CODEVS3123 a*b problem plus (FFT)

type xh=record
        x,y:double;
        end;
      arr=array[0..1000008] 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 1<<t长度
var i,p:longint;
    cnt:xh;
begin
    if n>>t=1 then exit;
    dft(a,s,t+1); dft(a,s+1<<t,t+1);
    for i:=0 to n>>t>>1-1 do
    begin
        p:=i<<t<<1+s;
        cnt:=w[i<<t]*a[p+1<<t];
        g[i]:=a[p]+cnt;
        g[i+n>>t>>1]:=a[p]-cnt;
    end;
    for i:=0 to n>>t-1 do a[s+i<<t]:=g[i];
end;
procedure clr(var a:arr);
begin
    fillchar(a,sizeof(a),0);
end;
procedure FFT;
var i,len:longint;
    lx:longint;
begin
    n:=1;
    while n<m<<1 do n:=n<<1;
    for i:=0 to n-1 do w[i].x:=cos(pi*2*i/n);
    for i:=0 to n-1 do w[i].y:=sin(pi*2*i/n);
    dft(a,0,0); dft(b,0,0);
    for i:=0 to n-1 do a[i]:=a[i]*b[i];
    for i:=0 to n-1 do w[i].y:=-w[i].y;  //！！！！
    dft(a,0,0);
    for i:=m<<1-1 downto 0 do a[i].x:=a[i].x/n;//！！！！
    for i:=0 to m<<1-1 do
    begin
        lx:=round(a[i].x);
        a[i+1].x:=a[i+1].x+lx div 10;
        a[i].x:=lx mod 10;
    end;
        len:=m<<1-1;
        while a[len].x<1e-12 do dec(len);
    for i:=len downto 0 do write(a[i].x:0:0);
    writeln;
end;
procedure main;
var i:longint;
begin
    read(ch);
    while ord(ch)>=48 do begin s1:=s1+ch; read(ch); end;
    readln(s2);
    clr(a); clr(b);
    for i:=length(s1) downto 1 do a[length(s1)-i].x:=ord(s1[i])-48;
    for i:=length(s2) downto 1 do b[length(s2)-i].x:=ord(s2[i])-48;
    if length(s1)>length(s2) then m:=length(s1) else m:=length(s2);
    FFT;
end;
begin
    main;
end.