UVa 109 - SCUD Busters

// UVa 109 - SCUD Busters
#include <iostream>
#include <stdio.h>
#include <vector>
#include <algorithm>
#include <string.h>
#include <cmath>
using namespace std;

struct point {
	int x, y;
};

point piv;

int cross(point c, point a, point b) {
	int x1 = a.x - c.x;
	int x2 = b.x - c.x;
	int y1 = a.y - c.y;
	int y2 = b.y - c.y;
	return x1 * y2 - x2 * y1;
}

bool operator <(point a, point b) {
	return cross(piv, a, b) < 0;
}

int main() {
	int n = 0, m;
	cin >> m;
	vector<point> p[20];
	while (m != -1) {
		for (int j = 0; j < m; j++) {
			point pp;
			cin >> pp.x >> pp.y;
			p[n].push_back(pp);
		}
		n++;
		cin >> m;
	}

	double area[20];
	point hull[20][101];
	int h[20];
	for (int i = 0; i < n; i++) {
		int mn = 0;
		for (int j = 1; j < p[i].size(); j++)
			if (p[i][j].x < p[i][mn].x || p[i][j].x == p[i][mn].x && p[i][j].y < p[i][mn].y)
				mn = j;
		piv = p[i][mn];
		p[i][mn] = p[i][0];
		p[i][0] = piv;
		sort(p[i].begin() + 1, p[i].end());

		p[i].push_back(piv);
		hull[i][0] = p[i][0];
		hull[i][1] = p[i][1];
		h[i] = 2;
		for (int j = 2; j < p[i].size(); j++) {
			while (h[i] >= 2 && cross(hull[i][h[i] - 2], hull[i][h[i] - 1], p[i][j]) > 0)
				h[i]--;
			hull[i][h[i]++] = p[i][j];
		}
		area[i] = 0;
		for (int j = 1; j < h[i]; j++)
			area[i] += hull[i][j - 1].x * hull[i][j].y - hull[i][j].x * hull[i][j - 1].y;
		area[i] /= 2;
	}

	double sol = 0;
	point fire;
	bool alive[101];
	memset(alive, true, sizeof(alive));
	while (cin >> fire.x >> fire.y) {
		for (int i = 0; i < n; i++)
			if (alive[i]) {
				bool mark = true;
				for (int j = 1; j < h[i]; j++) {
					if (cross(hull[i][j - 1], hull[i][j], fire) > 0) {
						mark = false;
						break;
					}
				}
				if (mark) {
					alive[i] = false;
					sol += area[i];
				}
			}
	}
	printf("%.2f\n", abs(sol));

	return 0;
}