public long Eval(string input)
{
input = input.Trim().Replace(" ", string.Empty);
ValidateInput(input);
var tokens = Tokenizer.Parse(input)
.Where(t => t.IsMathOperator || t.Type == TokenType.Number)
.ToList();
var s = new Stack <Token>();
foreach (var t in tokens)
{
if (t.IsMathOperator)
{
s.Push(t);
}
else
{
s.Push(t);
bool canProcess = true;
while (canProcess)
{
var t1 = s.Pop();
var t2 = s.Pop();
if (t2.Type == TokenType.Number)
{
var operation = s.Pop();
var r = GetCalculation(t1, t2, operation);
s.Push(r);
if (s.Count == 1)
{
canProcess = false;
}
}
else
{
s.Push(t2);
s.Push(t1);
canProcess = false;
}
}
}
}
return(s.Pop().Value.Value);
}