private Statement ParseStatement()
{
if (index == Tokens.Count)
{
throw new ParserException("Unexpected EOF - Expected Statement");
}
Statement result;
var currentToken = Tokens[index];
if (currentToken.Item1 == TokenType.Keyword && (Keywords)currentToken.Item2.Lexeme == Keywords.print)
{
// print statement
index++;
var endIndex = index;
while (Tokens[endIndex].Item1 != TokenType.Semicolon)
{
endIndex++;
}
var print = new Print { Expr = ParseExpression(endIndex) };
result = print;
}
else if (currentToken.Item1 == TokenType.Keyword && (Keywords)currentToken.Item2.Lexeme == Keywords.var)
{
// variable declaration
index++;
var declareVar = new DeclareVariable();
if (index < Tokens.Count && Tokens[index].Item1 == TokenType.Identifier)
{
declareVar.Ident = (string)Tokens[index].Item2.Lexeme;
}
else
{
throw new ParserException("Expected variable name after 'var' keyword");
}
index++;
if (index == Tokens.Count || Tokens[index].Item1 != TokenType.EqualOp)
{
throw new ParserException("Expected assigning in variable declaration");
}
index++;
var endIndex = index;
while (Tokens[endIndex].Item1 != TokenType.Semicolon)
{
endIndex++;
}
declareVar.Expr = ParseExpression(endIndex);
result = declareVar;
}
else if (currentToken.Item1 == TokenType.Keyword && (Keywords)currentToken.Item2.Lexeme == Keywords.read_integer)
{
// read_integer statement
index++;
var readInt = new ReadInteger();
if (index < Tokens.Count && Tokens[index].Item1 == TokenType.Identifier)
{
readInt.Ident = (string)Tokens[index++].Item2.Lexeme;
result = readInt;
}
else
{
throw new ParserException("Expected variable name after 'read_integer' expression");
}
}
else if (currentToken.Item1 == TokenType.Keyword && (Keywords)currentToken.Item2.Lexeme == Keywords.@for)
{
// for-loop statement
index++;
var forLoop = new ForLoop();
if (index < Tokens.Count && Tokens[index].Item1 == TokenType.Identifier)
{
forLoop.Ident = (string)Tokens[index].Item2.Lexeme;
}
else
{
throw new ParserException("Expected loop counter-variable after 'for'");
}
index++;
if (index == Tokens.Count || Tokens[index].Item1 != TokenType.EqualOp)
{
throw new ParserException("Missing '=' in for loop");
}
index++;
forLoop.From = ParseExpression();
if (index == Tokens.Count ||
(Tokens[index].Item1 != TokenType.Keyword || (Tokens[index].Item1 == TokenType.Keyword &&
(Keywords) Tokens[index].Item2.Lexeme !=
Keywords.to)))
{
throw new ParserException("Expected 'to' keyword in for-loop statement");
}
index++;
forLoop.To = ParseExpression();
if (index == Tokens.Count ||
(Tokens[index].Item1 != TokenType.Keyword || (Tokens[index].Item1 == TokenType.Keyword &&
(Keywords) Tokens[index].Item2.Lexeme !=
Keywords.@do)))
{
throw new ParserException("Expected 'do' keyword in for-loop statement");
}
index++;
forLoop.Body = ParseStatement();
result = forLoop;
if (index == Tokens.Count ||
(Tokens[index].Item1 != TokenType.Keyword || (Tokens[index].Item1 == TokenType.Keyword &&
(Keywords) Tokens[index].Item2.Lexeme !=
Keywords.end)))
{
throw new ParserException("Unterminated for-loop body");
}
index++;
}
else if (Tokens[index].Item1 == TokenType.Identifier)
{
// variable assigning statement
var assignStatement = new Assign { Ident = (string)Tokens[index++].Item2.Lexeme };
if (index == Tokens.Count || Tokens[index].Item1 != TokenType.EqualOp)
{
throw new ParserException("Expected '=' operator");
}
index++;
var endIndex = index;
while (Tokens[endIndex].Item1 != TokenType.Semicolon)
{
endIndex++;
}
assignStatement.Expr = ParseExpression(endIndex);
result = assignStatement;
}
else
{
// unknown statement
throw new ParserException("Parse error at token " + index + ": " + Tokens[index]);
}
if (index < Tokens.Count && Tokens[index].Item1 == TokenType.Semicolon)
{
index++;
if (index < Tokens.Count)
{
if (Tokens[index].Item1 != TokenType.Keyword || (Tokens[index].Item1 == TokenType.Keyword &&
(Keywords) Tokens[index].Item2.Lexeme != Keywords.end))
{
var sequenceStatement = new StatementList {First = result, Second = ParseStatement()};
result = sequenceStatement;
}
}
}
return result;
}