Cod sursa(job #2680705)

Utilizator Botzki17Botocan Cristian-Alexandru Botzki17 Data 3 decembrie 2020 23:22:10
Problema Cele mai apropiate puncte din plan Scor 0
Compilator cpp-64 Status done
Runda Arhiva educationala Marime 4.7 kb
// 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;
}