1043: [HAOI2008]下落的圆盘 - BZOJ

Description

有n个圆盘从天而降，后面落下的可以盖住前面的。求最后形成的封闭区域的周长。看下面这副图, 所有的红色线条的总长度即为所求.
Input

n ri xi y1 ... rn xn yn
Output

最后的周长，保留三位小数
Sample Input
2
1 0 0
1 1 0
Sample Output
10.472

数据规模
n<=1000

调了一晚上外加下午（弱菜写计算几何，tan函数都抄错了）

对每一个圆计算覆盖区间，然后排序，再区间合并，注意范围，我是全部弄到0到2π里

再膜拜一下z55250825，最后还是他看出错误来的

 uses math;
 const
     maxn=;
     eps=1e-7;
 var
     r,x,y,ll,rr:array[..maxn*]of double;
     n,tot:longint;
     ans:double;

 procedure init;
 var
     i:longint;
 begin
     read(n);
     for i:= to n do
       read(r[i],x[i],y[i]);
 end;

 function angle(a,b,c:double):double;
 begin
     exit(arccos((a*a+b*b-c*c)/(*a*b)));
 end;

 function tan(x,y:double):double;
 var
     a:double;
 begin
     if abs(x)<eps then
     begin
       if y>-eps then exit(pi/)
       else exit(pi*/);
     end;
     if abs(y)<eps then
     begin
       if x>-eps then exit()
       else exit(pi);
     end;
     a:=arctan(y/x);
     if a<-eps then a:=-a;
     if x>-eps then
       if y>-eps then exit(a)
       else exit(*pi-a)
     else
       if y>-eps then exit(pi-a)
       else exit(pi+a);
 end;

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

 procedure sort(l,r:longint);
 var
     i,j:longint;
     y:double;
 begin
     i:=l;
     j:=r;
     y:=ll[(l+r)>>];
     repeat
       while ll[i]+eps<y do
         inc(i);
       while ll[j]>y+eps do
         dec(j);
       if i<=j then
       begin
         swap(ll[i],ll[j]);
         swap(rr[i],rr[j]);
         inc(i);
         dec(j);
       end;
     until i>j;
     if i<r then sort(i,r);
     if l<j then sort(l,j);
 end;

 procedure work;
 var
     i,j:longint;
     d,a,b,sum,xx,yy:double;
     cover:boolean;
 begin
     for i:= to n do
       begin
         tot:=;
         cover:=false;
         for j:=i+ to n do
           begin
             d:=sqrt((x[i]-x[j])*(x[i]-x[j])+(y[i]-y[j])*(y[i]-y[j]));
             if (r[i]+r[j]-d<eps)or(d+r[j]-r[i]<eps) then continue;
             if d+r[i]-r[j]<eps then
             begin
               cover:=true;
               break;
             end;
             a:=tan(x[j]-x[i],y[j]-y[i]);
             b:=angle(d,r[i],r[j]);
             inc(tot);
             ll[tot]:=a-b;
             rr[tot]:=a+b;
             if ll[tot]<-eps then
             begin
               ll[tot+]:=*pi+ll[tot];
               rr[tot+]:=*pi;
               ll[tot]:=;
               inc(tot);
             end;
             if rr[tot]>*pi+eps then
             begin
               rr[tot+]:=rr[tot]-*pi;
               ll[tot+]:=;
               rr[tot]:=*pi;
               inc(tot);
             end;
           end;
         if cover then continue;
         sort(,tot);
         inc(tot);
         ll[tot]:=;
         rr[tot]:=;
         sum:=;
         xx:=;
         yy:=;
         for j:= to tot do
           if ll[j]+eps<=yy then
           begin
             if rr[j]+eps<=yy then continue;
             yy:=rr[j];
           end
           else
           begin
             sum:=sum+yy-xx;
             xx:=ll[j];
             yy:=rr[j];
           end;
         ans:=ans+(*pi-sum)*r[i];
       end;
     write(ans::);
 end;

 begin
     init;
     work;
 end.