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With inspiration from other libraries
There is a newer version: 2.12.15
/*
 * 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.tool;


import org.antlr.v4.tool.ast.ActionAST;
import org.antlr.v4.tool.ast.AltAST;
import org.antlr.v4.tool.ast.GrammarAST;
import org.antlr.v4.tool.ast.TerminalAST;
import org.stringtemplate.v4.misc.MultiMap;

import java.util.ArrayList;
import java.util.List;

/** An outermost alternative for a rule.  We don't track inner alternatives. */
public class Alternative implements AttributeResolver {
    public Rule rule;

	public AltAST ast;

	/** What alternative number is this outermost alt? 1..n */
	public int altNum;

    // token IDs, string literals in this alt
    public MultiMap tokenRefs = new MultiMap();

	// does not include labels
	public MultiMap tokenRefsInActions = new MultiMap();

    // all rule refs in this alt
    public MultiMap ruleRefs = new MultiMap();

	// does not include labels
	public MultiMap ruleRefsInActions = new MultiMap();

    /** A list of all LabelElementPair attached to tokens like id=ID, ids+=ID */
    public MultiMap labelDefs = new MultiMap();

    // track all token, rule, label refs in rewrite (right of ->)
    //public List rewriteElements = new ArrayList();

    /** Track all executable actions other than named actions like @init
     *  and catch/finally (not in an alt). Also tracks predicates, rewrite actions.
     *  We need to examine these actions before code generation so
     *  that we can detect refs to $rule.attr etc...
	 *
	 *  This tracks per alt
     */
    public List actions = new ArrayList();

    public Alternative(Rule r, int altNum) { this.rule = r; this.altNum = altNum; }

	@Override
	public boolean resolvesToToken(String x, ActionAST node) {
		if ( tokenRefs.get(x)!=null ) return true;
		LabelElementPair anyLabelDef = getAnyLabelDef(x);
		if ( anyLabelDef!=null && anyLabelDef.type==LabelType.TOKEN_LABEL ) return true;
		return false;
	}

	@Override
	public boolean resolvesToAttributeDict(String x, ActionAST node) {
		if ( resolvesToToken(x, node) ) return true;
        if ( ruleRefs.get(x)!=null ) return true; // rule ref in this alt?
        LabelElementPair anyLabelDef = getAnyLabelDef(x);
        if ( anyLabelDef!=null && anyLabelDef.type==LabelType.RULE_LABEL ) return true;
		return false;
	}

	/**  $x		Attribute: rule arguments, return values, predefined rule prop.
	 */
	@Override
	public Attribute resolveToAttribute(String x, ActionAST node) {
		return rule.resolveToAttribute(x, node); // reuse that code
	}

	/** $x.y, x can be surrounding rule, token/rule/label ref. y is visible
	 *  attr in that dictionary.  Can't see args on rule refs.
	 */
	@Override
	public Attribute resolveToAttribute(String x, String y, ActionAST node) {
        if ( tokenRefs.get(x)!=null ) { // token ref in this alt?
            return rule.getPredefinedScope(LabelType.TOKEN_LABEL).get(y);
        }
        if ( ruleRefs.get(x)!=null ) {  // rule ref in this alt?
            // look up rule, ask it to resolve y (must be retval or predefined)
			return rule.g.getRule(x).resolveRetvalOrProperty(y);
		}
		LabelElementPair anyLabelDef = getAnyLabelDef(x);
		if ( anyLabelDef!=null && anyLabelDef.type==LabelType.RULE_LABEL ) {
			return rule.g.getRule(anyLabelDef.element.getText()).resolveRetvalOrProperty(y);
		}
		else if ( anyLabelDef!=null ) {
			AttributeDict scope = rule.getPredefinedScope(anyLabelDef.type);
			if (scope == null) {
				return null;
			}

			return scope.get(y);
		}
		return null;
	}

	@Override
	public boolean resolvesToLabel(String x, ActionAST node) {
		LabelElementPair anyLabelDef = getAnyLabelDef(x);
		return anyLabelDef!=null &&
			   (anyLabelDef.type==LabelType.TOKEN_LABEL ||
				anyLabelDef.type==LabelType.RULE_LABEL);
	}

	@Override
	public boolean resolvesToListLabel(String x, ActionAST node) {
		LabelElementPair anyLabelDef = getAnyLabelDef(x);
		return anyLabelDef!=null &&
			   (anyLabelDef.type==LabelType.RULE_LIST_LABEL ||
				anyLabelDef.type==LabelType.TOKEN_LIST_LABEL);
	}

	public LabelElementPair getAnyLabelDef(String x) {
		List labels = labelDefs.get(x);
		if ( labels!=null ) return labels.get(0);
		return null;
	}

	/** x can be ruleref or rule label. */
	public Rule resolveToRule(String x) {
        if ( ruleRefs.get(x)!=null ) return rule.g.getRule(x);
		LabelElementPair anyLabelDef = getAnyLabelDef(x);
		if ( anyLabelDef!=null && anyLabelDef.type==LabelType.RULE_LABEL ) {
            return rule.g.getRule(anyLabelDef.element.getText());
        }
        return null;
    }
}