Closest Pair of Points in C

// C program to find the smallest distance from a given set of points using Divide and Conquer algorithm.

#include 
#include 
#include 
#include 

// 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
float dist(Point p1, Point p2)
{
    return sqrt((p1.x - p2.x) * (p1.x - p2.x) + (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
float bruteForce(Point P[], int n)
{
    float min = FLT_MAX;
    for (int i = 0; i < n; ++i)
        for (int j = i + 1; j < n; ++j)
            if (dist(P[i], P[j]) < min)
                min = dist(P[i], P[j]);
    return min;
}

// A utility function to find a minimum of two float values
float min(float x, float 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
float stripClosest(Point strip[], int size, float d)
{
    // Initialize the minimum distance as d
    float min = d;

    qsort(strip, size, sizeof(Point), compareY);

    // 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 < size; ++i)
        for (int j = i + 1; j < size && (strip[j].y - strip[i].y) < min; ++j)
            if (dist(strip[i], strip[j]) < min)
                min = dist(strip[i], strip[j]);

    return min;
}

// A recursive function to find the smallest distance. The array P contains all points sorted according to x
// coordinate
float closestUtil(Point P[], int n)
{
    // If there are 2 or 3 points, then use brute force
    if (n <= 3)
        return bruteForce(P, n);

    // Find the middle point
    int mid = n / 2;
    Point midPoint = P[mid];

    // Consider the vertical line passing through the middle point
    // calculate the smallest distance dl on left of middle point and
    // dr on right side
    float dl = closestUtil(P, mid);
    float dr = closestUtil(P + mid, n - mid);

    // Find the smaller of two distances
    float d = min(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(P[i].x - midPoint.x) < d)
            strip[j] = P[i], j++;

    // Find the closest points in strip. Return the minimum of d and closest distance in strip[]
    return min(d, stripClosest(strip, j, d));
}

// The main function that finds the smallest distance
// This method mainly uses closestUtil()
float closest(Point P[], int n)
{
    qsort(P, n, sizeof(Point), compareX);

    // Use recursive function closestUtil() to find the smallest distance
    return closestUtil(P, n);
}

// Driver program to test above functions
int main()
{
    Point P[] = {{2, 3}, {12, 30}, {40, 50}, {5, 1}, {12, 10}, {3, 4}};
    int n = sizeof(P) / sizeof(P[0]);
    printf("The smallest distance is %f ", closest(P, n));
    return 0;
}