bzoj2829

裸题，直接上凸包，然后加上一个圆周即可

只是在这之前没写过旋转而已

 const pi=3.14159265358979323;
       eps=1e-8;
 type point=record
        x,y:double;
      end;

 var p:array[..] of point;
     q:array[..] of longint;
     n,i,k,t:longint;
     ans,a,b,x,y,an,r:double;

 procedure swap(var a,b:point);
   var c:point;
   begin
     c:=a;
     a:=b;
     b:=c;
   end;

 function cmp(a,b:point):boolean;
   begin
     if abs(a.x-b.x)<eps then exit(a.y<b.y);
     exit(a.x<b.x);
   end;

 procedure sort(l,r:longint);
   var i,j:longint;
       x:point;
   begin
     i:=l;
     j:=r;
     x:=p[(l+r) shr ];
     repeat
       while cmp(p[i],x) do inc(i);
       while cmp(x,p[j]) do dec(j);
       if not(i>j) then
       begin
         swap(p[i],p[j]);
         inc(i);
         dec(j);
       end;
     until i>j;
     if l<j then sort(l,j);
     if i<r then sort(i,r);
   end;

 function cross(i,j,k:longint):double;
   begin
     exit((p[i].x-p[k].x)*(p[j].y-p[k].y)-(p[j].x-p[k].x)*(p[i].y-p[k].y));
   end;

 function dis(i,j:longint):double;
   begin
     exit(sqrt(sqr(p[i].x-p[j].x)+sqr(p[i].y-p[j].y)));
   end;

 function get(x,y,x0,y0:double):point;
   begin
     get.x:=cos(an)*x-sin(an)*y+x0;
     get.y:=sin(an)*x+cos(an)*y+y0;
   end;

 begin
   readln(n);
   readln(b,a,r);
   a:=a-*r;
   b:=b-*r;
   for i:= to n do
   begin
     readln(x,y,an);
     inc(t); p[t]:=get(a/,-b/,x,y);
     inc(t); p[t]:=get(a/,b/,x,y);
     inc(t); p[t]:=get(-a/,b/,x,y);
     inc(t); p[t]:=get(-a/,-b/,x,y);
   end;
   sort(,t);
   n:=t;
   t:=;
   q[]:=;
   for i:= to n do
   begin
     while (t>) and (cross(i,q[t],q[t-])>=-eps) do dec(t);
     inc(t);
     q[t]:=i;
   end;
   k:=t;
   for i:=n- downto  do
   begin
     while (t>k) and (cross(i,q[t],q[t-])>=-eps) do dec(t);
     inc(t);
     q[t]:=i;
   end;
   for i:= to t do
     ans:=ans+dis(q[i],q[i-]);

   writeln(ans+*r*pi::);
 end.