ParserATNSimulator.java

/*
 [The "BSD license"]
  Copyright (c) 2011 Terence Parr
  All rights reserved.

  Redistribution and use in source and binary forms, with or without
  modification, are permitted provided that the following conditions
  are met:

  1. Redistributions of source code must retain the above copyright
     notice, this list of conditions and the following disclaimer.
  2. Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in the
     documentation and/or other materials provided with the distribution.
  3. The name of the author may not be used to endorse or promote products
     derived from this software without specific prior written permission.

  THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
  IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
  INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
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 */

package org.antlr.v4.runtime.atn;

import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.dfa.DFA;
import org.antlr.v4.runtime.dfa.DFAState;
import org.antlr.v4.runtime.misc.IntervalSet;
import org.antlr.v4.runtime.misc.NotNull;
import org.antlr.v4.runtime.misc.Nullable;
import org.antlr.v4.runtime.misc.Utils;
import org.stringtemplate.v4.misc.MultiMap;

import java.util.*;

/**
 The embodiment of the adaptive LL(*) parsing strategy.

 The basic complexity of the adaptive strategy makes it harder to
 understand. We begin with ATN simulation to build paths in a
 DFA. Subsequent prediction requests go through the DFA first. If
 they reach a state without an edge for the current symbol, the
 algorithm fails over to the ATN simulation to complete the DFA
 path for the current input (until it finds a conflict state or
 uniquely predicting state).

 All of that is done without using the outer context because we
 want to create a DFA that is not dependent upon the rule
 invocation stack when we do a prediction.  One DFA works in all
 contexts. We avoid using context not necessarily because it
 slower, although it can be, but because of the DFA caching
 problem.  The closure routine only considers the rule invocation
 stack created during prediction beginning in the entry rule.  For
 example, if prediction occurs without invoking another rule's
 ATN, there are no context stacks in the configurations. When this
 leads to a conflict, we don't know if it's an ambiguity or a
 weakness in the strong LL(*) parsing strategy (versus full
 LL(*)).

 So, we simply retry the ATN simulation again, this time
 using full outer context and filling a dummy DFA (to avoid
 polluting the context insensitive DFA). Configuration context
 stacks will be the full invocation stack from the start rule. If
 we get a conflict using full context, then we can definitively
 say we have a true ambiguity for that input sequence. If we don't
 get a conflict, it implies that the decision is sensitive to the
 outer context. (It is not context-sensitive in the sense of
 context sensitive grammars.) We create a special DFA accept state
 that maps rule context to a predicted alternative. That is the
 only modification needed to handle full LL(*) prediction. In
 general, full context prediction will use more lookahead than
 necessary, but it pays to share the same DFA. For a schedule
 proof that full context prediction uses that most the same amount
 of lookahead as a context insensitive prediction, see the comment
 on method retryWithContext().

 So, the strategy is complex because we bounce back and forth from
 the ATN to the DFA, simultaneously performing predictions and
 extending the DFA according to previously unseen input
 sequences. The retry with full context is a recursive call to the
 same function naturally because it does the same thing, just with
 a different initial context. The problem is, that we need to pass
 in a "full context mode" parameter so that it knows to report
 conflicts differently. It also knows not to do a retry, to avoid
 infinite recursion, if it is already using full context.

 Retry a simulation using full outer context.
	 *
	 *  One of the key assumptions here is that using full context
	 *  can use at most the same amount of input as a simulation
	 *  that is not useful context (i.e., it uses all possible contexts
	 *  that could invoke our entry rule. I believe that this is true
	 *  and the proof might go like this.
	 *
	 *  THEOREM:  The amount of input consumed during a full context
	 *  simulation is at most the amount of input consumed during a
	 *  non full context simulation.
	 *
	 *  PROOF: Let D be the DFA state at which non-context simulation
	 *  terminated. That means that D does not have a configuration for
	 *  which we can legally pursue more input. (It is legal to work only
	 *  on configurations for which there is no conflict with another
	 *  configuration.) Now we restrict ourselves to following ATN edges
	 *  associated with a single context. Choose any DFA state D' along
	 *  the path (same input) to D. That state has either the same number
	 *  of configurations or fewer. (If the number of configurations is
	 *  the same, then we have degenerated to the non-context case.) Now
	 *  imagine that we restrict to following edges associated with
	 *  another single context and that we reach DFA state D'' for the
	 *  same amount of input as D'. The non-context simulation merges D'
	 *  and D''. The union of the configuration sets either has the same
	 *  number of configurations as both D' and D'' or it has more. If it
	 *  has the same number, we are no worse off and the merge does not
	 *  force us to look for more input than we would otherwise have to
	 *  do. If the union has more configurations, it can introduce
	 *  conflicts but not new alternatives--we cannot conjure up alternatives
	 *  by computing closure on the DFA state.  Here are the cases for
	 *  D' union D'':
	 *
	 *  1. No increase in configurations, D' = D''
	 *  2. Add configuration that introduces a new alternative number.
	 *     This cannot happen because no new alternatives are introduced
	 *     while computing closure, even during start state computation.
	 *  3. D'' adds a configuration that does not conflict with any
	 *     configuration in D'.  Simulating without context would then have
	 *     forced us to use more lookahead than D' (full context) alone.
	 *  3. D'' adds a configuration that introduces a conflict with a
	 *     configuration in D'. There are 2 cases:
	 *     a. The conflict does not cause termination (D' union D''
	 *        is added to the work list). Again no context simulation requires
	 *        more input.
	 *     b. The conflict does cause termination, but this cannot happen.
	 *        By definition, we know that with ALL contexts merged we
	 *        don't terminate until D and D' uses less input than D. Therefore
	 *        no context simulation requires more input than full context
	 *        simulation.
	 *
	 *  We have covered all the cases and there is never a situation where
	 *  a single, full context simulation requires more input than a
	 *  no context simulation.

	 I spent a bunch of time thinking about this problem after finding
	 a case where context-sensitive ATN simulation looks beyond what they
	 no context simulation uses. the no context simulation for if then else
	 stops at the else whereas full context scans through to the end of the
	 statement to decide that the "else statement" clause is ambiguous. And
	 sometimes it is not ambiguous! Ok, I made an untrue assumption in my
	 proof which I won't bother going to. the important thing is what I'm
	 going to do about it. I thought I had a simple answer, but nope. It
	 turns out that the if then else case is perfect example of something
	 that has the following characteristics:

	 * no context conflicts at k=1
	 * full context at k=(1 + length of statement) can be both ambiguous and not
	   ambiguous depending on the input, though I think from different contexts.

	 But, the good news is that the k=1 case is a special case in that
	 SLL(1) and LL(1) have exactly the same power so we can conclude that
	 conflicts at k=1 are true ambiguities and we do not need to pursue
	 context-sensitive parsing. That covers a huge number of cases
	 including the if then else clause and the predicated precedence
	 parsing mechanism. whew! because that could be extremely expensive if
	 we had to do context.

	 Further, there is no point in doing full context if none of the
	 configurations dip into the outer context. This nicely handles cases
	 such as super constructor calls versus function calls. One grammar
	 might look like this:

	 ctorBody : '{' superCall? stat* '}' ;

	 Or, you might see something like

	 stat : superCall ';' | expression ';' | ... ;

	 In both cases I believe that no closure operations will dip into the
	 outer context. In the first case ctorBody in the worst case will stop
	 at the '}'. In the 2nd case it should stop at the ';'. Both cases
	 should stay within the entry rule and not dip into the outer context.

	 So, we now cover what I hope is the vast majority of the cases (in
	 particular the very important precedence parsing case). Anything that
	 needs k>1 and dips into the outer context requires a full context
	 retry. In this case, I'm going to start out with a brain-dead solution
	 which is to mark the DFA state as context-sensitive when I get a
	 conflict. Any further DFA simulation that reaches that state will
	 launch an ATN simulation to get the prediction, without updating the
	 DFA or storing any context information. Later, I can make this more
	 efficient, but at least in this case I can guarantee that it will
	 always do the right thing. We are not making any assumptions about
	 lookahead depth.

	 Ok, writing this up so I can put in a comment.

	 Upon conflict in the no context simulation:

	 * if k=1, report ambiguity and resolve to the minimum conflicting alternative

	 * if k=1 and predicates, no report and include the predicate to
	   predicted alternative map in the DFA state

	 * if k=* and we did not dip into the outer context, report ambiguity
	   and resolve to minimum conflicting alternative

	 * if k>1 and we dip into outer context, retry with full context
		 * if conflict, report ambiguity and resolve to minimum conflicting
		   alternative, mark DFA as context-sensitive
		 * If no conflict, report ctx sensitivity and mark DFA as context-sensitive
		 * Technically, if full context k is less than no context k, we can
			 reuse the conflicting DFA state so we don't have to create special
			 DFA paths branching from context, but we can leave that for
			 optimization later if necessary.

	 * if non-greedy, no report and resolve to the exit alternative
 *
 * 	By default we do full context-sensitive LL(*) parsing not
 	 *  Strong LL(*) parsing. If we fail with Strong LL(*) we
 	 *  try full LL(*). That means we rewind and use context information
 	 *  when closure operations fall off the end of the rule that
 	 *  holds the decision were evaluating
*/
public class ParserATNSimulator<Symbol extends Token> extends ATNSimulator {
	public static boolean debug = false;
	public static boolean dfa_debug = false;
	public static boolean retry_debug = false;

	public boolean disable_global_context = false;
	public boolean force_global_context = false;
	public boolean always_try_local_context = true;

	public boolean optimize_ll1 = true;

	public static boolean optimize_closure_busy = true;

	public static int ATN_failover = 0;
	public static int predict_calls = 0;
	public static int retry_with_context = 0;
	public static int retry_with_context_indicates_no_conflict = 0;

	@Nullable
	protected final Parser parser;

	@NotNull
	public final DFA[] decisionToDFA;

	/** By default we do full context-sensitive LL(*) parsing not
	 *  Strong LL(*) parsing. If we fail with Strong LL(*) we
	 *  try full LL(*). That means we rewind and use context information
	 *  when closure operations fall off the end of the rule that
	 *  holds the decision were evaluating.
	 */
	protected boolean userWantsCtxSensitive = true;

	protected final Map<Integer, Integer> LL1Table = new HashMap<Integer, Integer>();

	/** Testing only! */
	public ParserATNSimulator(@NotNull ATN atn) {
		this(null, atn);
	}

	public ParserATNSimulator(@Nullable Parser parser, @NotNull ATN atn) {
		super(atn);
		this.parser = parser;
//		ctxToDFAs = new HashMap<RuleContext, DFA[]>();
		// TODO (sam): why distinguish on parser != null?
		decisionToDFA = new DFA[atn.getNumberOfDecisions() + (parser != null ? 1 : 0)];
		//		DOTGenerator dot = new DOTGenerator(null);
		//		System.out.println(dot.getDOT(atn.rules.get(0), parser.getRuleNames()));
		//		System.out.println(dot.getDOT(atn.rules.get(1), parser.getRuleNames()));
	}

	@Override
	public void reset() {
	}

	public int adaptivePredict(@NotNull SymbolStream<? extends Symbol> input, int decision,
							   @Nullable ParserRuleContext<?> outerContext)
	{
		return adaptivePredict(input, decision, outerContext, false);
	}

	public int adaptivePredict(@NotNull SymbolStream<? extends Symbol> input,
							   int decision,
							   @Nullable ParserRuleContext<?> outerContext,
							   boolean useContext)
	{
		predict_calls++;
		DFA dfa = decisionToDFA[decision];
		if (optimize_ll1 && dfa != null) {
			int ll_1 = input.LA(1);
			if (ll_1 >= 0 && ll_1 <= Short.MAX_VALUE) {
				int key = (decision << 16) + ll_1;
				Integer alt = LL1Table.get(key);
				if (alt != null) {
					return alt;
				}
			}
		}

		if (force_global_context) {
			useContext = true;
		}
		else if (!always_try_local_context) {
			useContext |= dfa != null && dfa.isContextSensitive();
		}

		userWantsCtxSensitive = useContext || (!disable_global_context && (outerContext != null));
		if (outerContext == null) {
			outerContext = ParserRuleContext.EMPTY;
		}

		SimulatorState state = null;
		if (dfa != null) {
			state = getStartState(dfa, input, outerContext, useContext);
		}

		if ( state==null ) {
			if ( dfa==null ) {
				DecisionState startState = atn.decisionToState.get(decision);
				decisionToDFA[decision] = dfa = new DFA(startState, decision);
			}

			return predictATN(dfa, input, outerContext, useContext);
		}
		else {
			//dump(dfa);
			// start with the DFA
			int m = input.mark();
			int index = input.index();
			try {
				int alt = execDFA(dfa, input, index, state);
				return alt;
			}
			finally {
				input.seek(index);
				input.release(m);
			}
		}
	}

	public SimulatorState getStartState(@NotNull DFA dfa,
										@NotNull SymbolStream<?> input,
										@NotNull ParserRuleContext<?> outerContext,
										boolean useContext) {

		if (!useContext) {
			if (dfa.s0 == null) {
				return null;
			}

			return new SimulatorState(outerContext, dfa.s0, false, outerContext);
		}

		RuleContext remainingContext = outerContext;
		assert outerContext != null;
		DFAState s0 = dfa.s0;
		while (remainingContext != null && s0 != null && s0.isCtxSensitive) {
			s0 = s0.getContextTarget(remainingContext.invokingState);
			remainingContext = remainingContext.parent;
		}

		if (s0 == null) {
			return null;
		}

		return new SimulatorState(outerContext, s0, useContext, (ParserRuleContext<?>)remainingContext);
	}

	public int predictATN(@NotNull DFA dfa, @NotNull SymbolStream<? extends Symbol> input,
						  @Nullable ParserRuleContext<?> outerContext,
						  boolean useContext)
	{
		if ( outerContext==null ) outerContext = ParserRuleContext.EMPTY;
		if ( debug ) System.out.println("ATN decision "+dfa.decision+
										" exec LA(1)=="+ getLookaheadName(input) +
										", outerContext="+outerContext.toString(parser));

		int alt = 0;
		int m = input.mark();
		int index = input.index();
		try {
			SimulatorState state = computeStartState(dfa, outerContext, useContext);
			alt = execATN(dfa, input, index, state);
		}
		catch (NoViableAltException nvae) {
			if ( debug ) dumpDeadEndConfigs(nvae);
			throw nvae;
		}
		finally {
			input.seek(index);
			input.release(m);
		}
		if ( debug ) System.out.println("DFA after predictATN: "+dfa.toString(parser.getTokenNames(), parser.getRuleNames()));
		return alt;
	}

	public int execDFA(@NotNull DFA dfa,
					   @NotNull SymbolStream<? extends Symbol> input, int startIndex,
					   @NotNull SimulatorState state)
    {
		ParserRuleContext<?> outerContext = state.outerContext;
		if ( dfa_debug ) System.out.println("DFA decision "+dfa.decision+
											" exec LA(1)=="+ getLookaheadName(input) +
											", outerContext="+outerContext.toString(parser));
		if ( dfa_debug ) System.out.print(dfa.toString(parser.getTokenNames(), parser.getRuleNames()));
		DFAState acceptState = null;
		DFAState s = state.s0;

		int t = input.LA(1);
		ParserRuleContext<?> remainingOuterContext = (ParserRuleContext<?>)state.remainingOuterContext;

	loop:
		while ( true ) {
			if ( dfa_debug ) System.out.println("DFA state "+s.stateNumber+" LA(1)=="+getLookaheadName(input));
			if ( state.useContext ) {
				while ( s.isCtxSensitive && s.contextSymbols.contains(t) ) {
					DFAState next = s.getContextTarget(remainingOuterContext.invokingState);
					if ( next == null ) {
						// fail over to ATN
						SimulatorState initialState = new SimulatorState(state.outerContext, s, state.useContext, remainingOuterContext);
						return execATN(dfa, input, startIndex, initialState);
					}

					remainingOuterContext = (ParserRuleContext<?>)remainingOuterContext.parent;
					s = next;
				}
			}
			if ( s.isAcceptState ) {
				if ( s.predicates!=null ) {
					if ( dfa_debug ) System.out.println("accept "+s);
				}
				else {
					if ( dfa_debug ) System.out.println("accept; predict "+s.prediction +" in state "+s.stateNumber);
				}
				acceptState = s;
				// keep going unless we're at EOF or state only has one alt number
				// mentioned in configs; check if something else could match
				// TODO: don't we always stop? only lexer would keep going
				// TODO: v3 dfa don't do this.
				break;
			}
			// if no edge, pop over to ATN interpreter, update DFA and return
			DFAState target = s.getTarget(t);
			if ( target == null ) {
				if ( dfa_debug && t>=0 ) System.out.println("no edge for "+parser.getTokenNames()[t]);
				int alt;
				if ( dfa_debug ) {
					System.out.println("ATN exec upon "+
                                       parser.getInputString(startIndex) +
									   " at DFA state "+s.stateNumber);
				}

				SimulatorState initialState = new SimulatorState(outerContext, s, state.useContext, remainingOuterContext);
				alt = execATN(dfa, input, startIndex, initialState);
				// this adds edge even if next state is accept for
				// same alt; e.g., s0-A->:s1=>2-B->:s2=>2
				// TODO: This next stuff kills edge, but extra states remain. :(
				if ( s.isAcceptState && alt!=-1 ) {
					DFAState d = s.getTarget(input.LA(1));
					if ( d.isAcceptState && d.prediction==s.prediction ) {
						// we can carve it out.
						s.setTarget(input.LA(1), ERROR); // IGNORE really not error
					}
				}
				if ( dfa_debug ) {
					System.out.println("back from DFA update, alt="+alt+", dfa=\n"+dfa.toString(parser.getTokenNames(), parser.getRuleNames()));
					//dump(dfa);
				}
				// action already executed
				if ( dfa_debug ) System.out.println("DFA decision "+dfa.decision+
													" predicts "+alt);
				return alt; // we've updated DFA, exec'd action, and have our deepest answer
			}
			else if ( target == ERROR ) {
				throw noViableAlt(input, outerContext, s.configset, startIndex);
			}
			s = target;
			if ( dfa_debug ) {
				if (s.isAcceptState && (!state.useContext || !s.isCtxSensitive)) {
					System.out.println("TODO (performance): consumed an unnecessary symbol in execDFA" +
							" - make sure to update k below when this is fixed.");
				}
			}
			input.consume();
			t = input.LA(1);
		}
//		if ( acceptState==null ) {
//			if ( debug ) System.out.println("!!! no viable alt in dfa");
//			return -1;
//		}

		if ( acceptState.configset.getConflictingAlts()!=null ) {
			if ( dfa.atnStartState instanceof DecisionState && ((DecisionState)dfa.atnStartState).isGreedy ) {
				// formula for k here is different than execATN because the loop above
				int k = input.index() - startIndex; // how much input we used
				if ( k == 1 || // SLL(1) == LL(1)
					!userWantsCtxSensitive ||
					!acceptState.configset.getDipsIntoOuterContext() )
				{
					// we don't report the ambiguity again
					//if ( !acceptState.configset.hasSemanticContext() ) {
					//	reportAmbiguity(dfa, acceptState, startIndex, input.index(), acceptState.configset.getConflictingAlts(), acceptState.configset);
					//}
				}
				else {
					assert !state.useContext;

					// Before attempting full context prediction, check to see if there are
					// disambiguating or validating predicates to evaluate which allow an
					// immediate decision
					if ( acceptState.predicates!=null ) {
						// rewind input so pred's LT(i) calls make sense
						input.seek(startIndex);
						int predictedAlt = evalSemanticContext(s.predicates, outerContext, true);
						if ( predictedAlt!=ATN.INVALID_ALT_NUMBER ) {
							return predictedAlt;
						}
					}

					input.seek(startIndex);
					dfa.setContextSensitive(true);
					return adaptivePredict(input, dfa.decision, outerContext, true);
				}
			}
		}

		// Before jumping to prediction, check to see if there are
		// disambiguating or validating predicates to evaluate
		if ( s.predicates!=null ) {
			// rewind input so pred's LT(i) calls make sense
			input.seek(startIndex);
			int predictedAlt = evalSemanticContext(s.predicates, outerContext, false);
			if ( predictedAlt!=ATN.INVALID_ALT_NUMBER ) {
				return predictedAlt;
			}
			throw noViableAlt(input, outerContext, s.configset, startIndex);
		}

		if ( dfa_debug ) System.out.println("DFA decision "+dfa.decision+
											" predicts "+acceptState.prediction);
		return acceptState.prediction;
	}

	/** Performs ATN simulation to compute a predicted alternative based
	 *  upon the remaining input, but also updates the DFA cache to avoid
	 *  having to traverse the ATN again for the same input sequence.

	 There are some key conditions we're looking for after computing a new
	 set of ATN configs (proposed DFA state):
	       * if the set is empty, there is no viable alternative for current symbol
	       * does the state uniquely predict an alternative?
	       * does the state have a conflict that would prevent us from
	         putting it on the work list?
	       * if in non-greedy decision is there a config at a rule stop state?

	 We also have some key operations to do:
	       * add an edge from previous DFA state to potentially new DFA state, D,
	         upon current symbol but only if adding to work list, which means in all
	         cases except no viable alternative (and possibly non-greedy decisions?)
	       * collecting predicates and adding semantic context to DFA accept states
	       * adding rule context to context-sensitive DFA accept states
	       * consuming an input symbol
	       * reporting a conflict
	       * reporting an ambiguity
	       * reporting a context sensitivity
	       * reporting insufficient predicates

	 We should isolate those operations, which are side-effecting, to the
	 main work loop. We can isolate lots of code into other functions, but
	 they should be side effect free. They can return package that
	 indicates whether we should report something, whether we need to add a
	 DFA edge, whether we need to augment accept state with semantic
	 context or rule invocation context. Actually, it seems like we always
	 add predicates if they exist, so that can simply be done in the main
	 loop for any accept state creation or modification request.

	 cover these cases:
	    dead end
	    single alt
	    single alt + preds
	    conflict
	    conflict + preds

	 TODO: greedy + those

	 */
	public int execATN(@NotNull DFA dfa,
					   @NotNull SymbolStream<? extends Symbol> input, int startIndex,
					   @NotNull SimulatorState initialState)
	{
		if ( debug ) System.out.println("execATN decision "+dfa.decision+" exec LA(1)=="+ getLookaheadName(input));
		ATN_failover++;

		final ParserRuleContext<?> outerContext = initialState.outerContext;
		final boolean useContext = initialState.useContext;

		int t = input.LA(1);

        DecisionState decState = atn.getDecisionState(dfa.decision);
		boolean greedy = decState.isGreedy;
		SimulatorState previous = initialState;

		PredictionContextCache contextCache = new PredictionContextCache(dfa.isContextSensitive());
		while (true) { // while more work
			SimulatorState nextState = computeReachSet(dfa, previous, t, greedy, contextCache);
			if (nextState == null) throw noViableAlt(input, outerContext, previous.s0.configset, startIndex);
			DFAState D = nextState.s0;
			ATNConfigSet reach = nextState.s0.configset;

			int predictedAlt = getUniqueAlt(reach);
			if ( predictedAlt!=ATN.INVALID_ALT_NUMBER ) {
				D.isAcceptState = true;
				D.prediction = predictedAlt;

				if (optimize_ll1
					&& input.index() == startIndex
					&& nextState.outerContext == nextState.remainingOuterContext
					&& dfa.decision >= 0
					&& greedy
					&& !D.configset.hasSemanticContext())
				{
					if (t >= 0 && t <= Short.MAX_VALUE) {
						int key = (dfa.decision << 16) + t;
						LL1Table.put(key, predictedAlt);
					}
				}

				if (useContext && always_try_local_context) {
					retry_with_context_indicates_no_conflict++;
					reportContextSensitivity(dfa, nextState, startIndex, input.index());
				}
			}
			else {
				D.configset.setConflictingAlts(getConflictingAlts(reach));
				if ( D.configset.getConflictingAlts()!=null ) {
					if ( greedy ) {
						D.isAcceptState = true;
						D.prediction = predictedAlt = resolveToMinAlt(D, D.configset.getConflictingAlts());

						int k = input.index() - startIndex + 1; // how much input we used
//						System.out.println("used k="+k);
						if ( k == 1 || // SLL(1) == LL(1)
							 !userWantsCtxSensitive ||
							 !D.configset.getDipsIntoOuterContext() )
						{
							if ( !D.configset.hasSemanticContext() ) {
								reportAmbiguity(dfa, D, startIndex, input.index(), D.configset.getConflictingAlts(), D.configset);
							}
						}
						else {
							int ambigIndex = input.index();

							if ( D.isAcceptState && D.configset.hasSemanticContext() ) {
								int nalts = decState.getNumberOfTransitions();
								List<DFAState.PredPrediction> predPredictions =
									predicateDFAState(D, D.configset, outerContext, nalts);
								if (predPredictions != null) {
									IntervalSet conflictingAlts = getConflictingAltsFromConfigSet(D.configset);
									if ( D.predicates.size() < conflictingAlts.size() ) {
										reportInsufficientPredicates(dfa, startIndex, ambigIndex,
																	conflictingAlts,
																	decState,
																	getPredsForAmbigAlts(conflictingAlts, D.configset, nalts),
																	D.configset,
																	false);
									}
									input.seek(startIndex);
									predictedAlt = evalSemanticContext(predPredictions, outerContext, true);
									if ( predictedAlt!=ATN.INVALID_ALT_NUMBER ) {
										return predictedAlt;
									}
								}
							}

							if ( debug ) System.out.println("RETRY with outerContext="+outerContext);
							dfa.setContextSensitive(true);
							SimulatorState fullContextState = computeStartState(dfa, outerContext, true);
							reportAttemptingFullContext(dfa, fullContextState, startIndex, ambigIndex);
							input.seek(startIndex);
							return execATN(dfa, input, startIndex, fullContextState);
						}
					}
					else {
						// upon ambiguity for nongreedy, default to exit branch to avoid inf loop
						// this handles case where we find ambiguity that stops DFA construction
						// before a config hits rule stop state. Was leaving prediction blank.
						int exitAlt = 2;
						D.isAcceptState = true; // when ambig or ctx sens or nongreedy or .* loop hitting rule stop
						D.prediction = predictedAlt = exitAlt;
					}
				}
			}

			if ( !greedy ) {
				int exitAlt = 2;
				if ( predictedAlt != ATN.INVALID_ALT_NUMBER && configWithAltAtStopState(reach, 1) ) {
					if ( debug ) System.out.println("nongreedy loop but unique alt "+D.configset.getUniqueAlt()+" at "+reach);
					// reaches end via .* means nothing after.
					D.isAcceptState = true;
					D.prediction = predictedAlt = exitAlt;
				}
				else {// if we reached end of rule via exit branch and decision nongreedy, we matched
					if ( configWithAltAtStopState(reach, exitAlt) ) {
						if ( debug ) System.out.println("nongreedy at stop state for exit branch");
						D.isAcceptState = true;
						D.prediction = predictedAlt = exitAlt;
					}
				}
			}

			if ( D.isAcceptState && D.configset.hasSemanticContext() ) {
				int nalts = decState.getNumberOfTransitions();
				List<DFAState.PredPrediction> predPredictions =
					predicateDFAState(D, D.configset, outerContext, nalts);
				if ( predPredictions!=null ) {
					IntervalSet conflictingAlts = getConflictingAltsFromConfigSet(D.configset);
					if ( D.predicates.size() < conflictingAlts.size() ) {
						reportInsufficientPredicates(dfa, startIndex, input.index(),
													conflictingAlts,
													decState,
													getPredsForAmbigAlts(conflictingAlts, D.configset, nalts),
													D.configset,
													false);
					}
					input.seek(startIndex);
					predictedAlt = evalSemanticContext(predPredictions, outerContext, false);
					if ( predictedAlt!=ATN.INVALID_ALT_NUMBER ) {
						return predictedAlt;
					}
				}

				if (D.prediction == ATN.INVALID_ALT_NUMBER) {
					throw noViableAlt(input, outerContext, D.configset, startIndex);
				}

				predictedAlt = D.prediction;
			}

			if ( D.isAcceptState ) return predictedAlt;

			previous = nextState;
			input.consume();
			t = input.LA(1);
		}
	}

	protected SimulatorState computeReachSet(DFA dfa, SimulatorState previous, int t, boolean greedy, PredictionContextCache contextCache) {
		final boolean useContext = previous.useContext;
		RuleContext remainingGlobalContext = previous.remainingOuterContext;
		List<ATNConfig> closureConfigs = new ArrayList<ATNConfig>(previous.s0.configset);
		List<Integer> contextElements = null;
		ATNConfigSet reach = new ATNConfigSet(!useContext);
		boolean stepIntoGlobal;
		do {
			boolean hasMoreContext = !useContext || remainingGlobalContext != null;

			ATNConfigSet reachIntermediate = new ATNConfigSet(!useContext);
			int ncl = closureConfigs.size();
			for (int ci=0; ci<ncl; ci++) { // TODO: foreach
				ATNConfig c = closureConfigs.get(ci);
				if ( debug ) System.out.println("testing "+getTokenName(t)+" at "+c.toString());
				int n = c.state.getNumberOfTransitions();
				for (int ti=0; ti<n; ti++) {               // for each transition
					Transition trans = c.state.transition(ti);
					ATNState target = getReachableTarget(trans, t);
					if ( target!=null ) {
						reachIntermediate.add(new ATNConfig(c, target));
					}
				}
			}

			final boolean collectPredicates = false;
			stepIntoGlobal = closure(reachIntermediate, reach, collectPredicates, dfa.isContextSensitive(), greedy, contextCache.isContextSensitive(), hasMoreContext, contextCache);

			if (previous.useContext && stepIntoGlobal) {
				reach.clear();

				int nextContextElement = remainingGlobalContext.isEmpty() ? PredictionContext.EMPTY_STATE_KEY : remainingGlobalContext.invokingState;
				if (contextElements == null) {
					contextElements = new ArrayList<Integer>();
				}

				if (remainingGlobalContext.isEmpty()) {
					remainingGlobalContext = null;
				} else {
					remainingGlobalContext = remainingGlobalContext.parent;
				}

				contextElements.add(nextContextElement);
				if (nextContextElement != PredictionContext.EMPTY_STATE_KEY) {
					for (int i = 0; i < closureConfigs.size(); i++) {
						closureConfigs.set(i, closureConfigs.get(i).appendContext(nextContextElement));
					}
				}
			}
		} while (useContext && stepIntoGlobal);

		if (reach.isEmpty()) {
			return null;
		}

		DFAState dfaState = null;
		if (previous.s0 != null) {
			dfaState = addDFAEdge(dfa, previous.s0.configset, t, contextElements, reach);
		}

		return new SimulatorState(previous.outerContext, dfaState, useContext, (ParserRuleContext<?>)remainingGlobalContext);
	}

	@NotNull
	public SimulatorState computeStartState(DFA dfa,
											ParserRuleContext<?> globalContext,
											boolean useContext)
	{
		DFAState s0 = dfa.s0;
		if (s0 != null) {
			if (!useContext) {
				return new SimulatorState(globalContext, s0, useContext, globalContext);
			}

			s0.setContextSensitive(atn);
		}

		final int decision = dfa.decision;
		@NotNull
		final ATNState p = dfa.atnStartState;

		int previousContext = 0;
		RuleContext remainingGlobalContext = globalContext;
		PredictionContext initialContext = PredictionContext.EMPTY; // always at least the implicit call to start rule
		if (useContext) {
			while (s0 != null && s0.isCtxSensitive && remainingGlobalContext != null) {
				DFAState next;
				if (remainingGlobalContext.isEmpty()) {
					next = s0.getContextTarget(PredictionContext.EMPTY_STATE_KEY);
					previousContext = PredictionContext.EMPTY_STATE_KEY;
					remainingGlobalContext = null;
				}
				else {
					next = s0.getContextTarget(remainingGlobalContext.invokingState);
					previousContext = remainingGlobalContext.invokingState;
					initialContext = initialContext.appendContext(remainingGlobalContext.invokingState);
					remainingGlobalContext = remainingGlobalContext.parent;
				}

				if (next == null) {
					break;
				}

				s0 = next;
			}
		}

		if (s0 != null && !s0.isCtxSensitive) {
			return new SimulatorState(globalContext, s0, useContext, (ParserRuleContext<?>)remainingGlobalContext);
		}

		ATNConfigSet configs = new ATNConfigSet(!useContext);

		DecisionState decState = null;
		if ( atn.decisionToState.size()>0 ) {
			decState = atn.decisionToState.get(decision);
		}

		PredictionContextCache contextCache = new PredictionContextCache(dfa.isContextSensitive());
		boolean greedy = decState == null || decState.isGreedy;
		while (true) {
			ATNConfigSet reachIntermediate = new ATNConfigSet(!useContext);
			int n = p.getNumberOfTransitions();
			for (int ti=0; ti<n; ti++) {
				// for each transition
				ATNState target = p.transition(ti).target;
				reachIntermediate.add(new ATNConfig(target, ti + 1, initialContext));
			}

			boolean hasMoreContext = remainingGlobalContext != null;
			final boolean collectPredicates = true;
			boolean stepIntoGlobal = closure(reachIntermediate, configs, collectPredicates, dfa.isContextSensitive(), greedy, contextCache.isContextSensitive(), hasMoreContext, contextCache);

			DFAState next = addDFAState(dfa, configs);
			if (s0 == null) {
				dfa.s0 = next;
			}
			else {
				s0.setContextTarget(previousContext, next);
			}
			s0 = next;

			if (!useContext || !stepIntoGlobal) {
				break;
			}

			// TODO: make sure it distinguishes empty stack states
			next.setContextSensitive(atn);

			configs.clear();
			int nextContextElement = remainingGlobalContext.isEmpty() ? PredictionContext.EMPTY_STATE_KEY : remainingGlobalContext.invokingState;

			if (remainingGlobalContext.isEmpty()) {
				remainingGlobalContext = null;
			} else {
				remainingGlobalContext = remainingGlobalContext.parent;
			}

			if (nextContextElement != PredictionContext.EMPTY_STATE_KEY) {
				initialContext = initialContext.appendContext(nextContextElement);
			}

			previousContext = nextContextElement;
		}

		return new SimulatorState(globalContext, s0, useContext, (ParserRuleContext<?>)remainingGlobalContext);
	}

	@Nullable
	public ATNState getReachableTarget(@NotNull Transition trans, int ttype) {
		if ( trans instanceof AtomTransition ) {
			AtomTransition at = (AtomTransition)trans;
			if ( at.label == ttype ) {
				return at.target;
			}
		}
		else if ( trans instanceof SetTransition ) {
			SetTransition st = (SetTransition)trans;
			boolean not = trans instanceof NotSetTransition;
			if ( !not && st.set.contains(ttype) || not && !st.set.contains(ttype) ) {
				return st.target;
			}
		}
		else if ( trans instanceof RangeTransition ) {
			RangeTransition rt = (RangeTransition)trans;
			if ( ttype>=rt.from && ttype<=rt.to ) return rt.target;
		}
		else if ( trans instanceof WildcardTransition && ttype!=Token.EOF ) {
			return trans.target;
		}
		return null;
	}

	/** collect and set D's semantic context */
	public List<DFAState.PredPrediction> predicateDFAState(DFAState D,
														   ATNConfigSet configs,
														   RuleContext outerContext,
														   int nalts)
	{
		IntervalSet conflictingAlts = getConflictingAltsFromConfigSet(configs);
		if ( debug ) System.out.println("predicateDFAState "+D);
		SemanticContext[] altToPred = getPredsForAmbigAlts(conflictingAlts, configs, nalts);
		// altToPred[uniqueAlt] is now our validating predicate (if any)
		List<DFAState.PredPrediction> predPredictions = null;
		if ( altToPred!=null ) {
			// we have a validating predicate; test it
			// Update DFA so reach becomes accept state with predicate
			predPredictions = getPredicatePredictions(conflictingAlts, altToPred);
			D.predicates = predPredictions;
			D.prediction = ATN.INVALID_ALT_NUMBER; // make sure we use preds
		}
		return predPredictions;
	}

	public SemanticContext[] getPredsForAmbigAlts(@NotNull IntervalSet ambigAlts,
												  @NotNull ATNConfigSet configs,
												  int nalts)
	{
		// REACH=[1|1|[]|0:0, 1|2|[]|0:1]
		SemanticContext[] altToPred = new SemanticContext[nalts +1];
		int n = altToPred.length;
		for (ATNConfig c : configs) {
			if ( ambigAlts.contains(c.alt) ) {
				altToPred[c.alt] = SemanticContext.or(altToPred[c.alt], c.semanticContext);
			}
		}

		int nPredAlts = 0;
		for (int i = 0; i < n; i++) {
			if (altToPred[i] == null) {
				altToPred[i] = SemanticContext.NONE;
			}
			else if (altToPred[i] != SemanticContext.NONE) {
				nPredAlts++;
			}
		}

		// Optimize away p||p and p&&p
		for (int i = 0; i < altToPred.length; i++) {
			if ( altToPred[i]!=null ) altToPred[i] = altToPred[i].optimize();
		}

		// nonambig alts are null in altToPred
		if ( nPredAlts==0 ) altToPred = null;
		if ( debug ) System.out.println("getPredsForAmbigAlts result "+Arrays.toString(altToPred));
		return altToPred;
	}

	public List<DFAState.PredPrediction> getPredicatePredictions(IntervalSet ambigAlts, SemanticContext[] altToPred) {
		List<DFAState.PredPrediction> pairs = new ArrayList<DFAState.PredPrediction>();
		int firstUnpredicated = ATN.INVALID_ALT_NUMBER;
		for (int i = 1; i < altToPred.length; i++) {
			SemanticContext pred = altToPred[i];
			// find first unpredicated but ambig alternative, if any.
			// Only ambiguous alternatives will have SemanticContext.NONE.
			// Any unambig alts or ambig naked alts after first ambig naked are ignored
			// (null, i) means alt i is the default prediction
			// if no (null, i), then no default prediction.
			if ( ambigAlts!=null && ambigAlts.contains(i) &&
				 pred==SemanticContext.NONE && firstUnpredicated==ATN.INVALID_ALT_NUMBER )
			{
				firstUnpredicated = i;
			}
			if ( pred!=null && pred!=SemanticContext.NONE ) {
				pairs.add(new DFAState.PredPrediction(pred, i));
			}
		}
		if ( pairs.isEmpty() ) pairs = null;
		else if ( firstUnpredicated!=ATN.INVALID_ALT_NUMBER ) {
			// add default prediction if we found null predicate
			pairs.add(new DFAState.PredPrediction(null, firstUnpredicated));
		}
//		System.out.println(Arrays.toString(altToPred)+"->"+pairs);
		return pairs;
	}

	/** Look through a list of predicate/alt pairs, returning alt for the
	 *  first pair that wins. A null predicate indicates the default
	 *  prediction for disambiguating predicates.
	 *
	 * @param checkUniqueMatch If true, {@link ATN#INVALID_ALT_NUMBER} will be returned
	 *      if the match in not unique.
	 */
	public int evalSemanticContext(@NotNull List<DFAState.PredPrediction> predPredictions,
								   ParserRuleContext<?> outerContext,
								   boolean checkUniqueMatch)
	{
		int predictedAlt = ATN.INVALID_ALT_NUMBER;
//		List<DFAState.PredPrediction> predPredictions = D.predicates;
//		if ( D.isCtxSensitive ) predPredictions = D.ctxToPredicates.get(outerContext);
		for (DFAState.PredPrediction pair : predPredictions) {
			if ( pair.pred==null ) {
				if (checkUniqueMatch && predictedAlt != ATN.INVALID_ALT_NUMBER) {
					// multiple predicates succeeded.
					return ATN.INVALID_ALT_NUMBER;
				}
				else {
					predictedAlt = pair.alt; // default prediction
					if (checkUniqueMatch) {
						continue;
					}
					else {
						break;
					}
				}
			}

			boolean evaluatedResult = pair.pred.eval(parser, outerContext);
			if ( debug || dfa_debug ) {
				System.out.println("eval pred "+pair+"="+evaluatedResult);
			}
			if ( evaluatedResult ) {
				if ( debug || dfa_debug ) System.out.println("PREDICT "+pair.alt);
				if (checkUniqueMatch && predictedAlt != ATN.INVALID_ALT_NUMBER) {
					// multiple predicates succeeded.
					return ATN.INVALID_ALT_NUMBER;
				}
				else {
					predictedAlt = pair.alt;
					if (!checkUniqueMatch) {
						break;
					}
				}
			}
		}
		// if no prediction: either all predicates are false and
		// all alternatives were guarded, or a validating predicate failed.
		return predictedAlt;
	}


	/* TODO: If we are doing predicates, there is no point in pursuing
		 closure operations if we reach a DFA state that uniquely predicts
		 alternative. We will not be caching that DFA state and it is a
		 waste to pursue the closure. Might have to advance when we do
		 ambig detection thought :(
		  */

	protected boolean closure(ATNConfigSet sourceConfigs,
						   @NotNull ATNConfigSet configs,
						   boolean collectPredicates,
						   boolean contextSensitiveDfa,
						   boolean greedy, boolean loopsSimulateTailRecursion,
						   boolean hasMoreContext,
						   PredictionContextCache contextCache)
	{
		boolean stepIntoGlobal = false;
		ATNConfigSet currentConfigs = sourceConfigs;
		Set<ATNConfig> closureBusy = new HashSet<ATNConfig>();
		while (currentConfigs.size() > 0) {
			ATNConfigSet intermediate = new ATNConfigSet(!contextSensitiveDfa);
			for (ATNConfig config : currentConfigs) {
				stepIntoGlobal |= closure(config, configs, intermediate, closureBusy, collectPredicates, greedy, loopsSimulateTailRecursion, hasMoreContext, contextCache, 0);
			}

			currentConfigs = intermediate;
		}

		return stepIntoGlobal;
	}

	protected boolean closure(@NotNull ATNConfig config,
						   @NotNull ATNConfigSet configs,
						   @Nullable ATNConfigSet intermediate,
						   @NotNull Set<ATNConfig> closureBusy,
						   boolean collectPredicates,
						   boolean greedy, boolean loopsSimulateTailRecursion,
						   boolean hasMoreContexts,
						   @Nullable PredictionContextCache contextCache,
						   int depth)
	{
		if ( debug ) System.out.println("closure("+config.toString(parser,true)+")");

		if ( !optimize_closure_busy && !closureBusy.add(config) ) {
			return false; // avoid infinite recursion
		}

		boolean stepIntoGlobal = false;
		boolean hasEmpty = config.context == null || config.context.isEmpty();
		if ( config.state instanceof RuleStopState ) {
			if ( !greedy ) {
				// don't see past end of a rule for any nongreedy decision
				if ( debug ) System.out.println("NONGREEDY at stop state of "+
												getRuleName(config.state.ruleIndex));
				configs.add(config, contextCache);
				return stepIntoGlobal;
			}
			// We hit rule end. If we have context info, use it
			if ( config.context!=null && !config.context.isEmpty() ) {
				for (int i = 0; i < config.context.parents.length; i++) {
					if (config.context.invokingStates[i] == PredictionContext.EMPTY_STATE_KEY) {
						hasEmpty = true;
						continue;
					}

					PredictionContext newContext = config.context.parents[i]; // "pop" invoking state
					ATNState invokingState = atn.states.get(config.context.invokingStates[i]);
					RuleTransition rt = (RuleTransition)invokingState.transition(0);
					ATNState retState = rt.followState;
					ATNConfig c = new ATNConfig(retState, config.alt, newContext, config.semanticContext);
					// While we have context to pop back from, we may have
					// gotten that context AFTER having fallen off a rule.
					// Make sure we track that we are now out of context.
					c.reachesIntoOuterContext = config.reachesIntoOuterContext;
					if (optimize_closure_busy && c.context.isEmpty() && !closureBusy.add(c)) {
						continue;
					}

					stepIntoGlobal |= closure(c, configs, intermediate, closureBusy, collectPredicates, greedy, loopsSimulateTailRecursion, hasMoreContexts, contextCache, depth - 1);
				}

				if (!hasEmpty || !hasMoreContexts) {
					return stepIntoGlobal;
				}

				config = new ATNConfig(config, config.state, PredictionContext.EMPTY);
			}
			else if (!hasMoreContexts) {
				configs.add(config, contextCache);
				return stepIntoGlobal;
			}
			else {
				// else if we have no context info, just chase follow links (if greedy)
				if ( debug ) System.out.println("FALLING off rule "+
												getRuleName(config.state.ruleIndex));
			}
		}
		else if ( loopsSimulateTailRecursion ) {
			if ( config.state.getClass()==StarLoopbackState.class ||
			config.state.getClass()==PlusLoopbackState.class )
			{
				config.context = config.context.getChild(config.state.stateNumber);
				// alter config; it's ok, since all calls to closure pass in a fresh config for us to chase
				if ( debug ) System.out.println("Loop back; push "+config.state.stateNumber+", stack="+config.context);
			}
			else if ( config.state.getClass()==LoopEndState.class ) {
				//throw new UnsupportedOperationException("Not implemented yet.");
				if ( debug ) System.out.println("Loop end; pop, stack="+config.context);
				PredictionContext p = config.context;
				LoopEndState end = (LoopEndState) config.state;
				while ( !p.isEmpty() ) {
					if ( p.invokingStates.length == 1 && p.invokingStates[0] == end.loopBackStateNumber ) {
						p = config.context = config.context.parents[0]; // pop loopback state
						continue;
					}
					else if ( p.invokingStates.length > 1 ) {
						int loopbackIndex = Arrays.binarySearch(p.invokingStates, end.loopBackStateNumber);
						if ( loopbackIndex >= 0 ) {
							PredictionContext remainingContext = null;
							for (int i = 0; i < p.invokingStates.length; i++) {
								if (p.invokingStates[i] == end.loopBackStateNumber) {
									continue;
								}

								if (remainingContext == null) {
									remainingContext = p.parents[i];
								}
								else {
									remainingContext = PredictionContext.join(remainingContext, p.parents[i], true);
								}
							}

							ATNConfig extraConfig = new ATNConfig(config, config.state, remainingContext);
							stepIntoGlobal |= closure(extraConfig, configs, intermediate, closureBusy, collectPredicates, greedy, loopsSimulateTailRecursion, hasMoreContexts, contextCache, depth);
							p = config.context = config.context.parents[loopbackIndex];
							continue;
						}
					}

					break;
				}
			}
		}

		ATNState p = config.state;
		// optimization
		if ( !p.onlyHasEpsilonTransitions() ) {
            configs.add(config, contextCache);
            if ( debug ) System.out.println("added config "+configs);
        }

        for (int i=0; i<p.getNumberOfTransitions(); i++) {
            Transition t = p.transition(i);
            boolean continueCollecting =
				!(t instanceof ActionTransition) && collectPredicates;
            ATNConfig c = getEpsilonTarget(config, t, continueCollecting, depth == 0, contextCache);
			if ( c!=null ) {
				if (optimize_closure_busy) {
					boolean checkClosure = false;
					if (c.state instanceof StarLoopEntryState || c.state instanceof BlockEndState) {
						checkClosure = true;
					}
					else if (c.state instanceof PlusBlockStartState) {
						checkClosure = true;
					}
					else if (config.state instanceof RuleStopState && c.context.isEmpty()) {
						checkClosure = true;
					}

					if (checkClosure && !closureBusy.add(c)) {
						continue;
					}
				}

				int newDepth = depth;
				if ( config.state instanceof RuleStopState ) {
					// target fell off end of rule; mark resulting c as having dipped into outer context
					// We can't get here if incoming config was rule stop and we had context
					// track how far we dip into outer context.  Might
					// come in handy and we avoid evaluating context dependent
					// preds if this is > 0.
					c.reachesIntoOuterContext++;
					stepIntoGlobal = true;
					newDepth--;
					if ( debug ) System.out.println("dips into outer ctx: "+c);
				}
				else if (t instanceof RuleTransition) {
					if (intermediate != null && !collectPredicates) {
						intermediate.add(c, contextCache);
						continue;
					}

					if (newDepth >= 0) {
						newDepth++;
					}
				}

				stepIntoGlobal |= closure(c, configs, intermediate, closureBusy, continueCollecting, greedy, loopsSimulateTailRecursion, hasMoreContexts, contextCache, newDepth);
			}
		}

		return stepIntoGlobal;
	}

	@NotNull
	public String getRuleName(int index) {
		if ( parser!=null && index>=0 ) return parser.getRuleNames()[index];
		return "<rule "+index+">";
	}

	@Nullable
	public ATNConfig getEpsilonTarget(@NotNull ATNConfig config, @NotNull Transition t, boolean collectPredicates, boolean inContext, PredictionContextCache contextCache) {
		if ( t instanceof RuleTransition ) {
			return ruleTransition(config, t, contextCache);
		}
		else if ( t instanceof PredicateTransition ) {
			return predTransition(config, (PredicateTransition)t, collectPredicates, inContext);
		}
		else if ( t instanceof ActionTransition ) {
			return actionTransition(config, (ActionTransition)t);
		}
		else if ( t.isEpsilon() ) {
			return new ATNConfig(config, t.target);
		}
		return null;
	}

	@NotNull
	public ATNConfig actionTransition(@NotNull ATNConfig config, @NotNull ActionTransition t) {
		if ( debug ) System.out.println("ACTION edge "+t.ruleIndex+":"+t.actionIndex);
		return new ATNConfig(config, t.target);
	}

	@Nullable
	public ATNConfig predTransition(@NotNull ATNConfig config,
									@NotNull PredicateTransition pt,
									boolean collectPredicates,
									boolean inContext)
	{
		if ( debug ) {
			System.out.println("PRED (collectPredicates="+collectPredicates+") "+
                    pt.ruleIndex+":"+pt.predIndex+
					", ctx dependent="+pt.isCtxDependent);
			if ( parser != null ) {
                System.out.println("context surrounding pred is "+
                                   parser.getRuleInvocationStack());
            }
		}

        ATNConfig c;
        if ( collectPredicates &&
			 (!pt.isCtxDependent || (pt.isCtxDependent&&inContext)) )
		{
            SemanticContext newSemCtx = SemanticContext.and(config.semanticContext, pt.getPredicate());
            c = new ATNConfig(config, pt.target, newSemCtx);
        }
		else {
			c = new ATNConfig(config, pt.target);
		}

		if ( debug ) System.out.println("config from pred transition="+c);
        return c;
	}

	@NotNull
	public ATNConfig ruleTransition(@NotNull ATNConfig config, @NotNull Transition t, @Nullable PredictionContextCache contextCache) {
		if ( debug ) {
			System.out.println("CALL rule "+getRuleName(t.target.ruleIndex)+
							   ", ctx="+config.context);
		}
		ATNState p = config.state;
		PredictionContext newContext;
		if (contextCache != null) {
			newContext = contextCache.getChild(config.context, p.stateNumber);
		}
		else {
			newContext = config.context.getChild(p.stateNumber);
		}

		return new ATNConfig(config, t.target, newContext);
	}

	/**
	 * From grammar:

	 s' : s s ;
	 s : x? | x ;
	 x : 'a' ;

	 config list: (4,1), (11,1,4), (7,1), (3,1,1), (4,1,1), (8,1,1), (7,1,1),
	 (8,2), (11,2,8), (11,1,[8 1])

	 state to config list:

	 3  -> (3,1,1)
	 4  -> (4,1), (4,1,1)
	 7  -> (7,1), (7,1,1)
	 8  -> (8,1,1), (8,2)
	 11 -> (11,1,4), (11,2,8), (11,1,8 1)

	 Walk and find state config lists with > 1 alt. If none, no conflict. return null. Here, states 11
	 and 8 have lists with both alts 1 and 2. Must check these config lists for conflicting configs.

	 Sam pointed out a problem with the previous definition, v3, of
	 ambiguous states. If we have another state associated with conflicting
	 alternatives, we should keep going. For example, the following grammar

	 s : (ID | ID ID?) ';' ;

	 When the ATN simulation reaches the state before ';', it has a DFA
	 state that looks like: [12|1|[], 6|2|[], 12|2|[]]. Naturally
	 12|1|[] and 12|2|[] conflict, but we cannot stop processing this node
	 because alternative to has another way to continue, via [6|2|[]].
	 The key is that we have a single state that has config's only associated
	 with a single alternative, 2, and crucially the state transitions
	 among the configurations are all non-epsilon transitions. That means
	 we don't consider any conflicts that include alternative 2. So, we
	 ignore the conflict between alts 1 and 2. We ignore a set of
	 conflicting alts when there is an intersection with an alternative
	 associated with a single alt state in the state->config-list map.

	 It's also the case that we might have two conflicting configurations but
	 also a 3rd nonconflicting configuration for a different alternative:
	 [1|1|[], 1|2|[], 8|3|[]]. This can come about from grammar:

	 a : A | A | A B ;

	 After matching input A, we reach the stop state for rule A, state 1.
	 State 8 is the state right before B. Clearly alternatives 1 and 2
	 conflict and no amount of further lookahead will separate the two.
	 However, alternative 3 will be able to continue and so we do not
	 stop working on this state. In the previous example, we're concerned
	 with states associated with the conflicting alternatives. Here alt
	 3 is not associated with the conflicting configs, but since we can continue
	 looking for input reasonably, I don't declare the state done. We
	 ignore a set of conflicting alts when we have an alternative
	 that we still need to pursue.

	 So, in summary, as long as there is a single configuration that is
	 not conflicting with any other configuration for that state, then
	 there is more input we can use to keep going. E.g.,
	 s->[(s,1,[x]), (s,2,[x]), (s,2,[y])]
	 s->[(s,1,_)]
	 s->[(s,1,[y]), (s,2,[x])]
	 Regardless of what goes on for the other states, this is
	 sufficient to force us to add this new state to the ATN-to-DFA work list.

	 TODO: split into "has nonconflict config--add to work list" and getambigalts
	 functions
	 */
	@Nullable
	public IntervalSet getConflictingAlts(@NotNull ATNConfigSet configs) {
		if ( debug ) System.out.println("### check ambiguous  "+configs);
		// First get a list of configurations for each state.
		// Most of the time, each state will have one associated configuration.
		MultiMap<Integer, ATNConfig> stateToConfigListMap = new MultiMap<Integer, ATNConfig>();
		Map<Integer, IntervalSet> stateToAltListMap = new HashMap<Integer, IntervalSet>();

		for (ATNConfig c : configs) {
			stateToConfigListMap.map(c.state.stateNumber, c);
			IntervalSet alts = stateToAltListMap.get(c.state.stateNumber);
			if ( alts==null ) {
				alts = new IntervalSet();
				stateToAltListMap.put(c.state.stateNumber, alts);
			}
			alts.add(c.alt);
		}
		// potential conflicts are states, s, with > 1 configurations and diff alts
		// find all alts with potential conflicts
		int numPotentialConflicts = 0;
		IntervalSet altsToIgnore = new IntervalSet();
		for (int state : stateToConfigListMap.keySet()) { // for each state
			IntervalSet alts = stateToAltListMap.get(state);
			if ( alts.size()==1 ) {
				if ( !atn.states.get(state).onlyHasEpsilonTransitions() ) {
					List<ATNConfig> configsPerState = stateToConfigListMap.get(state);
					ATNConfig anyConfig = configsPerState.get(0);
					altsToIgnore.add(anyConfig.alt);
					if ( debug ) System.out.println("### one alt and all non-ep: "+configsPerState);
				}
				// remove state's configurations from further checking; no issues with them.
				// (can't remove as it's concurrent modification; set to null)
//				return null;
				stateToConfigListMap.put(state, null);
			}
			else {
				numPotentialConflicts++;
			}
		}

		if ( debug ) System.out.println("### altsToIgnore: "+altsToIgnore);
		if ( debug ) System.out.println("### stateToConfigListMap="+stateToConfigListMap);

		if ( numPotentialConflicts==0 ) {
			return null;
		}

		// compare each pair of configs in sets for states with > 1 alt in config list, looking for
		// (s, i, ctx) and (s, j, ctx') where ctx==ctx' or one is suffix of the other.
		IntervalSet ambigAlts = new IntervalSet();
		for (int state : stateToConfigListMap.keySet()) {
			List<ATNConfig> configsPerState = stateToConfigListMap.get(state);
			if (configsPerState == null) continue;
			IntervalSet alts = stateToAltListMap.get(state);
// Sam's correction to ambig def is here:
			if ( !altsToIgnore.isNil() && alts.and(altsToIgnore).size()<=1 ) {
//				System.err.println("ignoring alt since "+alts+"&"+altsToIgnore+
//								   ".size is "+alts.and(altsToIgnore).size());
				continue;
			}
			int size = configsPerState.size();
			for (int i = 0; i < size; i++) {
				ATNConfig c = configsPerState.get(i);
				for (int j = i+1; j < size; j++) {
					ATNConfig d = configsPerState.get(j);
					if ( c.alt != d.alt ) {
						boolean conflicting =
							c.context.equals(d.context);
						if ( conflicting ) {
							if ( debug ) {
								System.out.println("we reach state "+c.state.stateNumber+
												   " in rule "+
												   (parser !=null ? getRuleName(c.state.ruleIndex) :"n/a")+
												   " alts "+c.alt+","+d.alt+" from ctx "+Utils.join(c.context.toStrings(parser, c.state.stateNumber), "")
												   +" and "+ Utils.join(d.context.toStrings(parser, d.state.stateNumber), ""));
							}
							ambigAlts.add(c.alt);
							ambigAlts.add(d.alt);
						}
					}
				}
			}
		}

		if ( debug ) System.out.println("### ambigAlts="+ambigAlts);

		if ( ambigAlts.isNil() ) return null;

		return ambigAlts;
	}

	protected IntervalSet getConflictingAltsFromConfigSet(ATNConfigSet configs) {
		IntervalSet conflictingAlts;
		if ( configs.getUniqueAlt()!= ATN.INVALID_ALT_NUMBER ) {
			conflictingAlts = IntervalSet.of(configs.getUniqueAlt());
		}
		else {
			conflictingAlts = configs.getConflictingAlts();
		}
		return conflictingAlts;
	}

	protected int resolveToMinAlt(@NotNull DFAState D, IntervalSet conflictingAlts) {
		// kill dead alts so we don't chase them ever
//		killAlts(conflictingAlts, D.configset);
		D.prediction = conflictingAlts.getMinElement();
		if ( debug ) System.out.println("RESOLVED TO "+D.prediction+" for "+D);
		return D.prediction;
	}

	@NotNull
	public String getTokenName(int t) {
		if ( t==Token.EOF ) return "EOF";
		if ( parser!=null && parser.getTokenNames()!=null ) {
			String[] tokensNames = parser.getTokenNames();
			if ( t>=tokensNames.length ) {
				System.err.println(t+" ttype out of range: "+ Arrays.toString(tokensNames));
				System.err.println(((CommonTokenStream)parser.getInputStream()).getTokens());
			}
			else {
				return tokensNames[t]+"<"+t+">";
			}
		}
		return String.valueOf(t);
	}

	public String getLookaheadName(SymbolStream<? extends Symbol> input) {
		return getTokenName(input.LA(1));
	}

	public void dumpDeadEndConfigs(@NotNull NoViableAltException nvae) {
		System.err.println("dead end configs: ");
		for (ATNConfig c : nvae.deadEndConfigs) {
			String trans = "no edges";
			if ( c.state.getNumberOfTransitions()>0 ) {
				Transition t = c.state.transition(0);
				if ( t instanceof AtomTransition) {
					AtomTransition at = (AtomTransition)t;
					trans = "Atom "+getTokenName(at.label);
				}
				else if ( t instanceof SetTransition ) {
					SetTransition st = (SetTransition)t;
					boolean not = st instanceof NotSetTransition;
					trans = (not?"~":"")+"Set "+st.set.toString();
				}
			}
			System.err.println(c.toString(parser, true)+":"+trans);
		}
	}

	@NotNull
	public NoViableAltException noViableAlt(@NotNull SymbolStream<? extends Symbol> input,
											@NotNull ParserRuleContext<?> outerContext,
											@NotNull ATNConfigSet configs,
											int startIndex)
	{
		return new NoViableAltException(parser, input,
											input.get(startIndex),
											input.LT(1),
											configs, outerContext);
	}

	public int getUniqueAlt(@NotNull Collection<ATNConfig> configs) {
		int alt = ATN.INVALID_ALT_NUMBER;
		for (ATNConfig c : configs) {
			if ( alt == ATN.INVALID_ALT_NUMBER ) {
				alt = c.alt; // found first alt
			}
			else if ( c.alt!=alt ) {
				return ATN.INVALID_ALT_NUMBER;
			}
		}
		return alt;
	}

	public boolean configWithAltAtStopState(@NotNull Collection<ATNConfig> configs, int alt) {
		for (ATNConfig c : configs) {
			if ( c.alt == alt ) {
				if ( c.state.getClass() == RuleStopState.class ) {
					return true;
				}
			}
		}
		return false;
	}

	@NotNull
	protected DFAState addDFAEdge(@NotNull DFA dfa,
								  @NotNull ATNConfigSet p,
								  int t,
								  List<Integer> contextTransitions,
								  @NotNull ATNConfigSet q)
	{
		assert dfa.isContextSensitive() || contextTransitions == null || contextTransitions.isEmpty();

		DFAState from = addDFAState(dfa, p);
		DFAState to = addDFAState(dfa, q);

		if (contextTransitions != null) {
			for (int context : contextTransitions) {
				if (!from.isCtxSensitive) {
					from.setContextSensitive(atn);
				}

				from.contextSymbols.add(t);
				DFAState next = from.getContextTarget(context);
				if (next != null) {
					from = next;
					continue;
				}

				next = addDFAContextState(dfa, from.configset, context);
				from.setContextTarget(context, next);
				from = next;
			}
		}

        if ( debug ) System.out.println("EDGE "+from+" -> "+to+" upon "+getTokenName(t));
		addDFAEdge(from, t, to);
		if ( debug ) System.out.println("DFA=\n"+dfa.toString(parser!=null?parser.getTokenNames():null, parser!=null?parser.getRuleNames():null));
		return to;
	}

	protected void addDFAEdge(@Nullable DFAState p, int t, @Nullable DFAState q) {
		if ( p!=null ) {
			p.setTarget(t, q);
		}
	}

	/** See comment on LexerInterpreter.addDFAState. */
	@NotNull
	protected DFAState addDFAContextState(@NotNull DFA dfa, @NotNull ATNConfigSet configs, int invokingContext) {
		if (invokingContext != PredictionContext.EMPTY_STATE_KEY) {
			ATNConfigSet contextConfigs = new ATNConfigSet(false);
			for (ATNConfig config : configs) {
				contextConfigs.add(config.appendContext(invokingContext));
			}

			return addDFAState(dfa, contextConfigs);
		}
		else {
			return addDFAState(dfa, configs);
		}
	}

	/** See comment on LexerInterpreter.addDFAState. */
	@NotNull
	protected DFAState addDFAState(@NotNull DFA dfa, @NotNull ATNConfigSet configs) {
		DFAState proposed = new DFAState(configs, -1, atn.maxTokenType);
		DFAState existing = dfa.states.get(proposed);
		if ( existing!=null ) return existing;

		DFAState newState = proposed;

		newState.stateNumber = dfa.states.size();
		configs.optimizeConfigs(this);
		newState.configset = configs.clone(true);
		dfa.states.put(newState, newState);
        if ( debug ) System.out.println("adding new DFA state: "+newState);
		return newState;
	}

//	public void reportConflict(int startIndex, int stopIndex,
//							   @NotNull IntervalSet alts,
//							   @NotNull ATNConfigSet configs)
//	{
//		if ( debug || retry_debug ) {
//			System.out.println("reportConflict "+alts+":"+configs+
//							   ", input="+parser.getInputString(startIndex, stopIndex));
//		}
//		if ( parser!=null ) parser.getErrorHandler().reportConflict(parser, startIndex, stopIndex, alts, configs);
//	}

	public void reportAttemptingFullContext(DFA dfa, SimulatorState initialState, int startIndex, int stopIndex) {
        if ( debug || retry_debug ) {
            System.out.println("reportAttemptingFullContext decision="+dfa.decision+":"+initialState.s0.configset+
                               ", input="+parser.getInputString(startIndex, stopIndex));
        }
        if ( parser!=null ) parser.getErrorHandler().reportAttemptingFullContext(parser, dfa, startIndex, stopIndex, initialState);
    }

	public void reportContextSensitivity(DFA dfa, SimulatorState acceptState, int startIndex, int stopIndex) {
        if ( debug || retry_debug ) {
            System.out.println("reportContextSensitivity decision="+dfa.decision+":"+acceptState.s0.configset+
                               ", input="+parser.getInputString(startIndex, stopIndex));
        }
        if ( parser!=null ) parser.getErrorHandler().reportContextSensitivity(parser, dfa, startIndex, stopIndex, acceptState);
    }

    /** If context sensitive parsing, we know it's ambiguity not conflict */
    public void reportAmbiguity(@NotNull DFA dfa, DFAState D, int startIndex, int stopIndex,
								@NotNull IntervalSet ambigAlts,
								@NotNull ATNConfigSet configs)
	{
		if ( debug || retry_debug ) {
//			ParserATNPathFinder finder = new ParserATNPathFinder(parser, atn);
//			int i = 1;
//			for (Transition t : dfa.atnStartState.transitions) {
//				System.out.println("ALT "+i+"=");
//				System.out.println(startIndex+".."+stopIndex+", len(input)="+parser.getInputStream().size());
//				TraceTree path = finder.trace(t.target, parser.getContext(), (TokenStream)parser.getInputStream(),
//											  startIndex, stopIndex);
//				if ( path!=null ) {
//					System.out.println("path = "+path.toStringTree());
//					for (TraceTree leaf : path.leaves) {
//						List<ATNState> states = path.getPathToNode(leaf);
//						System.out.println("states="+states);
//					}
//				}
//				i++;
//			}
			System.out.println("reportAmbiguity "+
							   ambigAlts+":"+configs+
                               ", input="+parser.getInputString(startIndex, stopIndex));
        }
        if ( parser!=null ) parser.getErrorHandler().reportAmbiguity(parser, dfa, startIndex, stopIndex,
                                                                     ambigAlts, configs);
    }

    public void reportInsufficientPredicates(@NotNull DFA dfa, int startIndex, int stopIndex,
											 @NotNull IntervalSet ambigAlts,
											 DecisionState decState,
											 @NotNull SemanticContext[] altToPred,
											 @NotNull ATNConfigSet configs,
											 boolean fullContextParse)
    {
        if ( debug || retry_debug ) {
            System.out.println("reportInsufficientPredicates "+
                               ambigAlts+", decState="+decState+": "+Arrays.toString(altToPred)+
                               parser.getInputString(startIndex, stopIndex));
        }
        if ( parser!=null ) {
            parser.getErrorHandler().reportInsufficientPredicates(parser, dfa, startIndex, stopIndex, ambigAlts,
																  decState, altToPred, configs, fullContextParse);
        }
    }

}