poj1279 求核面积

初学半平面交算法，本来想自己敲一遍的，但是今晚确实比较晚了，于是直接套了昨天手敲的模版，竟然1A了，嘿嘿...今天总算还是做了一题。

#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; //大于0取半平面的左半，小于0取右半    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;  //与上面的注释处的大于小于符号相反，大于0，是p在向量l0.a->l0.b的左边，小于0是在右边，当p不在半平面l0内时，返回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\n", getArea());    }    return 0;}