jgit/org.eclipse.jgit/src/org/eclipse/jgit/api/DescribeCommand.java

/*
 * Copyright (C) 2013, CloudBees, Inc.
 * and other copyright owners as documented in the project's IP log.
 *
 * This program and the accompanying materials are made available
 * under the terms of the Eclipse Distribution License v1.0 which
 * accompanies this distribution, is reproduced below, and is
 * available at http://www.eclipse.org/org/documents/edl-v10.php
 *
 * All rights reserved.
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 * without modification, are permitted provided that the following
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 *
 * - Redistributions of source code must retain the above copyright
 *   notice, this list of conditions and the following disclaimer.
 *
 * - Redistributions in binary form must reproduce the above
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package org.eclipse.jgit.api;

import org.eclipse.jgit.api.errors.GitAPIException;
import org.eclipse.jgit.api.errors.JGitInternalException;
import org.eclipse.jgit.api.errors.RefNotFoundException;
import org.eclipse.jgit.errors.IncorrectObjectTypeException;
import org.eclipse.jgit.errors.MissingObjectException;
import org.eclipse.jgit.internal.JGitText;
import org.eclipse.jgit.lib.Constants;
import org.eclipse.jgit.lib.ObjectId;
import org.eclipse.jgit.lib.Ref;
import org.eclipse.jgit.lib.Repository;
import org.eclipse.jgit.revwalk.*;

import java.io.IOException;
import java.text.MessageFormat;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import static org.eclipse.jgit.lib.Constants.R_TAGS;

/**
 * Given a commit, show the most recent tag that is reachable from a commit.
 *
 * @since 3.2
 */
public class DescribeCommand extends GitCommand<String> {
	private final RevWalk w;

	/**
	 * Commit to describe.
	 */
	private RevCommit target;

	/**
	 * How many tags we'll consider as candidates.
	 * This can only go up to the number of flags JGit can support in a walk,
	 * which is 24.
	 */
	private int maxCandidates = 10;

	/**
	 *
	 * @param repo
	 */
	protected DescribeCommand(Repository repo) {
		super(repo);
		w = new RevWalk(repo);
		w.setRetainBody(false);
	}

	/**
	 * Sets the commit to be described.
	 *
	 * @param target
	 * 		A non-null object ID to be described.
	 * @return {@code this}
	 * @throws MissingObjectException
	 *             the supplied commit does not exist.
	 * @throws IncorrectObjectTypeException
	 *             the supplied id is not a commit or an annotated tag.
	 * @throws IOException
	 *             a pack file or loose object could not be read.
	 */
	public DescribeCommand setTarget(ObjectId target) throws IOException {
		this.target = w.parseCommit(target);
		return this;
	}

	/**
	 * Sets the commit to be described.
	 *
	 * @param rev
	 * 		Commit ID, tag, branch, ref, etc.
	 * 		See {@link Repository#resolve(String)} for allowed syntax.
	 * @return {@code this}
	 * @throws IncorrectObjectTypeException
	 *             the supplied id is not a commit or an annotated tag.
	 * @throws RefNotFoundException
	 * 				the given rev didn't resolve to any object.
	 * @throws IOException
	 *             a pack file or loose object could not be read.
	 */
	public DescribeCommand setTarget(String rev) throws IOException,
			RefNotFoundException {
		ObjectId id = repo.resolve(rev);
		if (id == null)
			throw new RefNotFoundException(MessageFormat.format(JGitText.get().refNotResolved, rev));
		return setTarget(id);
	}

	/**
	 * Describes the specified commit. Target defaults to HEAD if no commit was
	 * set explicitly.
	 *
	 * @return if there's a tag that points to the commit being described, this
	 *         tag name is returned. Otherwise additional suffix is added to the
	 *         nearest tag, just like git-describe(1).
	 *         <p>
	 *         If none of the ancestors of the commit being described has any
	 *         tags at all, then this method returns null, indicating that
	 *         there's no way to describe this tag.
	 */
	@Override
	public String call() throws GitAPIException {
		try {
			checkCallable();

			if (target == null)
				setTarget(Constants.HEAD);

			Map<ObjectId, Ref> tags = new HashMap<ObjectId, Ref>();

			for (Ref r : repo.getRefDatabase().getRefs(R_TAGS).values()) {
				ObjectId key = repo.peel(r).getPeeledObjectId();
				if (key == null)
					key = r.getObjectId();
				tags.put(key, r);
			}

			// combined flags of all the candidate instances
			final RevFlagSet allFlags = new RevFlagSet();

			/**
			 * Tracks the depth of each tag as we find them.
			 */
			class Candidate {
				final Ref tag;
				final RevFlag flag;

				/**
				 * This field counts number of commits that are reachable from
				 * the tip but not reachable from the tag.
				 */
				int depth;

				Candidate(RevCommit commit, Ref tag) {
					this.tag = tag;
					this.flag = w.newFlag(tag.getName());
					// we'll mark all the nodes reachable from this tag accordingly
					allFlags.add(flag);
					w.carry(flag);
					commit.add(flag);
					// As of this writing, JGit carries a flag from a child to its parents
					// right before RevWalk.next() returns, so all the flags that are added
					// must be manually carried to its parents. If that gets fixed,
					// this will be unnecessary.
					commit.carry(flag);
				}

				/**
				 * Does this tag contain the given commit?
				 */
				boolean reaches(RevCommit c) {
					return c.has(flag);
				}

				String describe(ObjectId tip) throws IOException {
					return String.format("%s-%d-g%s", tag.getName().substring(R_TAGS.length()), //$NON-NLS-1$
							Integer.valueOf(depth), w.getObjectReader().abbreviate(tip).name());
				}
			}
			List<Candidate> candidates = new ArrayList<Candidate>();    // all the candidates we find

			// is the target already pointing to a tag? if so, we are done!
			Ref lucky = tags.get(target);
			if (lucky != null)
				return lucky.getName().substring(R_TAGS.length());

			w.markStart(target);

			int seen = 0;   // commit seen thus far
			RevCommit c;
			while ((c = w.next()) != null) {
				if (!c.hasAny(allFlags)) {
					// if a tag already dominates this commit,
					// then there's no point in picking a tag on this commit
					// since the one that dominates it is always more preferable
					Ref t = tags.get(c);
					if (t != null) {
						Candidate cd = new Candidate(c, t);
						candidates.add(cd);
						cd.depth = seen;
					}
				}

				// if the newly discovered commit isn't reachable from a tag that we've seen
				// it counts toward the total depth.
				for (Candidate cd : candidates) {
					if (!cd.reaches(c))
						cd.depth++;
				}

				// if we have search going for enough tags, we will start
				// closing down. JGit can only give us a finite number of bits,
				// so we can't track all tags even if we wanted to.
				if (candidates.size() >= maxCandidates)
					break;

				// TODO: if all the commits in the queue of RevWalk has allFlags
				// there's no point in continuing search as we'll not discover any more
				// tags. But RevWalk doesn't expose this.
				seen++;
			}

			// at this point we aren't adding any more tags to our search,
			// but we still need to count all the depths correctly.
			while ((c = w.next()) != null) {
				if (c.hasAll(allFlags)) {
					// no point in visiting further from here, so cut the search here
					for (RevCommit p : c.getParents())
						p.add(RevFlag.SEEN);
				} else {
					for (Candidate cd : candidates) {
						if (!cd.reaches(c))
							cd.depth++;
					}
				}
			}

			// if all the nodes are dominated by all the tags, the walk stops
			if (candidates.isEmpty())
				return null;

			Candidate best = Collections.min(candidates, new Comparator<Candidate>() {
				public int compare(Candidate o1, Candidate o2) {
					return o1.depth - o2.depth;
				}
			});

			return best.describe(target);
		} catch (IOException e) {
			throw new JGitInternalException(e.getMessage(), e);
		} finally {
			setCallable(false);
			w.release();
		}
	}
}