operadores$ = "*/+-"
espacios$ = "                                                                                "

clear screen
print "24 Game"
print "============\n"
print "The player is provided with 4 numbers with which to perform operations"
print "of addition (+), subtraction (-), multiplication (*) or division (/) to attempt"
print "to get 24 as result."
print "Use reverse Polish notation (first the operands and then the operators)."
print "For example: instead of 2 + 4, type 2 4 +\n\n"

repeat
	print at(0,9) espacios$, espacios$, espacios$, espacios$, espacios$, espacios$
	print at(0,9);
	serie$ = ordenaCadena$(genSerie$())
	validos$ = serie$+operadores$
	line input "Enter your formula in reverse Polish notation: " entrada$
	entrada$ = quitaEspacios$(entrada$)
	entradaOrd$ = ordenaCadena$(entrada$)
	if (right$(entradaOrd$,4) <> serie$) or (len(entradaOrd$)<>7) then
		print "Error in the entered series"
	else
		resultado = evaluaEntrada(entrada$)
		print "The result is = ",resultado," "
		if resultado = 24 then
			print "Correct!"
		else
			print "Error!"
		end if
	end if
	print "Want to try again? (press N to exit, or another key to continue)"
until(upper$(left$(inkey$(),1)) = "N")

sub genSerie$()
	local i, c$, s$
	
	print "The numbers to be used are: ";
	i = ran()
	for i = 1 to 4
		c$ = str$(int(ran(9))+1)
		print c$," ";
		s$ = s$ + c$
	next i
	print
	return s$
end sub


sub evaluaEntrada(entr$)
	local d1, d2, c$, n(4), i
	
	while(entr$<>"")
		c$ = left$(entr$,1)
		entr$ = mid$(entr$,2)		
		if instr(serie$,c$) then
			i = i + 1
			n(i) = val(c$)
		elseif instr(operadores$,c$) then
			d2 = n(i)
			n(i) = 0
			i = i - 1
			d1 = n(i)
			n(i) = evaluador(d1, d2, c$)
		else
			print "Invalid sign"
			return
		end if
	wend
	
	return n(i)
		
end sub


sub evaluador(d1, d2, op$)
	local t
	
	switch op$
		case "+": t = d1 + d2 : break
		case "-": t = d1 - d2 : break
		case "*": t = d1 * d2 : break
		case "/": t = d1 / d2 : break
	end switch
	
	return t
end sub


sub quitaEspacios$(entr$)
	local n, i, s$, t$(1)
	
	n = token(entr$,t$()," ")
	
	for i=1 to n
		s$ = s$ + t$(i)
	next i
	return s$
end sub


sub ordenaCadena$(cadena$)
	local testigo, n, fin, c$
	
	fin = len(cadena$)-1
	repeat
		testigo = false
		for n = 1 to fin
			if mid$(cadena$,n,1) > mid$(cadena$,n+1,1) then
				testigo = true
				c$ = mid$(cadena$,n,1)
				mid$(cadena$,n,1) = mid$(cadena$,n+1,1)
				mid$(cadena$,n+1,1) = c$
			end if
		next n
	until(testigo = false)
	return cadena$
end sub