BitTorrent task solution

gistfile1.c

#include<stdio.h>

#define BOWLS 40
#define ORANGES 9


int count = 0;
// All the places in which an orange can't be placed are 1's
int used[BOWLS];
// A list of the oranges already placed.
int oranges[ORANGES];

int turn;
int y;
int tmp_index;

void check(int x) {
  if (turn == ORANGES) {
    ++count;
    return;
  }

  for(; x < BOWLS; ++x) {
    // Only check available positions.
    if (!used[x]) {
      // Mark this position as used.
      used[x] += 1;
      // This position is called "B"
      // For every A, make sure no C exists such that |A-B| = |B-C|
      // I.e. mark them as used even though no oranges are placed there.
      for(y = 0; y < turn; ++y) {
        tmp_index = x + x - oranges[y];
        if (tmp_index >= 0 && tmp_index < BOWLS) {
          used[tmp_index] += 1;
        }
      }

      // The orange on this turn is placed on "x"
      oranges[turn] = x;
      ++turn;
      check(x+1);
      --turn;
      // Revert the "used" markers placed this turn.
      for(y = 0; y < turn; ++y) {
        tmp_index = x + x - oranges[y];
        if (tmp_index >= 0 && tmp_index < BOWLS) {
          used[tmp_index] -= 1;
        }
      }
      used[x] -= 1;
    }
  }
}



int main() {
  check(0);
  printf("%d\n", count);
}