private AstNode Expression()
{
// EXPRESSION = TERM (ADD | SUBTRACT TERM)* (COMPARISON TERM)
var node = Term();
var ops = new[] { Tokens.Add, Tokens.Subtract };
while (ops.Contains(_currentToken.Type))
{
var token = _currentToken;
Eat(token.Type);
if (token.Type == Tokens.Add)
{
node = new AddNode(node, Term());
}
else if (token.Type == Tokens.Subtract)
{
node = new SubtractNode(node, Term());
}
}
if (Tokens.IsComparison(_currentToken.Type))
{
var token = _currentToken;
if (token.Type == Tokens.Equal)
{
Eat(Tokens.Equal);
node = new EqualToNode(node, Term());
}
else if (token.Type == Tokens.NotEqual)
{
Eat(Tokens.NotEqual);
node = new NotEqualToNode(node, Term());
}
else if (token.Type == Tokens.GreaterThan)
{
Eat(Tokens.GreaterThan);
node = new GreaterThanNode(node, Term());
}
else if (token.Type == Tokens.GreaterThanOrEqualTo)
{
Eat(Tokens.GreaterThanOrEqualTo);
node = new GreaterThanOrEqualToNode(node, Term());
}
else if (token.Type == Tokens.LessThan)
{
Eat(Tokens.LessThan);
node = new LessThanNode(node, Term());
}
else if (token.Type == Tokens.LessThanOrEqualTo)
{
Eat(Tokens.LessThanOrEqualTo);
node = new LessThanOrEqualToNode(node, Term());
}
}
return(node);
}
private bool Visit(LessThanOrEqualToNode node)
{
var left = Visit(node.Left);
var right = Visit(node.Right);
if (left is int && right is int)
{
return((int)left <= (int)right);
}
else if (left is float && right is float)
{
return((float)left <= (float)right);
}
return(false);
}