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The ANTLR 4 Runtime
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/*
 * [The "BSD license"]
 *  Copyright (c) 2012 Terence Parr
 *  Copyright (c) 2012 Sam Harwell
 *  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
 *  NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 *  DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 *  THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 *  (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 *  THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

package org.antlr.v4.runtime.atn;

import org.antlr.v4.runtime.IntStream;
import org.antlr.v4.runtime.Token;
import org.antlr.v4.runtime.misc.Args;
import org.antlr.v4.runtime.misc.IntervalSet;
import org.antlr.v4.runtime.misc.NotNull;
import org.antlr.v4.runtime.misc.Nullable;

import java.util.HashSet;
import java.util.Set;

public class LL1Analyzer {
	/** Special value added to the lookahead sets to indicate that we hit
	 *  a predicate during analysis if seeThruPreds==false.
	 */
	public static final int HIT_PRED = Token.INVALID_TYPE;

	@NotNull
	public final ATN atn;

	public LL1Analyzer(@NotNull ATN atn) { this.atn = atn; }

	/**
	 * From an ATN state, {@code s}, find the set of all labels reachable from
	 * {@code s} at depth k. Only for DecisionStates.
	 */
	@Nullable
	public IntervalSet[] getDecisionLookahead(@Nullable ATNState s) {
//		System.out.println("LOOK("+s.stateNumber+")");
		if ( s==null ) return null;
		IntervalSet[] look = new IntervalSet[s.getNumberOfTransitions()+1];
		for (int alt=1; alt<=s.getNumberOfTransitions(); alt++) {
			look[alt] = new IntervalSet();
			Set lookBusy = new HashSet();
			boolean seeThruPreds = false; // fail to get lookahead upon pred
			_LOOK(s.transition(alt - 1).target,
				  PredictionContext.EMPTY_FULL,
				  look[alt], lookBusy, seeThruPreds, false);
			// Wipe out lookahead for this alternative if we found nothing
			// or we had a predicate when we !seeThruPreds
			if ( look[alt].size()==0 || look[alt].contains(HIT_PRED) ) {
				look[alt] = null;
			}
		}
		return look;
	}

	/**
	 * Get lookahead, using {@code ctx} if we reach end of rule. If {@code ctx}
	 * is {@code PredictionContext#EMPTY_LOCAL} or
	 * {@link PredictionContext#EMPTY_FULL}, don't chase FOLLOW. If {@code ctx}
	 * is {@code PredictionContext#EMPTY_LOCAL}, {@link Token#EPSILON EPSILON}
	 * is in set if we can reach end of rule. If {@code ctx} is
	 * {@link PredictionContext#EMPTY_FULL}, {@link IntStream#EOF EOF} is in set
	 * if we can reach end of rule.
	 */
    @NotNull
   	public IntervalSet LOOK(@NotNull ATNState s, @NotNull PredictionContext ctx) {
		Args.notNull("ctx", ctx);
   		IntervalSet r = new IntervalSet();
		boolean seeThruPreds = true; // ignore preds; get all lookahead
   		_LOOK(s, ctx, r, new HashSet(), seeThruPreds, true);
   		return r;
   	}

    /** Compute set of tokens that can come next. If the context is {@link PredictionContext#EMPTY_FULL},
     *  then we don't go anywhere when we hit the end of the rule. We have
     *  the correct set.  If the context is {@link PredictionContext#EMPTY_LOCAL},
	 *  that means that we did not want any tokens following this rule--just the
	 *  tokens that could be found within this rule. Add EPSILON to the set
	 *  indicating we reached the end of the ruled out having to match a token.
     */
    protected void _LOOK(@NotNull ATNState s, @NotNull PredictionContext ctx,
						 @NotNull IntervalSet look,
                         @NotNull Set lookBusy,
						 boolean seeThruPreds, boolean addEOF)
	{
//		System.out.println("_LOOK("+s.stateNumber+", ctx="+ctx);
        ATNConfig c = ATNConfig.create(s, 0, ctx);
        if ( !lookBusy.add(c) ) return;

        if ( s instanceof RuleStopState ) {
            if ( PredictionContext.isEmptyLocal(ctx) ) {
                look.add(Token.EPSILON);
                return;
            } else if (ctx.isEmpty() && addEOF) {
				look.add(Token.EOF);
				return;
			}

			for (int i = 0; i < ctx.size(); i++) {
				if ( ctx.getReturnState(i)!=PredictionContext.EMPTY_FULL_STATE_KEY ) {
					ATNState returnState = atn.states.get(ctx.getReturnState(i));
//			System.out.println("popping back to "+retState);
					for (int j = 0; j < ctx.size(); j++) {
						_LOOK(returnState, ctx.getParent(j), look, lookBusy, seeThruPreds, addEOF);
					}
					return;
				}
			}
        }

        int n = s.getNumberOfTransitions();
        for (int i=0; i