Graham Scan display Hull Points.cpp

#include <iostream>
#include <stack>
#include <stdlib.h>
using namespace std;
struct Point
{
    int x, y;
};
Point p0;

Point nextToTop(stack<Point> &S)
{
    Point p = S.top();
    S.pop();
    Point res = S.top();
    S.push(p);
    return res;
}
void swap(Point &p1, Point &p2)
{
    Point temp = p1;
    p1 = p2;
    p2 = temp;
}

int distSq(Point p1, Point p2)
{
    return (p1.x - p2.x) * (p1.x - p2.x) +
           (p1.y - p2.y) * (p1.y - p2.y);
}
int orientation(Point p, Point q, Point r)
{
    int val = (q.y - p.y) * (r.x - q.x) -
              (q.x - p.x) * (r.y - q.y);
    if (val == 0)
        return 0;             
    return (val > 0) ? 1 : 2; 
}
int compare(const void *vp1, const void *vp2)
{
    Point *p1 = (Point *)vp1;
    Point *p2 = (Point *)vp2;

    int o = orientation(p0, *p1, *p2);
    if (o == 0)
        return (distSq(p0, *p2) >= distSq(p0, *p1)) ? -1 : 1;
    return (o == 2) ? -1 : 1;
}

void convexHull(Point points[], int n)
{

    int ymin = points[0].y, min = 0;
    for (int i = 1; i < n; i++)
    {
        int y = points[i].y;
        if ((y < ymin) || (ymin == y &&
                           points[i].x < points[min].x))
            ymin = points[i].y, min = i;
    }

    swap(points[0], points[min]);
    p0 = points[0];
    qsort(&points[1], n - 1, sizeof(Point), compare);
    int m = 1; 
    for (int i = 1; i < n; i++)
    {

        while (i < n - 1 && orientation(p0, points[i], points[i + 1]) == 0)
            i++;
        points[m] = points[i];
        m++; 
    }
    if (m < 3)
        return;
    stack<Point> S;
    S.push(points[0]);
    S.push(points[1]);
    S.push(points[2]);

    for (int i = 3; i < m; i++)
    {
        while (S.size() > 1 && orientation(nextToTop(S), S.top(), points[i]) != 2)
            S.pop();
        S.push(points[i]);
    }

    while (!S.empty())
    {
        Point p = S.top();
        cout << "(" << p.x << ", " << p.y << ")" << endl;
        S.pop();
    }
}

int main()
{
    Point points[] = {{0, 3}, {1, 1}, {2, 2}, {4, 4}, {0, 0}, {1, 2}, {3, 1}, {3, 3}};
    int n = sizeof(points) / sizeof(points[0]);
    convexHull(points, n);
    return 0;
}