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With inspiration from other libraries
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/*
 * Copyright (c) 2012-2017 The ANTLR Project. All rights reserved.
 * Use of this file is governed by the BSD 3-clause license that
 * can be found in the LICENSE.txt file in the project root.
 */

package org.antlr.v4.analysis;

import org.antlr.v4.runtime.atn.ATN;
import org.antlr.v4.runtime.atn.ATNState;
import org.antlr.v4.runtime.atn.RuleStartState;
import org.antlr.v4.runtime.atn.RuleStopState;
import org.antlr.v4.runtime.atn.RuleTransition;
import org.antlr.v4.runtime.atn.Transition;
import org.antlr.v4.runtime.misc.OrderedHashSet;
import org.antlr.v4.tool.Grammar;
import org.antlr.v4.tool.Rule;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class LeftRecursionDetector {
	Grammar g;
	public ATN atn;

	/** Holds a list of cycles (sets of rule names). */
	public List> listOfRecursiveCycles = new ArrayList>();

	/** Which rule start states have we visited while looking for a single
	 * 	left-recursion check?
	 */
	Set rulesVisitedPerRuleCheck = new HashSet();

	public LeftRecursionDetector(Grammar g, ATN atn) {
		this.g = g;
		this.atn = atn;
	}

	public void check() {
		for (RuleStartState start : atn.ruleToStartState) {
			//System.out.print("check "+start.rule.name);
			rulesVisitedPerRuleCheck.clear();
			rulesVisitedPerRuleCheck.add(start);
			//FASerializer ser = new FASerializer(atn.g, start);
			//System.out.print(":\n"+ser+"\n");

			check(g.getRule(start.ruleIndex), start, new HashSet());
		}
		//System.out.println("cycles="+listOfRecursiveCycles);
		if ( !listOfRecursiveCycles.isEmpty() ) {
			g.tool.errMgr.leftRecursionCycles(g.fileName, listOfRecursiveCycles);
		}
	}

	/** From state s, look for any transition to a rule that is currently
	 *  being traced.  When tracing r, visitedPerRuleCheck has r
	 *  initially.  If you reach a rule stop state, return but notify the
	 *  invoking rule that the called rule is nullable. This implies that
	 *  invoking rule must look at follow transition for that invoking state.
	 *
	 *  The visitedStates tracks visited states within a single rule so
	 *  we can avoid epsilon-loop-induced infinite recursion here.  Keep
	 *  filling the cycles in listOfRecursiveCycles and also, as a
	 *  side-effect, set leftRecursiveRules.
	 */
	public boolean check(Rule enclosingRule, ATNState s, Set visitedStates) {
		if ( s instanceof RuleStopState) return true;
		if ( visitedStates.contains(s) ) return false;
		visitedStates.add(s);

		//System.out.println("visit "+s);
		int n = s.getNumberOfTransitions();
		boolean stateReachesStopState = false;
		for (int i=0; i());
					// we're back from visiting that rule
					rulesVisitedPerRuleCheck.remove((RuleStartState)t.target);
					if ( nullable ) {
						stateReachesStopState |= check(enclosingRule, rt.followState, visitedStates);
					}
				}
			}
			else if ( t.isEpsilon() ) {
				stateReachesStopState |= check(enclosingRule, t.target, visitedStates);
			}
			// else ignore non-epsilon transitions
		}
		return stateReachesStopState;
	}

	/** enclosingRule calls targetRule. Find the cycle containing
	 *  the target and add the caller.  Find the cycle containing the caller
	 *  and add the target.  If no cycles contain either, then create a new
	 *  cycle.
	 */
	protected void addRulesToCycle(Rule enclosingRule, Rule targetRule) {
		//System.err.println("left-recursion to "+targetRule.name+" from "+enclosingRule.name);
		boolean foundCycle = false;
		for (Set rulesInCycle : listOfRecursiveCycles) {
			// ensure both rules are in same cycle
			if (rulesInCycle.contains(targetRule)) {
				rulesInCycle.add(enclosingRule);
				foundCycle = true;
			}
			if (rulesInCycle.contains(enclosingRule)) {
				rulesInCycle.add(targetRule);
				foundCycle = true;
			}
		}
		if ( !foundCycle ) {
			Set cycle = new OrderedHashSet();
			cycle.add(targetRule);
			cycle.add(enclosingRule);
			listOfRecursiveCycles.add(cycle);
		}
	}
}