Indetectables

program Project1;

{$APPTYPE CONSOLE}
{$WriteableConst On}//esta directiva es necearia para modificar constantes


uses
  SysUtils,Math;


// Grammar:
        // expression = term | expression `+` term | expression `-` term
        // term = factor | term `*` factor | term `/` factor | term brackets
        // factor = brackets | number | factor `^` factor
        // brackets = `(` expression `)`

//Esta funcion utliza recursividad mutua


function EvaluarString(const str:string):double;
const
pos:integer=0;//inicia en cero para despues convertirse en 1
var
c:integer;
procedure eatChar; //va obteniendo caracter por caracter excepto los vacios
begin
  inc(pos);
  if pos<=length(str) then c:=ord(str[pos])
  else c:=0;
end;
procedure eatSpace; //come los espacions en blanco
begin
  while c=32 do eatChar;
end;
function parseExpresion:double; forward; //prototipo de la funcion
function parseTerm:double; forward;  //prototipo de la funcion
function parseFactor:double; forward;  //prototipo de la funcion
function parse:double;
var
v:double;
begin
  eatChar();
  v:=parseExpresion();

  if c<>0 then
  begin
    writeln('Error en el simbolo ',char(c));
    Result:=0.0;
    Exit;
  end;
  Result:=v;
end;
function parseExpresion:double;
var
v:double;
begin
   v:=parseTerm();

   while(true)do
   begin

     eatSpace();

     if c=ord('+') then //suma
     begin
       eatChar();
       v:=v+parseTerm();
     end
     else if c=ord('-') then  //resta
     begin
       eatChar();
       v:=v-parseTerm();
     end
     else
     begin
       Result:=v;
       break;//necesario si no entra en un bucle sin fin
     end;
   end;
end;
function parseTerm:double;
var
v:double;
begin
  v:=parseFactor();
  while True do
  begin
    eatSpace();

    if c=ord('/') then
    begin
      eatChar;
      v:=v/parseFactor;
    end
    else if((c=ord('*')) or (c=ord('('))) then  //multiplicacion
    begin
       if c=ord('*') then eatChar;
       v:=v*parseFactor();
    end
    else
    begin
      Result:=v;
      break;//necesario si no entra en un bucle sin fin
    end;
  end;
end;
function parseFactor:double;
var
v:double;
negate:boolean;
sb:string;
begin
  negate:=false;
  eatSpace;

  if c=ord('(') then
  begin
    eatChar;
    v:=parseExpresion;

    if c=ord(')') then eatChar;
  end
  else   //numeros
  begin
    if ((c=ord('+')) or (c=ord('-'))) then  //simbolos unarios positivo y negativo
    begin
      negate:=c=ord('-');
      eatChar;
      eatSpace;
    end;

    sb:='';
    while (((c>=ord('0'))and (c<=ord('9'))) or (c=ord('.'))) do
    begin
      sb:=sb+chr(c);
      eatChar;
    end;


    if length(sb)=0 then
    begin
      writeln('Error no existen caracterese en sb');
      result:=0.0;
      Exit;
    end;

    v:=strtofloat(sb);
  end;
  eatSpace;
  if c=ord('^') then  //exponente
  begin
    eatChar;
    v:=Math.Power(v,parseFactor);
  end;

  if negate then v:=-v;
  result:=v;
end;
begin
  result:=parse();//retorna la operacion
end;


var
e:double;
begin
  //Ejemplo de uso
  e:=EvaluarString('(5*3)+(5^2)');  //evaluar expresion
  writeln(e:0:2);//resultado 40
  readln;

end.