// Parse the given stream of tokens
public ParseTree Parse(IEnumerable <Token> tokenSource)
{
isValidParseTree = true;
errors = new List <Error>();
Stack <ISymbol> symbolStack = new Stack <ISymbol>();
symbolStack.Push(Terminal.EOF);
symbolStack.Push(start);
ParseTree parseTree = new ParseTree(start);
Stack <IParseNode> treeStack = new Stack <IParseNode>();
treeStack.Push(new ParseLeaf(Terminal.EOF));
treeStack.Push(parseTree);
IEnumerator <Token> tokenStream = tokenSource.GetEnumerator();
tokenStream.MoveNext();
while (symbolStack.Count > 0)
{
if (Program.debug)
{
Console.WriteLine("=========================================================");
Console.WriteLine(" PARSE: Stack " + SymbolsToString(symbolStack));
Console.WriteLine(" PARSE: expecting " + symbolStack.Peek());
Console.WriteLine(" PARSE: token " + tokenStream.Current);
}
// ignore error tokens
if (tokenStream.Current.Type == TokenType.ERROR)
{
if (Program.debug)
{
Console.WriteLine(" PARSE: skipping error token");
}
errors.Add(new LexicalError(tokenStream.Current));
tokenStream.MoveNext();
continue;
}
if (symbolStack.Peek() is Terminal)
{
Terminal term = symbolStack.Peek() as Terminal;
ParseLeaf leaf = treeStack.Peek() as ParseLeaf;
if (term == Terminal.EPSILON)
{
// epsilon production was used, exclude from parse tree
if (Program.debug)
{
Console.WriteLine(" PARSE: ignore epsilon");
}
symbolStack.Pop();
treeStack.Pop();
}
else if (term.Matches(tokenStream.Current))
{
// current token matches the top of the parse stack, add it to parse tree
if (Program.debug)
{
Console.WriteLine(" PARSE: Terminal match");
}
leaf.Token = tokenStream.Current;
tokenStream.MoveNext();
symbolStack.Pop();
treeStack.Pop();
}
else
{
// current token does no match, recover from error
if (Program.debug)
{
Console.WriteLine(" PARSE: Error, Terminal mismatch");
}
errors.Add(new SyntaxError(tokenStream.Current));
Synchronize(symbolStack, treeStack, tokenStream);
}
}
else // top of stack is a nonterminal
{
Nonterminal var = symbolStack.Pop() as Nonterminal;
IParseNode popped = treeStack.Pop();
ParseTree subtree = popped as ParseTree;
ISymbol[] production = table.Get(var, tokenStream.Current);
if (production == null)
{
// cannot derive the current token from the nonterminal at the top of the stack
if (Program.debug)
{
Console.WriteLine(" PARSE: Error, No such production");
}
symbolStack.Push(var);
treeStack.Push(popped);
errors.Add(new SyntaxError(tokenStream.Current));
Synchronize(symbolStack, treeStack, tokenStream);
}
else
{
// use the production specified by the parse table, add node to parse tree
if (Program.debug)
{
Console.WriteLine(" PARSE: Using production " + SymbolsToString(production));
}
for (int i = production.Length - 1; i >= 0; i--)
{
IParseNode treeChild;
if (production[i] is Terminal)
{
treeChild = new ParseLeaf(production[i] as Terminal);
}
else
{
treeChild = new ParseTree(production[i] as Nonterminal);
}
subtree.Children.Insert(0, treeChild);
treeStack.Push(treeChild);
symbolStack.Push(production[i]);
}
}
}
}
if (Program.debug)
{
Console.WriteLine(parseTree);
}
return(parseTree);
}
// Uses phrase-level error recovery with First and Follow sets to recover from bad state
private void Synchronize(Stack <ISymbol> symbolStack, Stack <IParseNode> treeStack, IEnumerator <Token> tokenStream)
{
// Loop until good state found (return statements) or no more symbols on stack.
sync : while (symbolStack.Count > 0)
{
if (Program.debug)
{
Console.WriteLine(" PARSE: Synchronize token " + tokenStream.Current + " symbol " + symbolStack.Peek());
}
// no recovery from end of file, empty the parse stack
if (tokenStream.Current.Type == TokenType.EOF)
{
while (symbolStack.Count > 0)
{
isValidParseTree = false;
symbolStack.Pop();
treeStack.Pop();
}
if (Program.debug)
{
Console.WriteLine("PARSE: Unexpected EOF");
}
return;
}
if (symbolStack.Peek() is Terminal)
{
Terminal t = symbolStack.Peek() as Terminal;
if (t.Matches(tokenStream.Current))
{
// good state reached
if (Program.debug)
{
Console.WriteLine(" PARSE: Token matches");
}
return;
}
if (syncTerm.Matches(tokenStream.Current))
{
// special case: we have a synchronising token like ';' that we do not want to skip over
// so remove symbol from parse stack
if (Program.debug)
{
Console.WriteLine(" PARSE: Discarding symbol " + symbolStack.Peek());
}
isValidParseTree = false;
symbolStack.Pop();
treeStack.Pop();
continue;
}
// normal case: discard token and continue with recovery
if (Program.debug)
{
Console.WriteLine(" PARSE: Discard token");
}
if (!tokenStream.MoveNext())
{
throw new Exception("OUT OF TOKENS");
}
continue;
}
else
{
Nonterminal v = symbolStack.Peek() as Nonterminal;
if (table.Get(v, tokenStream.Current) != null)
{
// good state reached (current token in First set of symbol at top of stack)
if (Program.debug)
{
Console.WriteLine(" PARSE: Valid production exists");
}
return;
}
// if current terminal could be matched by something in Follow set of
// symbol at top of stack, then skip the current symbol
foreach (ISymbol sym in grammar.Follow(v))
{
if (sym is Terminal)
{
Terminal followTerm = sym as Terminal;
if (followTerm.Matches(tokenStream.Current))
{
if (Program.debug)
{
Console.WriteLine(" PARSE: Discarding symbol " + symbolStack.Peek());
}
isValidParseTree = false;
symbolStack.Pop();
treeStack.Pop();
goto sync;
}
}
if (sym is Nonterminal)
{
Nonterminal followVar = sym as Nonterminal;
if (table.Get(followVar, tokenStream.Current) != null)
{
if (Program.debug)
{
Console.WriteLine(" PARSE: Discarding symbol " + symbolStack.Peek());
}
isValidParseTree = false;
symbolStack.Pop();
treeStack.Pop();
goto sync;
}
}
}
// default action: skip current terminal
if (Program.debug)
{
Console.WriteLine(" PARSE: Discard token");
}
if (!tokenStream.MoveNext())
{
throw new Exception("OUT OF TOKENS");
}
}
}
}