Solution to nine digits puzzle at compile time

main.cpp

#include <stdio.h>

#define SWAP_INT(x, y)                                                         \
  do {                                                                         \
    int tmp = (x);                                                             \
    (x) = (y);                                                                 \
    (y) = tmp;                                                                 \
  } while (0)

template <int N> struct ConstArray {
  constexpr ConstArray() : arr() {
    for (auto i = 0; i < N; i++) {
      arr[i] = i + 1;
    }
  }
  int arr[N];
};

constexpr int polydiv(int v, int d, ConstArray<9> n) {
  int s = d - 1;
  for (int i = s; i < 9; i++) {
    SWAP_INT(n.arr[s], n.arr[i]);
    int t = (v * 10) + n.arr[s];
    if (t % d == 0) {
      if (d == 9) {
        return t;
      }
      if ((t = polydiv(t, d + 1, n)) != 0) {
        return t;
      }
      // continue
    }
    SWAP_INT(n.arr[d - 1], n.arr[i]);
  }
  return 0;
}

constexpr auto n = ConstArray<9>();

constexpr auto p = polydiv(0, 1, n);

int main() {
  printf("%d\n", p);
  return 0;
}