/*
 * Copyright (C) 2008-2009, Google 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.
 *
 * Redistribution and use in source and binary forms, with or
 * without modification, are permitted provided that the following
 * conditions are met:
 *
 * - 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
 *   copyright notice, this list of conditions and the following
 *   disclaimer in the documentation and/or other materials provided
 *   with the distribution.
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 *   products derived from this software without specific prior
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 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
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package org.eclipse.jgit.merge;

import java.io.IOException;

import org.eclipse.jgit.errors.IncorrectObjectTypeException;
import org.eclipse.jgit.lib.AnyObjectId;
import org.eclipse.jgit.lib.Constants;
import org.eclipse.jgit.lib.ObjectId;
import org.eclipse.jgit.lib.ObjectWriter;
import org.eclipse.jgit.lib.Repository;
import org.eclipse.jgit.lib.WindowCursor;
import org.eclipse.jgit.revwalk.RevCommit;
import org.eclipse.jgit.revwalk.RevObject;
import org.eclipse.jgit.revwalk.RevTree;
import org.eclipse.jgit.revwalk.RevWalk;
import org.eclipse.jgit.revwalk.filter.RevFilter;
import org.eclipse.jgit.treewalk.AbstractTreeIterator;
import org.eclipse.jgit.treewalk.CanonicalTreeParser;
import org.eclipse.jgit.treewalk.EmptyTreeIterator;

/**
 * Instance of a specific {@link MergeStrategy} for a single {@link Repository}.
 */
public abstract class Merger {
	/** The repository this merger operates on. */
	protected final Repository db;

	/** A RevWalk for computing merge bases, or listing incoming commits. */
	protected final RevWalk walk;

	private ObjectWriter writer;

	/** The original objects supplied in the merge; this can be any tree-ish. */
	protected RevObject[] sourceObjects;

	/** If {@link #sourceObjects}[i] is a commit, this is the commit. */
	protected RevCommit[] sourceCommits;

	/** The trees matching every entry in {@link #sourceObjects}. */
	protected RevTree[] sourceTrees;

	/**
	 * Create a new merge instance for a repository.
	 *
	 * @param local
	 *            the repository this merger will read and write data on.
	 */
	protected Merger(final Repository local) {
		db = local;
		walk = new RevWalk(db);
	}

	/**
	 * @return the repository this merger operates on.
	 */
	public Repository getRepository() {
		return db;
	}

	/**
	 * @return an object writer to create objects in {@link #getRepository()}.
	 */
	public ObjectWriter getObjectWriter() {
		if (writer == null)
			writer = new ObjectWriter(getRepository());
		return writer;
	}

	/**
	 * Merge together two or more tree-ish objects.
	 * <p>
	 * Any tree-ish may be supplied as inputs. Commits and/or tags pointing at
	 * trees or commits may be passed as input objects.
	 *
	 * @param tips
	 *            source trees to be combined together. The merge base is not
	 *            included in this set.
	 * @return true if the merge was completed without conflicts; false if the
	 *         merge strategy cannot handle this merge or there were conflicts
	 *         preventing it from automatically resolving all paths.
	 * @throws IncorrectObjectTypeException
	 *             one of the input objects is not a commit, but the strategy
	 *             requires it to be a commit.
	 * @throws IOException
	 *             one or more sources could not be read, or outputs could not
	 *             be written to the Repository.
	 */
	public boolean merge(final AnyObjectId[] tips) throws IOException {
		sourceObjects = new RevObject[tips.length];
		for (int i = 0; i < tips.length; i++)
			sourceObjects[i] = walk.parseAny(tips[i]);

		sourceCommits = new RevCommit[sourceObjects.length];
		for (int i = 0; i < sourceObjects.length; i++) {
			try {
				sourceCommits[i] = walk.parseCommit(sourceObjects[i]);
			} catch (IncorrectObjectTypeException err) {
				sourceCommits[i] = null;
			}
		}

		sourceTrees = new RevTree[sourceObjects.length];
		for (int i = 0; i < sourceObjects.length; i++)
			sourceTrees[i] = walk.parseTree(sourceObjects[i]);

		return mergeImpl();
	}

	/**
	 * Create an iterator to walk the merge base of two commits.
	 *
	 * @param aIdx
	 *            index of the first commit in {@link #sourceObjects}.
	 * @param bIdx
	 *            index of the second commit in {@link #sourceObjects}.
	 * @return the new iterator
	 * @throws IncorrectObjectTypeException
	 *             one of the input objects is not a commit.
	 * @throws IOException
	 *             objects are missing or multiple merge bases were found.
	 */
	protected AbstractTreeIterator mergeBase(final int aIdx, final int bIdx)
			throws IOException {
		if (sourceCommits[aIdx] == null)
			throw new IncorrectObjectTypeException(sourceObjects[aIdx],
					Constants.TYPE_COMMIT);
		if (sourceCommits[bIdx] == null)
			throw new IncorrectObjectTypeException(sourceObjects[bIdx],
					Constants.TYPE_COMMIT);

		walk.reset();
		walk.setRevFilter(RevFilter.MERGE_BASE);
		walk.markStart(sourceCommits[aIdx]);
		walk.markStart(sourceCommits[bIdx]);
		final RevCommit base = walk.next();
		if (base == null)
			return new EmptyTreeIterator();
		final RevCommit base2 = walk.next();
		if (base2 != null) {
			throw new IOException("Multiple merge bases for:" + "\n  "
					+ sourceCommits[aIdx].name() + "\n  "
					+ sourceCommits[bIdx].name() + "found:" + "\n  "
					+ base.name() + "\n  " + base2.name());
		}
		return openTree(base.getTree());
	}

	/**
	 * Open an iterator over a tree.
	 *
	 * @param treeId
	 *            the tree to scan; must be a tree (not a treeish).
	 * @return an iterator for the tree.
	 * @throws IncorrectObjectTypeException
	 *             the input object is not a tree.
	 * @throws IOException
	 *             the tree object is not found or cannot be read.
	 */
	protected AbstractTreeIterator openTree(final AnyObjectId treeId)
			throws IncorrectObjectTypeException, IOException {
		final WindowCursor curs = new WindowCursor();
		try {
			return new CanonicalTreeParser(null, db, treeId, curs);
		} finally {
			curs.release();
		}
	}

	/**
	 * Execute the merge.
	 * <p>
	 * This method is called from {@link #merge(AnyObjectId[])} after the
	 * {@link #sourceObjects}, {@link #sourceCommits} and {@link #sourceTrees}
	 * have been populated.
	 *
	 * @return true if the merge was completed without conflicts; false if the
	 *         merge strategy cannot handle this merge or there were conflicts
	 *         preventing it from automatically resolving all paths.
	 * @throws IncorrectObjectTypeException
	 *             one of the input objects is not a commit, but the strategy
	 *             requires it to be a commit.
	 * @throws IOException
	 *             one or more sources could not be read, or outputs could not
	 *             be written to the Repository.
	 */
	protected abstract boolean mergeImpl() throws IOException;

	/**
	 * @return resulting tree, if {@link #merge(AnyObjectId[])} returned true.
	 */
	public abstract ObjectId getResultTreeId();
}