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// A divide and conquer program in C++ to find the smallest distance from a
// given set of points.
#include <fstream>
#include <algorithm>
#include <cmath>
#include <cfloat>
#include <iomanip>
using namespace std;
ifstream fin("cmap.in");
ofstream fout("cmap.out");
// A structure to represent a Point in 2D plane
struct Point
{
int x, y;
};
// Needed to sort array of points according to X coordinate
int compareX(const void* a, const void* b)
{
Point *p1 = (Point *)a, *p2 = (Point *)b;
return (p1->x - p2->x);
}
// Needed to sort array of points according to Y coordinate
int compareY(const void* a, const void* b)
{
Point *p1 = (Point *)a, *p2 = (Point *)b;
return (p1->y - p2->y);
}
// A utility function to find the distance between two points
double dist(Point p1, Point p2)
{
return sqrt( 1.0 *(p1.x - p2.x)*(p1.x - p2.x) +
1.0 * (p1.y - p2.y)*(p1.y - p2.y));
}
// A Brute Force method to return the smallest distance between two points
// in P[] of size n
double bruteForce(Point P[], int n)
{
double cmin = FLT_MAX;
for (int i = 0; i < n; ++i)
for (int j = i+1; j < n; ++j)
if (dist(P[i], P[j]) < cmin)
cmin = dist(P[i], P[j]);
return cmin;
}
// A utility function to find a minimum of two float values
double dmin(double x, double y)
{
return (x < y)? x : y;
}
// A utility function to find the distance between the closest points of
// strip of a given size. All points in strip[] are sorted according to
// y coordinate. They all have an upper bound on minimum distance as d.
// Note that this method seems to be a O(n^2) method, but it's a O(n)
// method as the inner loop runs at most 6 times
double stripClosest(Point strip[], int csize, double d)
{
double cmin = d; // Initialize the minimum distance as d
// Pick all points one by one and try the next points till the difference
// between y coordinates is smaller than d.
// This is a proven fact that this loop runs at most 6 times
for (int i = 0; i < csize; ++i)
for (int j = i+1; j < csize && (strip[j].y - strip[i].y) < cmin; ++j)
if (dist(strip[i],strip[j]) < cmin)
cmin = dist(strip[i], strip[j]);
return cmin;
}
// A recursive function to find the smallest distance. The array Px contains
// all points sorted according to x coordinates and Py contains all points
// sorted according to y coordinates
double closestUtil(Point Px[], Point Py[], int n)
{
// If there are 2 or 3 points, then use brute force
if (n <= 3)
return bruteForce(Px, n);
// Find the middle point
int mid = n/2;
Point midPoint = Px[mid];
// Divide points in y sorted array around the vertical line.
// Assumption: All x coordinates are distinct.
Point Pyl[mid]; // y sorted points on left of vertical line
Point Pyr[n-mid]; // y sorted points on right of vertical line
int li = 0, ri = 0; // indexes of left and right subarrays
for (int i = 0; i < n; i++)
{
if (Py[i].x <= midPoint.x && li<mid)
Pyl[li++] = Py[i];
else
Pyr[ri++] = Py[i];
}
// Consider the vertical line passing through the middle point
// calculate the smallest distance dl on left of middle point and
// dr on right side
double dl = closestUtil(Px, Pyl, mid);
double dr = closestUtil(Px + mid, Pyr, n-mid);
// Find the smaller of two distances
double d = dmin(dl, dr);
// Build an array strip[] that contains points close (closer than d)
// to the line passing through the middle point
Point strip[n];
int j = 0;
for (int i = 0; i < n; i++)
if (abs(Py[i].x - midPoint.x) < d)
strip[j] = Py[i], j++;
// Find the closest points in strip. Return the minimum of d and closest
// distance is strip[]
return stripClosest(strip, j, d);
}
// The main function that finds the smallest distance
// This method mainly uses closestUtil()
float closest(Point P[], int n)
{
Point Px[n];
Point Py[n];
for (int i = 0; i < n; i++)
{
Px[i] = P[i];
Py[i] = P[i];
}
qsort(Px, n, sizeof(Point), compareX);
qsort(Py, n, sizeof(Point), compareY);
// Use recursive function closestUtil() to find the smallest distance
return closestUtil(Px, Py, n);
}
// Driver program to test above functions
int main()
{
int n, x, y;
fin>>n;
Point P[n];
for(int i=0; i < n; i++)
{
fin>>x>>y;
P[i].x = x;
P[i].y = y;
}
fout<<fixed<<showpoint;
fout<<setprecision(17);
fout<<closest(P, n);
return 0;
}