问Pascal子串等价物

EN

    function substring(s: string; a, b: integer): string;
    var len: integer;
      procedure swap(var a, b: integer);
      var temp: integer;
      begin
        temp:= a;
        a:= b;
        b:= temp;
      end;
    begin
      if (a > b) then
        swap(a, b);
      len:= length(s);
      if ((len = 0) or ((a < 1) and (b < 1)) or 
          ((a > len) and (b > len))) then
      begin
        substring:= '';
      end
      else
      begin
        if (a < 1) then
          a:= 1;
        if (b > len) then
          b:= len;
        substring:= copy(s, a, b);
      end;
    end;