poj3819 Coverage （求直线与圆的交占直线的百分比 ）

#include <iostream>
#include <algorithm>
#include <cstdio>
#include <cstdlib>
#include <cmath>
using namespace  std;
const int MAX = 300;
struct Node
{
	double x, y;
};
struct Line
{
	Node s, e;
};
Line line;
Node p[MAX];
double A, B, C, delta;
double x, y, r;
double x11, y11, dx, dy;
Node tmp, cir;
double sqr(double x)
{
	return x * x;
}
int circle_cross_line(Node s, Node e, Node O, double r)//推断圆与直线是否有交点
{
	double x0 = O.x, y0=O.y;
	x11 = s.x, y11 = s.y;
	double x2 = e.x, y2 = e.y;
	dx = x2 - x11, dy = y2 - y11;
	A = dx * dx + dy*dy;
	B = 2 * dx * (x11 - x0) + 2 * dy * (y11 - y0);
	C = sqr(x11-x0) + sqr(y11-y0) - sqr(r);
	delta = sqr(B) - 4 * A * C;
	return delta > 0;
}
int cmp(Node a, Node b)
{
	if (a.x < b.x)
		return 1;
	return 0;
}
int main()
{
	int n, i, cnt;
	int flag, flagnum;
	double leng;
	while (scanf("%d", &n) && n)
	{
		flagnum = 0;
		scanf("%lf%lf%lf%lf", &line.s.x, &line.s.y, &line.e.x, &line.e.y);
		if (line.s.x!=line.e.x)
		{
			if (line.s.x < line.e.x)
			{
				tmp.x = line.s.x;
				tmp.y = line.e.x;
			}
			else
			{
				tmp.x = line.e.x;
				tmp.y = line.s.x;
			}
			flag = 0;
			leng = fabs(line.e.x - line.s.x);
		}
		else if (line.s.x==line.e.x && line.s.y!=line.e.y)
		{
			if (line.s.y < line.e.y)
			{
				tmp.x = line.s.y;
				tmp.y = line.e.y;
			}
			else
			{
				tmp.x = line.e.y;
				tmp.y = line.s.y;
			}
			flag = 1;
			leng = fabs(line.e.y - line.s.y);
		}
		else
			flagnum = 1;
		cnt = 0;
		for (i=0; i<n; i++)
		{
			scanf("%lf%lf%lf", &cir.x, &cir.y, &r);
			if (flagnum)
				continue;
			if (circle_cross_line(line.s, line.e, cir, r))
			{
				p[cnt].x = (-B-sqrt(delta))/(2.0*A);
				p[cnt].y = (-B+sqrt(delta))/(2.0*A);
				if (flag==0)
				{
					p[cnt].x = p[cnt].x * dx + x11;
					p[cnt].y = p[cnt].y * dx + x11;
				}
				else
				{
					p[cnt].x = p[cnt].x * dy + y11;
					p[cnt].y = p[cnt].y * dy + y11;
				}
				if (p[cnt].x>p[cnt].y)
				{
					double t = p[cnt].x;
					p[cnt].x = p[cnt].y;
					p[cnt].y = t;
				}
				if (p[cnt].x>tmp.y || p[cnt].y<tmp.x)
					continue;
				if (p[cnt].x<tmp.x)
					p[cnt].x = tmp.x;
				if (p[cnt].y>tmp.y)
					p[cnt].y = tmp.y; 
				cnt++;
			}
		}
		if (flagnum || cnt==0)
			printf("0.00\n");
		else
		{
			sort(p, p+cnt, cmp);
			double sum = 0;
			tmp = p[0];
			for (i=1; i<cnt; i++)
			{
				if (p[i].x < tmp.y)
				{
					if (p[i].y > tmp.y)
						tmp.y = p[i].y;
				}
				else
				{
					sum += tmp.y - tmp.x;
					tmp = p[i];
				}
			}
			sum += tmp.y - tmp.x;
			printf("%.2f\n", sum/leng*100.0);
		}
	}
	return 0;
}