public Expression Parse(string input)
{
if (string.IsNullOrWhiteSpace(input))
{
return(null);
}
ExpressionTokenizer tokenizer = new ExpressionTokenizer(input);
while (tokenizer.HasToken())
{
Token token = tokenizer.NextToken();
TokenType tokenType = token.Type;
string tokenValue = token.Value;
if (tokenType == TokenType.Operator)
{
switch (tokenValue)
{
case "(":
if (operatorStack.Contains(")"))
{
throw new Exception("Illigal expression");
}
operatorStack.Push(tokenValue);
break;
case ")":
if (!operatorStack.Contains("("))
{
throw new Exception("Illigal expression");
}
while (operatorStack.Count > 0)
{
string op = operatorStack.Pop();
if (op == "(")
{
break;
}
Expression right = operandStack.Pop();
Expression left = operandStack.Pop();
operandStack.Push(ParseBinaryExpression(op, left, right));
}
break;
case "+":
case "-":
if (operatorStack.Count == 0)
{
operatorStack.Push(tokenValue);
}
else
{
if (operatorStack.Peek() != "(")
{
string op = operatorStack.Pop();
Expression right = operandStack.Pop();
Expression left = operandStack.Pop();
operandStack.Push(ParseBinaryExpression(op, left, right));
}
operatorStack.Push(tokenValue);
}
break;
case "*":
case "/":
if (operatorStack.Count == 0)
{
operatorStack.Push(tokenValue);
}
else
{
var _op = operatorStack.Peek();
if (_op != "+" && _op != "-" && _op != "(")
{
string op = operatorStack.Pop();
Expression right = operandStack.Pop();
Expression left = operandStack.Pop();
operandStack.Push(ParseBinaryExpression(op, left, right));
}
operatorStack.Push(tokenValue);
}
break;
default:
break;
}
}
else
{
if (tokenType == TokenType.Literal)
{
operandStack.Push(new Literal(tokenValue));
}
else
{
operandStack.Push(new Variable(tokenValue));
}
}
}
while (operatorStack.Count > 0)
{
string op = operatorStack.Pop();
Expression right = operandStack.Pop();
Expression left = operandStack.Pop();
operandStack.Push(ParseBinaryExpression(op, left, right));
}
if (operandStack.Count != 1)
{
throw new Exception("Invalid expression");
}
return(operandStack.Pop());
}