Closest Pair of Points

/*** Divide & Conquer - Closest Pair of Points

We are given an array of n points in the plane, and the problem is to find out the closest pair of points in the array. This problem arises in a number of applications. For example, in air-traffic control, you may want to monitor planes that come too close together, since this may indicate a possible collision. Recall the following formula for distance between two points p and q.
       Euclidean Distance = sqrt((p1.x - p2.x)^2 + (p1.y - p2.y)^2);

***/

#include<iostream>
#include<cmath>
#include <vector>
#include<algorithm>
#include<utility>
#include<limits>


using namespace std;

struct point {
       int x;
       int y;
};

bool sX (point p1, point p2);
bool sY (point p1, point p2);

class closestPair {
public:
       closestPair(int (*a)[2], int n);
       ~closestPair(){ }
       void getClosestpair(int begin, int end);
       double minDist;
      
private:
       vector<point> points;
//     int minDist;
       pair<point, point> closest;
       double distance(point p1, point p2);
       void crossDist(int vline, int begin, int end);
};

closestPair::closestPair(int (*a)[2], int n) {
       point p;
       points.clear();
       for(int i = 0; i < n; i++) {
              p.x = a[i][0];
              p.y = a[i][1];
              points.push_back(p);
       }
       sort(points.begin(), points.end(), sX);
       minDist = INT_MAX;
}

void closestPair::getClosestpair(int begin, int end) {
       int mid;
       double dist;
       int vline;

       if(begin == end) return;

       if(begin == end + 1) {
              dist = distance(points[begin], points[end]);
              if(minDist > dist) {
                     minDist = dist;
                     closest = make_pair(points[begin], points[end]);
              }
       }

       mid = (begin + end) / 2;
       vline = points[mid].x;
       getClosestpair(begin, mid);
       getClosestpair(mid+1, end);
       crossDist(vline, begin, end);

}

bool sX (point p1, point p2) {
       return (p1.x < p2.x);
}

bool sY (point p1, point p2) {
       return (p1.y < p2.y);
}

double closestPair::distance(point p1, point p2) {
       return sqrt((p1.x - p2.x)*(p1.x - p2.x) + (p1.y - p2.y)*(p1.y - p2.y));
}

void closestPair::crossDist(int vline, int begin, int end) {
       int xd;
       double dist;
       vector<point> left, right;
       for(int i = begin; i <= end; i++) {
              xd = points[i].x - vline;
              if(xd > 0 && abs(xd) < minDist) {
                     right.push_back(points[i]);
              }
              if(xd <= 0 && abs(xd) < minDist) {
                     left.push_back(points[i]);
              }
       }

       sort(left.begin(), left.end(), sY);
       sort(right.begin(), right.end(), sY);

       for(int i = 0; i < left.size(); i++) {
              for(int j = 0; j < right.size(); j++) {
                     if(j > i+7) break;
                     dist = distance(left[i], right[j]);
                     if(dist < minDist) {
                           minDist = dist;
                           closest = make_pair(points[i], points[j]);
                     }
              }
       }            
}


int main()
{
       int a[][2] = {{2, 3}, {12, 30}, {40, 50}, {5, 1}, {12, 10}, {3, 4}};
       int n = sizeof(a) / sizeof(a[0]);
       closestPair cp(a, n);
       cp.getClosestpair(0, n-1);
       cout<< "min = "<< cp.minDist<<endl;
       return 0;
}


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