/*
 * Copyright (C) 2009, Google Inc.
 * 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;
import org.eclipse.jgit.errors.StopWalkException;
import org.eclipse.jgit.lib.ObjectId;
import org.eclipse.jgit.revwalk.filter.RevFilter;

/**
 * Default (and first pass) RevCommit Generator implementation for RevWalk.
 * <p>
 * This generator starts from a set of one or more commits and process them in
 * descending (newest to oldest) commit time order. Commits automatically cause
 * their parents to be enqueued for further processing, allowing the entire
 * commit graph to be walked. A {@link RevFilter} may be used to select a subset
 * of the commits and return them to the caller.
 */
class PendingGenerator extends Generator {
	private static final int PARSED = RevWalk.PARSED;

	private static final int SEEN = RevWalk.SEEN;

	private static final int UNINTERESTING = RevWalk.UNINTERESTING;

	/**
	 * Number of additional commits to scan after we think we are done.
	 * <p>
	 * This small buffer of commits is scanned to ensure we didn't miss anything
	 * as a result of clock skew when the commits were made. We need to set our
	 * constant to 1 additional commit due to the use of a pre-increment
	 * operator when accessing the value.
	 */
	static final int OVER_SCAN = 5 + 1;

	/** A commit near the end of time, to initialize {@link #last} with. */
	private static final RevCommit INIT_LAST;

	static {
		INIT_LAST = new RevCommit(ObjectId.zeroId());
		INIT_LAST.commitTime = Integer.MAX_VALUE;
	}

	private final RevWalk walker;

	private final DateRevQueue pending;

	private final RevFilter filter;

	private final int output;

	/** Last commit produced to the caller from {@link #next()}. */
	private RevCommit last = INIT_LAST;

	/**
	 * Number of commits we have remaining in our over-scan allotment.
	 * <p>
	 * Only relevant if there are {@link #UNINTERESTING} commits in the
	 * {@link #pending} queue.
	 */
	private int overScan = OVER_SCAN;

	boolean canDispose;

	PendingGenerator(final RevWalk w, final DateRevQueue p,
			final RevFilter f, final int out) {
		super(w.isFirstParent());
		walker = w;
		pending = p;
		filter = f;
		output = out;
		canDispose = true;
	}

	@Override
	int outputType() {
		return output | SORT_COMMIT_TIME_DESC;
	}

	@Override
	RevCommit next() throws MissingObjectException,
			IncorrectObjectTypeException, IOException {
		try {
			for (;;) {
				final RevCommit c = pending.next();
				if (c == null) {
					return null;
				}

				final boolean produce;
				if ((c.flags & UNINTERESTING) != 0)
					produce = false;
				else {
					if (filter.requiresCommitBody())
						c.parseBody(walker);
					produce = filter.include(walker, c);
				}

				int parentCount = c.getParentCount();
				for (int i = 0; i < parentCount; i++) {
					RevCommit p = c.getParent(i);
					// If the commit is uninteresting, don't try to prune
					// parents because we want the maximal uninteresting set.
					if (firstParent && i > 0 && (c.flags & UNINTERESTING) == 0) {
						continue;
					}
					if ((p.flags & SEEN) != 0)
						continue;
					if ((p.flags & PARSED) == 0)
						p.parseHeaders(walker);
					p.flags |= SEEN;
					pending.add(p);
				}
				walker.carryFlagsImpl(c);

				if ((c.flags & UNINTERESTING) != 0) {
					if (pending.everbodyHasFlag(UNINTERESTING)) {
						final RevCommit n = pending.peek();
						if (n != null && n.commitTime >= last.commitTime) {
							// This is too close to call. The next commit we
							// would pop is dated after the last one produced.
							// We have to keep going to ensure that we carry
							// flags as much as necessary.
							//
							overScan = OVER_SCAN;
						} else if (--overScan == 0)
							throw StopWalkException.INSTANCE;
					} else {
						overScan = OVER_SCAN;
					}
					if (canDispose)
						c.disposeBody();
					continue;
				}

				if (produce)
					return last = c;
				else if (canDispose)
					c.disposeBody();
			}
		} catch (StopWalkException swe) {
			pending.clear();
			return null;
		}
	}
}