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The ANTLR 4 grammar compiler.
There is a newer version: 4.13.2
/*
 * [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.automata;

import org.antlr.v4.runtime.atn.ATN;
import org.antlr.v4.runtime.atn.ATNState;
import org.antlr.v4.runtime.atn.BlockEndState;
import org.antlr.v4.runtime.atn.EpsilonTransition;
import org.antlr.v4.runtime.atn.PlusLoopbackState;
import org.antlr.v4.runtime.atn.RuleTransition;
import org.antlr.v4.runtime.atn.StarLoopbackState;
import org.antlr.v4.runtime.atn.Transition;

/**
 *
 * @author Terence Parr
 */
public class TailEpsilonRemover extends ATNVisitor {

	private final ATN _atn;

	public TailEpsilonRemover(ATN atn) {
		this._atn = atn;
	}

	@Override
	public void visitState(ATNState p) {
		if (p.getStateType() == ATNState.BASIC && p.getNumberOfTransitions() == 1) {
			ATNState q = p.transition(0).target;
			if (p.transition(0) instanceof RuleTransition) {
				q = ((RuleTransition) p.transition(0)).followState;
			}
			if (q.getStateType() == ATNState.BASIC) {
				// we have p-x->q for x in {rule, action, pred, token, ...}
				// if edge out of q is single epsilon to block end
				// we can strip epsilon p-x->q-eps->r
				Transition trans = q.transition(0);
				if (q.getNumberOfTransitions() == 1 && trans instanceof EpsilonTransition) {
					ATNState r = trans.target;
					if (r instanceof BlockEndState || r instanceof PlusLoopbackState || r instanceof StarLoopbackState) {
						// skip over q
						if (p.transition(0) instanceof RuleTransition) {
							((RuleTransition) p.transition(0)).followState = r;
						} else {
							p.transition(0).target = r;
						}
						_atn.removeState(q);
					}
				}
			}
		}
	}
}