CODEVS3123 a*b problem plus (FFT)

 type xh=record
         x,y:double;
         end;
       arr=array[..] of xh;
 var n,m:longint;
     s1,s2:ansistring;
     a,b,g,w:arr;
     ch:char;
 operator -(a,b:xh) c:xh;
 begin
     c.x:=a.x-b.x;
     c.y:=a.y-b.y;
 end;
 operator +(a,b:xh) c:xh;
 begin
     c.x:=a.x+b.x;
     c.y:=a.y+b.y;
 end;
 operator *(a,b:xh) c:xh;
 begin
     c.x:=a.x*b.x-a.y*b.y;
     c.y:=a.x*b.y+a.y*b.x;
 end;
 procedure dft(var a:arr;s,t:longint);//a待处理数组 s初始位置 <<t长度
 var i,p:longint;
     cnt:xh;
 begin
     if n>>t= then exit;
     dft(a,s,t+); dft(a,s+<<t,t+);
     for i:= to n>>t>>- do
     begin
         p:=i<<t<<+s;
         cnt:=w[i<<t]*a[p+<<t];
         g[i]:=a[p]+cnt;
         g[i+n>>t>>]:=a[p]-cnt;
     end;
     for i:= to n>>t- do a[s+i<<t]:=g[i];
 end;
 procedure clr(var a:arr);
 begin
     fillchar(a,sizeof(a),);
 end;
 procedure FFT;
 var i,len:longint;
     lx:longint;
 begin
     n:=;
     while n<m<< do n:=n<<;
     for i:= to n- do w[i].x:=cos(pi**i/n);
     for i:= to n- do w[i].y:=sin(pi**i/n);
     dft(a,,); dft(b,,);
     for i:= to n- do a[i]:=a[i]*b[i];
     for i:= to n- do w[i].y:=-w[i].y;  //！！！！
     dft(a,,);
     for i:=m<<- downto  do a[i].x:=a[i].x/n;//！！！！
     for i:= to m<<- do
     begin
         lx:=round(a[i].x);
         a[i+].x:=a[i+].x+lx div ;
         a[i].x:=lx mod ;
     end;
         len:=m<<-;
         while a[len].x<1e-12 do dec(len);
     for i:=len downto  do write(a[i].x::);
     writeln;
 end;
 procedure main;
 var i:longint;
 begin
     read(ch);
     while ord(ch)>= do begin s1:=s1+ch; read(ch); end;
     readln(s2);
     clr(a); clr(b);
     for i:=length(s1) downto  do a[length(s1)-i].x:=ord(s1[i])-;
     for i:=length(s2) downto  do b[length(s2)-i].x:=ord(s2[i])-;
     if length(s1)>length(s2) then m:=length(s1) else m:=length(s2);
     FFT;
 end;
 begin
     main;
 end.