POJ 1279 Art Gallery 半平面交 + 多边形面积

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模板题，没什么好说的。

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#include<stdio.h>#include<string.h>#include<math.h>#include<algorithm>using namespace std;#define eps 1e-8struct point{    double x, y;}p[1505], tmp[1505], q[1505];double a, b, c;void get_line(point p1, point p2){    a = p2.y - p1.y;    b = p1.x - p2.x;    c = p2.x * p1.y - p2.y * p1.x;}point intersect(point p1, point p2) {    double u = fabs(a * p1.x + b * p1.y + c);    double v = fabs(a * p2.x + b * p2.y + c);    point ret;    ret.x = (p1.x * v + p2.x * u) / (u + v);    ret.y = (p1.y * v + p2.y * u) / (u + v);    return ret;}int n, m;void cut() {    int tm = 0;    int i;    for(i = 1; i <= m; i++)    {        if(a * q[i].x + b * q[i].y + c >= 0)               tmp[++tm] = q[i];                            else        {            if(a * q[i-1].x + b * q[i-1].y + c > 0)                 tmp[++tm] = intersect(q[i-1], q[i]);              if(a * q[i+1].x + b * q[i+1].y + c > 0)                tmp[++tm] = intersect(q[i], q[i+1]);        }    }    for(i = 1; i <= tm; i++)        q[i] = tmp[i];    q[0] = tmp[tm];    q[tm + 1] = tmp[1];    m = tm;}void solve(){    int i;    for(i = 1; i <= n; i++)        q[i] = p[i];    p[n+1] = p[1];    q[0] = q[n];    q[n+1] = q[1];    m = n;    for(i = 1; i <= n; i++)    {        get_line(p[i], p[i+1]);         cut();                     }}double cal(int n, point *p){    int i;    double s = 0;    p[n+1] = p[1];    for(i = 1; i <= n; i++)        s += p[i].x * p[i+1].y - p[i].y * p[i+1].x;    return fabs(s) / 2;} int main(){    int i, j, cas;    scanf("%d", &cas);    while(cas--)    {        scanf("%d", &n);        for(i = 1;i <= n; i++)            scanf("%lf%lf", &p[i].x, &p[i].y);        solve();        if(m >= 3)printf("%.2f\n", cal(m, q) );        else printf("0.00\n");    }    return 0;}