poj 1279核面積を求める

#include <iostream>
#include <cstdio>
#include <cmath>
#include <algorithm>
using namespace std;

const int maxn = 1505;
const double eps = 1e-8;
int n, pn, dq[maxn], top, bot;
struct Point {
    double x, y;
} p[maxn];

struct Line {
    Point a, b;
    double angle;
    Line& operator= (Line l) {
       a.x = l.a.x; a.y = l.a.y;
       b.x = l.b.x; b.y = l.b.y;
       angle = l.angle;
       return *this;
    }
} l[maxn];

int dblcmp(double k) {
    if (fabs(k) < eps) return 0;
    return k > 0 ? 1 : -1;
}

double multi(Point p0, Point p1, Point p2) {
    return (p1.x-p0.x)*(p2.y-p0.y)-(p1.y-p0.y)*(p2.x-p0.x);
}

bool cmp(const Line& l1, const Line& l2) {
    int d = dblcmp(l1.angle-l2.angle);
    if (!d) return dblcmp(multi(l1.a, l2.a, l2.b)) < 0; //  0       ，  0   
    return d < 0;
}

void addLine(Line& l, double x1, double y1, double x2, double y2) {
    l.a.x = x1; l.a.y = y1;
    l.b.x = x2; l.b.y = y2;
    l.angle = atan2(y2-y1, x2-x1);
}

 void getIntersect(Line l1, Line l2, Point& p) {
    double A1 = l1.b.y - l1.a.y;
    double B1 = l1.a.x - l1.b.x;
    double C1 = (l1.b.x - l1.a.x) * l1.a.y - (l1.b.y - l1.a.y) * l1.a.x;
    double A2 = l2.b.y - l2.a.y;
    double B2 = l2.a.x - l2.b.x;
    double C2 = (l2.b.x - l2.a.x) * l2.a.y - (l2.b.y - l2.a.y) * l2.a.x;
    p.x = (C2 * B1 - C1 * B2) / (A1 * B2 - A2 * B1);
    p.y = (C1 * A2 - C2 * A1) / (A1 * B2 - A2 * B1);
}

bool judge(Line l0, Line l1, Line l2) {
     Point p;
     getIntersect(l1, l2, p);
     return dblcmp(multi(p, l0.a, l0.b)) > 0;  //                ，  0， p   l0.a->l0.b   ，  0    ， p     l0  ，  true
}

void HalfPlaneIntersect( ){
    int i, j;
    sort(l, l+n, cmp);
    for (i = 0, j = 0; i < n; i++)
        if (dblcmp(l[i].angle-l[j].angle) > 0)
            l[++j] = l[i];
    n = j + 1;
    dq[0] = 0;
    dq[1] = 1;
    top = 1;
    bot = 0;
    for (i = 2; i < n; i++) {
        while (top > bot && judge(l[i], l[dq[top]], l[dq[top-1]])) top--;
        while (top > bot && judge(l[i], l[dq[bot]], l[dq[bot+1]])) bot++;
        dq[++top] = i;
    }
    while (top > bot && judge(l[dq[bot]], l[dq[top]], l[dq[top-1]])) top--;
    while (top > bot && judge(l[dq[top]], l[dq[bot]], l[dq[bot+1]])) bot++;
    dq[++top] = dq[bot];
    for (pn = 0, i = bot; i < top; i++, pn++)
        getIntersect(l[dq[i+1]], l[dq[i]], p[pn]);
}

double getArea() {
    if (pn < 3) return 0;
    double area = 0;
    for (int i = 1; i < pn-1; i++)
        area += multi(p[0], p[i], p[i+1]);
    if (area < 0) area = -area;
    return area/2;
}

int main()
{
    int t, i;

    scanf ("%d", &t);
    while (t--) {
        scanf ("%d", &n);
        for (i = 0; i < n; i++)
            scanf ("%lf%lf", &p[i].x, &p[i].y);
        for (i = 0; i < n-1; i++)
            addLine(l[i], p[i].x, p[i].y, p[i+1].x, p[i+1].y);
        addLine(l[i], p[i].x, p[i].y, p[0].x, p[0].y);
        HalfPlaneIntersect();
        printf ("%.2lf
", getArea());
    }
    return 0;
}