POJ3525-Most Distant Point from the Sea（二分+半平面交）

Most Distant Point from the Sea

Time Limit: 5000MS		Memory Limit: 65536K
Total Submissions: 3955		Accepted: 1847		Special Judge

Description

The main land of Japan called Honshu is an island surrounded by the sea. In such an island, it is natural to ask a question: “Where is the most distant point from the sea?” The answer to this question for Honshu was found in 1996. The most distant point
is located in former Usuda Town, Nagano Prefecture, whose distance from the sea is 114.86 km.

In this problem, you are asked to write a program which, given a map of an island, finds the most distant point from the sea in the island, and reports its distance from the sea. In order to simplify the problem, we only consider maps representable by convex
polygons.

Input

The input consists of multiple datasets. Each dataset represents a map of an island, which is a convex polygon. The format of a dataset is as follows.

n
x1		y1
	⋮
xn		yn

Every input item in a dataset is a non-negative integer. Two input items in a line are separated by a space.

n in the first line is the number of vertices of the polygon, satisfying 3 ≤ n ≤ 100. Subsequent n lines are the x- and y-coordinates of the n vertices. Line segments (x_i, y_i)–(x_i+1, y_i+1)
(1 ≤ i ≤ n − 1) and the line segment (x_n, y_n)–(x₁, y₁) form the border of the polygon in counterclockwise order. That is, these line segments see the inside of
the polygon in the left of their directions. All coordinate values are between 0 and 10000, inclusive.

You can assume that the polygon is simple, that is, its border never crosses or touches itself. As stated above, the given polygon is always a convex one.

The last dataset is followed by a line containing a single zero.

Output

For each dataset in the input, one line containing the distance of the most distant point from the sea should be output. An output line should not contain extra characters such as spaces. The answer should not have an error greater than 0.00001 (10⁻⁵).
You may output any number of digits after the decimal point, provided that the above accuracy condition is satisfied.

Sample Input

4
0 0
10000 0
10000 10000
0 10000
3
0 0
10000 0
7000 1000
6
0 40
100 20
250 40
250 70
100 90
0 70
3
0 0
10000 10000
5000 5001
0

Sample Output

5000.000000
494.233641
34.542948
0.353553

题意：让你求凸包内的点距离凸包边上最大的距离。

思路：能够二分答案，求半平面交。

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <vector>
#include <algorithm>
using namespace std;
#define REP(_,a,b) for(int _ = (a); _ < (b); _++)
#define sz(s)  (int)((s).size())
typedef long long ll;
const double eps = 1e-9;
const int maxn = 100*2+10;
struct Point{
	double x,y;
	Point(double x=0.0,double y = 0.0):x(x),y(y){}
};
vector<Point> vP;
typedef Point Vector;
Point poly[maxn];
vector<Vector> vV1,vV2;
struct Line {
	Point P;
	Vector v;
	double ang;
	Line(){}
	Line(Point P,Vector v):P(P),v(v){
		ang = atan2(v.y,v.x);
	}
	bool operator <(const Line&L) const{
		return ang < L.ang;
	}
};
Line L[maxn];
Vector operator + (Vector A,Vector B) {
	return Vector(A.x+B.x,A.y+B.y);
}
Vector operator - (Vector A,Vector B){
	return Vector(A.x-B.x,A.y-B.y);
}
Vector operator * (Vector A,double p){
	return Vector(A.x*p,A.y*p);
}
Vector operator / (Vector A,double p){
	return Vector(A.x/p,A.y/p);
}
bool operator < (const Point &a,const Point &b){
	return a.x < b.x || (a.x==a.y && a.y < b.y);
}
int dcmp(double x){
	if(fabs(x) < eps) return 0;
	else return x < 0? -1:1;
}
bool operator == (const Point &a,const Point &b){
	return dcmp(a.x-b.x)==0&& dcmp(a.y-b.y)==0;
}
double Dot(Vector A,Vector B) {return A.x*B.x+A.y*B.y;}
double Length(Vector A) {return sqrt(Dot(A,A));}
double Angle(Vector A,Vector B) {return acos(Dot(A,B)/Length(A)/Length(B));}
double Cross(Vector A,Vector B) {return A.x*B.y-A.y*B.x;}
Vector Rotate(Vector A,double rad) {return Vector(A.x*cos(rad)-A.y*sin(rad),A.x*sin(rad)+A.y*cos(rad)); }
Vector Normal(Vector A) {
	double L = Length(A);
	return Vector(-A.y/L,A.x/L);
}
bool OnLeft(Line L,Point p){
	return Cross(L.v,p-L.P) > 0;
}
Point GetIntersection(Line a,Line b){
	Vector u = a.P-b.P;
	double t = Cross(b.v,u) / Cross(a.v,b.v);
	return a.P+a.v*t;
}
int HalfplaneIntersection(Line* L,int n,Point* poly){
	sort(L,L+n);
	int first,last;
	Point *p = new Point[n];
	Line *q = new Line[n];
	q[first=last=0] = L[0];
	for(int i = 1; i < n; i++){
		while(first < last && !OnLeft(L[i],p[last-1])) last--;
		while(first < last && !OnLeft(L[i],p[first])) first++;
		q[++last] = L[i];
		if(fabs(Cross(q[last].v,q[last-1].v))<eps) {
			last--;
			if(OnLeft(q[last],L[i].P)) q[last] = L[i];
		}
		if(first<last) p[last-1] = GetIntersection(q[last-1],q[last]);
	}
	while(first < last && !OnLeft(q[first],p[last-1])) last--;
	if(last - first <=1) return 0;
	p[last] = GetIntersection(q[last],q[first]);
	int m = 0;
	for(int i = first; i <= last; i++) poly[m++] = p[i];
	return m;
}
int n;
void init(){
	vP.clear();
	vV1.clear();
	vV2.clear();
}
void input(){
	REP(_,0,n){
		double x,y;
		scanf("%lf%lf",&x,&y);
		vP.push_back(Point(x,y));
	}
	#define next(i) ((i)+1)%n
	REP(_,0,n){
		vV1.push_back(vP[next(_)]-vP[(_)]);
		vV2.push_back(Normal(vP[next(_)]-vP[(_)]));
	}
}
void solve(){
	double l = 0,r = 40000;
	while(r-l > eps){
		double mid = (l+r)/2;
		REP(_,0,n) {
			L[_] = Line(vP[_]+vV2[_]*mid ,vV1[_]);
		}
		int m = HalfplaneIntersection(L,n,poly);
		if(m==0){
			r = mid;
		}else{
			l = mid;
		}
	}
	printf("%.7f\n",r);
}
int main(){

	while(~scanf("%d",&n) && n){
		init();
		input();
		solve();
	}
	return 0;
}