private void Expect(Token.TokenType tokenType)
{
if (!IsNext(tokenType))
{
throw new Exception($"Unexpected token {_lexer.Peek( )} at position {_lexer.Position}");
}
}
private bool TryParseVariable(out ASTNode node)
{
node = null;
if (IsNext(Token.TokenType.Identifier))
{
var token = _lexer.Peek();
var stEntry = _symbolTable.Get(token.Value);
if (stEntry == null)
{
throw new Exception($"Undefined Identifier {token.Value} at position {token.Position}");
}
if (stEntry.Type == SymbolTableEntry.EntryType.Variable)
{
node = new VariableIdentifierASTNode(Accept( ));
}
}
return(node != null);
}
/// <summary>
/// Parses the NUMBER Production Rule
/// NUMBER: [0-9]+
/// </summary>
/// <returns></returns>
private static ASTNode ParseNumber(Lexer lexer)
{
var token = lexer.Peek();
if (token.Type != Token.TokenType.Number)
{
throw new Exception($"Invalid Expression. NUMBER Expected at position {lexer.Position}");
}
lexer.Accept( ); //consume the token
return(new NumberASTNode(token));
}
/// <summary>
/// Parses the TERM Production Rule
/// TERM: FACTOR [('*'|'/') FACTOR]*
/// </summary>
private static ASTNode ParseTerm(Lexer lexer)
{
var left = ParseFactor(lexer);
var peekToken = lexer.Peek(); //look ahead 1 token
while (peekToken.Type == Token.TokenType.Multiplication || peekToken.Type == Token.TokenType.Division)
{
var op = lexer.ReadNext( ); //read the operator
var right = ParseFactor(lexer);
if (right == null)
{
throw new Exception($"Invalid Expression. FACTOR Expected at position {lexer.Position}");
}
left = CreateBinaryOperator(op, left, right);
peekToken = lexer.Peek( ); //lookahead of 1 token LL(k) where K = 1
}
return(left);
}