/*

 * Copyright (c) 2015, 2015, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

package org.graalvm.compiler.core.common.util;

import java.util.ArrayList;

import java.util.List;

/**

 * Creates an array of T objects order by the occurrence frequency of each object. The most

 * frequently used object is the first one, the least frequently used the last one. If {@code null}

 * is added, it is always the first element.

 *

 * Either object {@link #createIdentityEncoder() identity} or object {@link #createEqualityEncoder()

 * equality} can be used to build the array and count the frequency.

 */

public class FrequencyEncoder<T> {

    static class Entry<T> {

        protected final T object;

        protected int frequency;

        protected int index;

        protected Entry(T object) {

            this.object = object;

            this.index = -1;

        }

    }

    protected final EconomicMap<T, Entry<T>> map;

    protected boolean containsNull;

    /**

     * Creates an encoder that uses object identity.

     */

    public static <T> FrequencyEncoder<T> createIdentityEncoder() {

        return new FrequencyEncoder<>(EconomicMap.create(Equivalence.IDENTITY_WITH_SYSTEM_HASHCODE));

    }

    /**

     * Creates an encoder that uses {@link Object#equals(Object) object equality}.

     */

    public static <T> FrequencyEncoder<T> createEqualityEncoder() {

        return new FrequencyEncoder<>(EconomicMap.create(Equivalence.DEFAULT));

    }

    protected FrequencyEncoder(EconomicMap<T, Entry<T>> map) {

        this.map = map;

    }

    /**

     * Adds an object to the array.

     */

    public void addObject(T object) {

        if (object == null) {

            containsNull = true;

            return;

        }

        Entry<T> entry = map.get(object);

        if (entry == null) {

            entry = new Entry<>(object);

            map.put(object, entry);

        }

        entry.frequency++;

    }

    /**

     * Returns the index of an object in the array. The object must have been

     * {@link #addObject(Object) added} before.

     */

    public int getIndex(T object) {

        if (object == null) {

            assert containsNull;

            return 0;

        }

        Entry<T> entry = map.get(object);

        assert entry != null && entry.index >= 0;

        return entry.index;

    }

    /**

     * Returns the number of distinct objects that have been added, i.e., the length of the array.

     */

    public int getLength() {

        return map.size() + (containsNull ? 1 : 0);

    }

    /**

     * Fills the provided array with the added objects. The array must have the {@link #getLength()

     * correct length}.

     */

    public T[] encodeAll(T[] allObjects) {

        assert allObjects.length == getLength();

        List<Entry<T>> sortedEntries = new ArrayList<>(allObjects.length);

        for (Entry<T> value : map.getValues()) {

            sortedEntries.add(value);

        }

        sortedEntries.sort((e1, e2) -> -Integer.compare(e1.frequency, e2.frequency));

        int offset = 0;

        if (containsNull) {

            allObjects[0] = null;

            offset = 1;

        }

        for (int i = 0; i < sortedEntries.size(); i++) {

            Entry<T> entry = sortedEntries.get(i);

            int index = i + offset;

            entry.index = index;

            allObjects[index] = entry.object;

            assert entry.object != null;

        }

        return allObjects;

    }

}