ObjectIdSubclassMap.java

/*
 * Copyright (C) 2009, Google Inc.
 * Copyright (C) 2008, Marek Zawirski <marek.zawirski@gmail.com>
 * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org> and others
 *
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Distribution License v. 1.0 which is available at
 * https://www.eclipse.org/org/documents/edl-v10.php.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

package org.eclipse.jgit.lib;

import java.util.Iterator;
import java.util.NoSuchElementException;

/**
 * Fast, efficient map specifically for {@link org.eclipse.jgit.lib.ObjectId}
 * subclasses.
 * <p>
 * This map provides an efficient translation from any ObjectId instance to a
 * cached subclass of ObjectId that has the same value.
 * <p>
 * If object instances are stored in only one map,
 * {@link org.eclipse.jgit.lib.ObjectIdOwnerMap} is a more efficient
 * implementation.
 *
 * @param <V>
 *            type of subclass of ObjectId that will be stored in the map.
 */
public class ObjectIdSubclassMap<V extends ObjectId>
        implements Iterable<V>, ObjectIdSet {
    private static final int INITIAL_TABLE_SIZE = 2048;

    int size;

    private int grow;

    private int mask;

    V[] table;

    /**
     * Create an empty map.
     */
    public ObjectIdSubclassMap() {
        initTable(INITIAL_TABLE_SIZE);
    }

    /**
     * Remove all entries from this map.
     */
    public void clear() {
        size = 0;
        initTable(INITIAL_TABLE_SIZE);
    }

    /**
     * Lookup an existing mapping.
     *
     * @param toFind
     *            the object identifier to find.
     * @return the instance mapped to toFind, or null if no mapping exists.
     */
    public V get(AnyObjectId toFind) {
        final int msk = mask;
        int i = toFind.w1 & msk;
        final V[] tbl = table;
        V obj;

        while ((obj = tbl[i]) != null) {
            if (AnyObjectId.isEqual(obj, toFind)) {
                return obj;
            }
            i = (i + 1) & msk;
        }
        return null;
    }

    /**
     * {@inheritDoc}
     * <p>
     * Returns true if this map contains the specified object.
     */
    @Override
    public boolean contains(AnyObjectId toFind) {
        return get(toFind) != null;
    }

    /**
     * Store an object for future lookup.
     * <p>
     * An existing mapping for <b>must not</b> be in this map. Callers must
     * first call {@link #get(AnyObjectId)} to verify there is no current
     * mapping prior to adding a new mapping, or use
     * {@link #addIfAbsent(ObjectId)}.
     *
     * @param newValue
     *            the object to store.
     */
    public <Q extends V> void add(Q newValue) {
        if (++size == grow)
            grow();
        insert(newValue);
    }

    /**
     * Store an object for future lookup.
     * <p>
     * Stores {@code newValue}, but only if there is not already an object for
     * the same object name. Callers can tell if the value is new by checking
     * the return value with reference equality:
     *
     * <pre>
     * V obj = ...;
     * boolean wasNew = map.addIfAbsent(obj) == obj;
     * </pre>
     *
     * @param newValue
     *            the object to store.
     * @return {@code newValue} if stored, or the prior value already stored and
     *         that would have been returned had the caller used
     *         {@code get(newValue)} first.
     */
    public <Q extends V> V addIfAbsent(Q newValue) {
        final int msk = mask;
        int i = newValue.w1 & msk;
        final V[] tbl = table;
        V obj;

        while ((obj = tbl[i]) != null) {
            if (AnyObjectId.isEqual(obj, newValue))
                return obj;
            i = (i + 1) & msk;
        }

        if (++size == grow) {
            grow();
            insert(newValue);
        } else {
            tbl[i] = newValue;
        }
        return newValue;
    }

    /**
     * Get number of objects in map
     *
     * @return number of objects in map
     */
    public int size() {
        return size;
    }

    /**
     * Whether {@link #size()} is 0.
     *
     * @return true if {@link #size()} is 0.
     */
    public boolean isEmpty() {
        return size == 0;
    }

    /** {@inheritDoc} */
    @Override
    public Iterator<V> iterator() {
        return new Iterator<>() {
            private int found;

            private int i;

            @Override
            public boolean hasNext() {
                return found < size;
            }

            @Override
            public V next() {
                while (i < table.length) {
                    final V v = table[i++];
                    if (v != null) {
                        found++;
                        return v;
                    }
                }
                throw new NoSuchElementException();
            }

            @Override
            public void remove() {
                throw new UnsupportedOperationException();
            }
        };
    }

    private void insert(V newValue) {
        final int msk = mask;
        int j = newValue.w1 & msk;
        final V[] tbl = table;
        while (tbl[j] != null)
            j = (j + 1) & msk;
        tbl[j] = newValue;
    }

    private void grow() {
        final V[] oldTable = table;
        final int oldSize = table.length;

        initTable(oldSize << 1);
        for (int i = 0; i < oldSize; i++) {
            final V obj = oldTable[i];
            if (obj != null)
                insert(obj);
        }
    }

    private void initTable(int sz) {
        grow = sz >> 1;
        mask = sz - 1;
        table = createArray(sz);
    }

    @SuppressWarnings("unchecked")
    private final V[] createArray(int sz) {
        return (V[]) new ObjectId[sz];
    }
}