BlockRevQueue.java

/*
 * 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.revwalk;

import java.io.IOException;

import org.eclipse.jgit.errors.IncorrectObjectTypeException;
import org.eclipse.jgit.errors.MissingObjectException;

abstract class BlockRevQueue extends AbstractRevQueue {
    protected BlockFreeList free;

    /**
     * Create an empty revision queue.
     *
     * @param firstParent
     *            whether only first-parent links should be followed when
     *            walking
     */
    protected BlockRevQueue(boolean firstParent) {
        super(firstParent);
        free = new BlockFreeList();
    }

    BlockRevQueue(Generator s) throws MissingObjectException,
            IncorrectObjectTypeException, IOException {
        super(s.firstParent);
        free = new BlockFreeList();
        outputType = s.outputType();
        s.shareFreeList(this);
        for (;;) {
            final RevCommit c = s.next();
            if (c == null)
                break;
            add(c);
        }
    }

    /**
     * {@inheritDoc}
     * <p>
     * Reconfigure this queue to share the same free list as another.
     * <p>
     * Multiple revision queues can be connected to the same free list, making
     * it less expensive for applications to shuttle commits between them. This
     * method arranges for the receiver to take from / return to the same free
     * list as the supplied queue.
     * <p>
     * Free lists are not thread-safe. Applications must ensure that all queues
     * sharing the same free list are doing so from only a single thread.
     */
    @Override
    public void shareFreeList(BlockRevQueue q) {
        free = q.free;
    }

    static final class BlockFreeList {
        private Block next;

        Block newBlock() {
            Block b = next;
            if (b == null)
                return new Block();
            next = b.next;
            b.clear();
            return b;
        }

        void freeBlock(Block b) {
            b.next = next;
            next = b;
        }

        void clear() {
            next = null;
        }
    }

    static final class Block {
        static final int BLOCK_SIZE = 256;

        /** Next block in our chain of blocks; null if we are the last. */
        Block next;

        /** Our table of queued commits. */
        final RevCommit[] commits = new RevCommit[BLOCK_SIZE];

        /** Next valid entry in {@link #commits}. */
        int headIndex;

        /** Next free entry in {@link #commits} for addition at. */
        int tailIndex;

        boolean isFull() {
            return tailIndex == BLOCK_SIZE;
        }

        boolean isEmpty() {
            return headIndex == tailIndex;
        }

        boolean canUnpop() {
            return headIndex > 0;
        }

        void add(RevCommit c) {
            commits[tailIndex++] = c;
        }

        void unpop(RevCommit c) {
            commits[--headIndex] = c;
        }

        RevCommit pop() {
            return commits[headIndex++];
        }

        RevCommit peek() {
            return commits[headIndex];
        }

        void clear() {
            next = null;
            headIndex = 0;
            tailIndex = 0;
        }

        void resetToMiddle() {
            headIndex = tailIndex = BLOCK_SIZE / 2;
        }

        void resetToEnd() {
            headIndex = tailIndex = BLOCK_SIZE;
        }
    }
}