SRM641 Div1 250

#include <vector>
#include <algorithm>
using namespace std;

const int MAX = 2560;
typedef unsigned int ui;
ui s[MAX][MAX/32];

inline ui popcount(ui x) {
    ui count;
    for (count=0; x; count++)
        x &= x-1;
    return count;
}

class TrianglesContainOrigin
{
public:
  long long count(vector <int> x, vector <int> y)
  {
    ui i, j, k, N = x.size();
    vector <pair <int, int> > p(N);
    for (i=0; i<N; i++)
      p[i] = make_pair(x[i], y[i]);
    sort(p.begin(), p.end());

    for (i=0; i<N; i++)
      for (j=i+1; j<N; j++)
        if (p[j].second * p[i].first > p[i].second * p[j].first)
           s[i][j>>5] |= (1 << (j&31));

    long long ans = 0;
    for (i=0; i<N-2; i++) {
      for (j=i+1; j<N-1; j++) {
        ui bit = (1<<(j&31));
        ui up = (s[i][j>>5] & bit);
        ui mask = 0xFFFFFFFF - (bit-1);
        for (k=j>>5; k<(N>>5)+1; k++) {
          ui t = up ? (s[j][k] & (s[i][k]^mask)) : (s[i][k] & (s[j][k]^mask));
          ans += popcount(t);
          mask = 0xFFFFFFFF;
        }
      }
    }
    return ans;
  }
};