SOLUTIONS
#define FANCY_PRINT

static inline void print_array(uint8_t *a, uint8_t n) {
  putchar('[');
  for (uint8_t i = 0; i < n - 1; i++)
    printf("%d, ", a[i]);
  if (n > 0) printf("%d", a[n - 1]);
  puts("]");
}

static inline void print_board(uint8_t *a, uint8_t n) {
  for (uint8_t i = 0; i < n; i++) {
    for (uint8_t j = 0; j < a[i]; j++)
      printf("▄ ");
    printf("\e[38;5;2m▄ \e[0m");
    for (uint8_t j = a[i] + 1; j < n; j++)
      printf("▄ ");
    puts("");
  }
  puts("\n");
}

static size_t solve(a, n, l, m) uint8_t *a, n, l; int m; {
  size_t c = 0, s = 0;
  for (uint32_t z = m, f = (1UL << n) - 1; z != f;) {
    z |= (s = ~((-2 - z) | z));
    if (~m & s) {
      uint8_t i = __builtin_ffs(s) - 1, j;
      for (j = 0; j < l && i + j - l != a[j] && i != a[j] + j - l; j++);
      if (j == l) a[l] = i, c += l + 1 == n ?
#ifdef PRINT_SOLUTIONS
#  ifdef FANCY_PRINT
      print_board(a, n),
#  else
      print_array(a, n),
#  endif
#endif
        1 : solve(a, n, l + 1, m | s);
    }
  }
  return c;
}

static inline size_t n_queens(int8_t n) {
  uint8_t a[n];
  return solve(&a, n, 0, 0);
}

int main(int argc, char **argv) {
  const clock_t start = clock();
  printf("%sx%s: %lu solutions found\n",
         argv[1], argv[1], n_queens(strtol(argv[1], NULL, 10)));
  printf("finished in %lfs\n", (clock() - start) / (double) CLOCKS_PER_SEC);
}