operators$ = "*+-/"
space$ = "                                                                                "

sub present()
	clear screen
	print "24 Game"
	print "============\n"
	print "Computer provide 4 numbers (1 to 9). With operators +, -, * and / you try to\nobtain 24."
	print "Use Reverse Polish Notation (first operand and then the operators)"
	print "For example: instead of 2 + 4, type 2 4 +\n\n"
end sub

repeat
	present()
	serie$ = sortString$(genSerie$())
	valid$ = serie$+operators$
	print "If you give up, press ENTER and the program attempts to find a solution."
	line input "Write your solution: " input$
	if input$ = "" then
		print "Thinking ... "
		res$ = explorer$()
		if res$ = "" print "Can not get 24 with these numbers.."
	else
		input$ = delSpace$(input$)
		inputSort$ = sortString$(input$)
		if (right$(inputSort$,4) <> serie$) or (len(inputSort$)<>7) then
			print "Syntax error"
		else
			result = evalInput(input$)
			print "Your solution = ",result," is ";
			if result = 24 then
				print "Correct!"
			else
				print "Wrong!"
			end if
		end if
	end if
	print "\nDo you want to try again? (press N for exit, other key to continue)"
until(upper$(left$(inkey$(),1)) = "N")

exit

sub genSerie$()
	local i, c$, s$
	
	print "The numbers you should use 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 evalInput(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$)
		elseif instr(operators$,c$) then
			d2 = n(i)
			n(i) = 0
			i = i - 1
			if i = 0 return
			d1 = n(i)
			n(i) = evaluator(d1, d2, c$)
		else
			print "Invalid symbol"
			return
		end if
	wend
	
	return n(i)
		
end sub


sub evaluator(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 delSpace$(entr$)
	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 sortString$(string$)
	local signal, n, fin, c$
	
	fin = len(string$)-1
	repeat
		signal = false
		for n = 1 to fin
			if mid$(string$,n,1) > mid$(string$,n+1,1) then
				signal = true
				c$ = mid$(string$,n,1)
				mid$(string$,n,1) = mid$(string$,n+1,1)
				mid$(string$,n+1,1) = c$
			end if
		next n
	until(signal = false)
	return string$
end sub


sub explorer$()
	local d1,d2,o3,x4,x5,x6,o7,p$,result,solution,solutions$,n
	
	for d1 = 1 to 4
		for d2 = 1 to 4
			for o3 = 1 to 4
				for x4 = 1 to 8
					for x5 = 1 to 8
						for x6 = 1 to 8
							for o7 = 1 to 4
								p$ = mid$(serie$,d1,1)+mid$(serie$,d2,1)+mid$(operators$,o3,1)
								p$ = p$+mid$(valid$,x4,1)+mid$(valid$,x5,1)+mid$(valid$,x6,1)
								p$ = p$+mid$(operators$,o7,1)
								if not instr(solutions$,p$) then
									if validateInput(p$) then
										result = evalInput(p$)
										if result = 24 then
											solution = solution + 1
											print "Solution: ",solution," = ";
											solutions$ = solutions$ + p$
											for n = 1 to 7
												print mid$(p$,n,1)," ";
											next n
											print
										end if
									end if
								end if
							next o7
						next x6
					next x5
				next x4
			next o3
		next d2
	next d1
	return p$	
end sub


sub validateInput(e$)
	local n, inputSort$
	
	inputSort$ = sortString$(e$)
	if serie$ <> right$(inputSort$,4) return false
	for n=1 to 3
		if not instr(operators$,mid$(inputSort$,n,1)) then
			return false
		end if
	next n
	return true
end sub