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With inspiration from other libraries

There is a newer version: 2.12.15

package org.antlr.runtime.misc;

import java.util.*;

/** Sometimes we need to map a key to a value but key is two pieces of data.
 *  This nested hash table saves creating a single key each time we access
 *  map; avoids mem creation.
 */
public class DoubleKeyMap {
	Map> data = new LinkedHashMap>();

	public Value put(Key1 k1, Key2 k2, Value v) {
		Map data2 = data.get(k1);
		Value prev = null;
		if ( data2==null ) {
			data2 = new LinkedHashMap();
			data.put(k1, data2);
		}
		else {
			prev = data2.get(k2);
		}
		data2.put(k2, v);
		return prev;
	}

	public Value get(Key1 k1, Key2 k2) {
		Map data2 = data.get(k1);
		if ( data2==null ) return null;
		return data2.get(k2);
	}

	public Map get(Key1 k1) { return data.get(k1); }

	/** Get all values associated with primary key */
	public Collection values(Key1 k1) {
		Map data2 = data.get(k1);
		if ( data2==null ) return null;
		return data2.values();
	}

	/** get all primary keys */
	public Set keySet() {
		return data.keySet();
	}

	/** get all secondary keys associated with a primary key */
	public Set keySet(Key1 k1) {
		Map data2 = data.get(k1);
		if ( data2==null ) return null;
		return data2.keySet();
	}

	public Collection values() {
		Set s = new HashSet();
		for (Map k2 : data.values()) {
			for (Value v : k2.values()) {
				s.add(v);
			}
		}
		return s;
	}
}