with Ada.Text_IO;              use Ada.Text_IO;
with Ada.Integer_Text_IO;      use Ada.Integer_Text_IO;
with Ada.Text_IO.Unbounded_IO; use Ada.Text_IO.Unbounded_IO;
with Ada.Strings.Unbounded;    use Ada.Strings.Unbounded;

procedure parser is
  type TokenType     is (TokenTypeNone,TokenTypeInteger,TokenTypeOperator,TokenTypeError,TokenTypeEnd);
  type TokenOperator is (TokenOperatorAdd,TokenOperatorSub,TokenOperatorMul,TokenOperatorDiv);
  Input        : Unbounded_String:=To_Unbounded_String("10+4");
  InputPos     : Integer:=1;
  TokenInteger : Integer;
  TokenOp      : TokenOperator;
  CalcResult   : Integer;

  function GetToken return TokenType is

    Char     : Character;
	CurrType : TokenType:=TokenTypeNone;

  begin

    while InputPos<=Length(Input) loop
	  Char:=Element(Input,InputPos);
      Put("Input Pos:");
	  Put(InputPos);
	  Put("::");
	  Put(Char);
	  New_Line;

	  case Char is
	    when ' ' =>
		   if CurrType/=TokenTypeNone then
		     exit;
		   end if;
	    when '0'..'9' =>
		   case CurrType is
		     when TokenTypeNone    => CurrType     := TokenTypeInteger;
			                          TokenInteger := Character'Pos(Char)-Character'Pos('0');
			 when TokenTypeInteger => TokenInteger := TokenInteger*10+ (Character'Pos(Char)-Character'Pos('0'));
			 when others           => null;
		   end case;
		when '+' =>
		  if CurrType=TokenTypeNone then
		    TokenOp  := TokenOperatorAdd;
			CurrType := TokenTypeOperator;
			InputPos := InputPos+1;
		  end if;
		  exit;
		when others =>
		  CurrType:=TokenTypeError;
	  end case;

	  InputPos := InputPos+1;

	end loop;
	if CurrType=TokenTypeNone then
	  CurrType:=TokenTypeEnd;
	end if;
    return CurrType;
  end;

  function Process2 return Integer is
    LeftValue : Integer;
  begin
    return 0;
  end;

  function Process return Integer is
    LeftValue  : Integer;
	RightValue : Integer;
	Op         : TokenOperator;
  begin
	LeftValue:=Process2;
	while GetToken=TokenTypeOperator loop
	  Op := TokenOp;
	  if Op not in (TokenOperatorAdd,TokenOperatorSub) then
	    return LeftValue;
	  end if;
	  RightValue := Process2;
	end loop;
	return LeftValue;
  end;

begin
  Put("Input String:");
  Put(Input);
  New_Line;
  CalcResult:=Process;
  Put("Result");
  Put(CalcResult);
end;