Uyuw's Concert POJ2451

裸半平面交，以前没写过，先写一遍再说

我越来越不注意细节了，最后才发现空间稍微开小了（没有开那个零头，他又要多4条边，就WA了）

 const
     maxn=;
     eps=1e-7;
 type
     point=record
       x,y:double;
     end;
     poly=record
       x,y,w:point;
     end;

 var
     a:array[..maxn]of poly;
     n:longint;

 operator -(a,b:point)c:point;
 begin
     c.x:=a.x-b.x;
     c.y:=a.y-b.y;
 end;

 operator +(a,b:point)c:point;
 begin
     c.x:=a.x+b.x;
     c.y:=a.y+b.y;
 end;

 operator *(a:point;b:double)c:point;
 begin
     c.x:=a.x*b;
     c.y:=a.y*b;
 end;

 operator *(a,b:point)c:double;
 begin
     c:=a.x*b.y-a.y*b.x;
 end;

 function wh(a:point):longint;
 begin
     with a do
       begin
         if abs(x)<eps then
           if y>eps then exit()
           else exit()
         else
           if abs(y)<eps then
             if x>eps then exit()
             else exit()
           else
             if x>eps then
               if y>eps then exit()
               else exit()
             else
               if y>eps then exit()
               else exit();
       end;
 end;

 procedure swap(var x,y:poly);
 var
     t:poly;
 begin
     t:=x;x:=y;y:=t;
 end;

 procedure sort(l,r:longint);
 var
     i,j:longint;
     y:point;
 begin
     i:=l;
     j:=r;
     y:=a[(l+r)>>].w;
     repeat
       while (wh(a[i].w)<wh(y)) or ((wh(a[i].w)=wh(y)) and (y*a[i].w+eps<)) do
         inc(i);
       while (wh(a[j].w)>wh(y)) or ((wh(a[j].w)=wh(y)) and (y*a[j].w>eps)) do
         dec(j);
       if i<=j then
       begin
         swap(a[i],a[j]);
         inc(i);
         dec(j);
       end;
     until i>j;
     if i<r then sort(i,r);
     if j>l then sort(l,j);
 end;

 procedure init;
 var
     i:longint;
 begin
     read(n);
     for i:= to n do
       with a[i] do
         begin
           read(x.x,x.y,y.x,y.y);
           w:=y-x;
         end;
     a[n+].x.x:=;
     a[n+].x.x:=;
     a[n+].x.y:=;
     a[n+].x.y:=;
     for i:= to  do
       begin
         a[n+i+].y:=a[n+((i+)mod )+].x;
         a[n+i+].w:=a[n+i+].y-a[n+i+].x;
       end;
     inc(n,);
     sort(,n);
 end;

 function get(a,b:poly):point;
 var
     s1,s2:double;
 begin
     s1:=a.w*(b.y-a.x);
     s2:=a.w*(b.x-a.x);
     if s1*s2>eps then s1:=-s1
     else
       begin
         s1:=abs(s1);
         s2:=abs(s2);
       end;
     if abs(s1+s2)<eps then get:=b.x
     else get:=b.x+(b.w*(s2/(s1+s2)));
 end;

 var
     q:array[..maxn]of poly;
     d:array[..maxn]of point;
     l,r:longint;
     ans:double;

 procedure work;
 var
     i:longint;
 begin
     l:=;
     r:=;
     q[]:=a[];
     for i:= to n do
       begin
         while (l<r) and (a[i].w*(d[r-]-a[i].x)+eps<) do
           dec(r);
         while (l<r) and (a[i].w*(d[l]-a[i].x)+eps<) do
           inc(l);
         inc(r);
         q[r]:=a[i];
         if abs(q[r-].w*q[r].w)<eps then
         begin
           dec(r);
           if a[i].w*(q[r].x-a[i].x)< then q[r]:=a[i];
         end;
         d[r-]:=get(q[r],q[r-]);
       end;
     while (l<r) and (q[l].w*(d[r-]-q[l].x)+eps<) do
       dec(r);
     if l<r then d[r]:=get(q[l],q[r]);
     if r-l<= then writeln('0.0')
     else
       begin
         for i:=l to r- do
           ans:=ans+(d[i]*d[i+])/;
         ans:=ans+(d[r]*d[l])/;
         ans:=abs(ans);
         if ans<eps then writeln('0.0')
         else writeln(ans::);
       end;
 end;

 begin
     init;
     work;
 end.