/*
 * Copyright (C) 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.
 *
 * - Neither the name of the Eclipse Foundation, Inc. nor the
 *   names of its contributors may be used to endorse or promote
 *   products derived from this software without specific prior
 *   written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
 * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
 * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
 * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

package org.eclipse.jgit.lib;

import java.io.File;
import java.io.IOException;
import java.lang.ref.Reference;
import java.lang.ref.SoftReference;
import java.util.Iterator;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;

import org.eclipse.jgit.errors.RepositoryNotFoundException;
import org.eclipse.jgit.storage.file.FileRepository;
import org.eclipse.jgit.util.FS;
import org.eclipse.jgit.util.IO;
import org.eclipse.jgit.util.RawParseUtils;

/** Cache of active {@link Repository} instances. */
public class RepositoryCache {
	private static final RepositoryCache cache = new RepositoryCache();

	/**
	 * Open an existing repository, reusing a cached instance if possible.
	 * <p>
	 * When done with the repository, the caller must call
	 * {@link Repository#close()} to decrement the repository's usage counter.
	 *
	 * @param location
	 *            where the local repository is. Typically a {@link FileKey}.
	 * @return the repository instance requested; caller must close when done.
	 * @throws IOException
	 *             the repository could not be read (likely its core.version
	 *             property is not supported).
	 * @throws RepositoryNotFoundException
	 *             there is no repository at the given location.
	 */
	public static Repository open(final Key location) throws IOException,
			RepositoryNotFoundException {
		return open(location, true);
	}

	/**
	 * Open a repository, reusing a cached instance if possible.
	 * <p>
	 * When done with the repository, the caller must call
	 * {@link Repository#close()} to decrement the repository's usage counter.
	 *
	 * @param location
	 *            where the local repository is. Typically a {@link FileKey}.
	 * @param mustExist
	 *            If true, and the repository is not found, throws {@code
	 *            RepositoryNotFoundException}. If false, a repository instance
	 *            is created and registered anyway.
	 * @return the repository instance requested; caller must close when done.
	 * @throws IOException
	 *             the repository could not be read (likely its core.version
	 *             property is not supported).
	 * @throws RepositoryNotFoundException
	 *             There is no repository at the given location, only thrown if
	 *             {@code mustExist} is true.
	 */
	public static Repository open(final Key location, final boolean mustExist)
			throws IOException {
		return cache.openRepository(location, mustExist);
	}

	/**
	 * Register one repository into the cache.
	 * <p>
	 * During registration the cache automatically increments the usage counter,
	 * permitting it to retain the reference. A {@link FileKey} for the
	 * repository's {@link Repository#getDirectory()} is used to index the
	 * repository in the cache.
	 * <p>
	 * If another repository already is registered in the cache at this
	 * location, the other instance is closed.
	 *
	 * @param db
	 *            repository to register.
	 */
	public static void register(final Repository db) {
		if (db.getDirectory() != null) {
			FileKey key = FileKey.exact(db.getDirectory(), db.getFS());
			cache.registerRepository(key, db);
		}
	}

	/**
	 * Remove a repository from the cache.
	 * <p>
	 * Removes a repository from the cache, if it is still registered here,
	 * permitting it to close.
	 *
	 * @param db
	 *            repository to unregister.
	 */
	public static void close(final Repository db) {
		if (db.getDirectory() != null) {
			FileKey key = FileKey.exact(db.getDirectory(), db.getFS());
			cache.unregisterRepository(key);
		}
	}

	/** Unregister all repositories from the cache. */
	public static void clear() {
		cache.clearAll();
	}

	private final ConcurrentHashMap<Key, Reference<Repository>> cacheMap;

	private final Lock[] openLocks;

	private RepositoryCache() {
		cacheMap = new ConcurrentHashMap<Key, Reference<Repository>>();
		openLocks = new Lock[4];
		for (int i = 0; i < openLocks.length; i++)
			openLocks[i] = new Lock();
	}

	private Repository openRepository(final Key location,
			final boolean mustExist) throws IOException {
		Reference<Repository> ref = cacheMap.get(location);
		Repository db = ref != null ? ref.get() : null;
		if (db == null) {
			synchronized (lockFor(location)) {
				ref = cacheMap.get(location);
				db = ref != null ? ref.get() : null;
				if (db == null) {
					db = location.open(mustExist);
					ref = new SoftReference<Repository>(db);
					cacheMap.put(location, ref);
				}
			}
		}
		db.incrementOpen();
		return db;
	}

	private void registerRepository(final Key location, final Repository db) {
		db.incrementOpen();
		SoftReference<Repository> newRef = new SoftReference<Repository>(db);
		Reference<Repository> oldRef = cacheMap.put(location, newRef);
		Repository oldDb = oldRef != null ? oldRef.get() : null;
		if (oldDb != null)
			oldDb.close();
	}

	private void unregisterRepository(final Key location) {
		Reference<Repository> oldRef = cacheMap.remove(location);
		Repository oldDb = oldRef != null ? oldRef.get() : null;
		if (oldDb != null)
			oldDb.close();
	}

	private void clearAll() {
		for (int stage = 0; stage < 2; stage++) {
			for (Iterator<Map.Entry<Key, Reference<Repository>>> i = cacheMap
					.entrySet().iterator(); i.hasNext();) {
				final Map.Entry<Key, Reference<Repository>> e = i.next();
				final Repository db = e.getValue().get();
				if (db != null)
					db.close();
				i.remove();
			}
		}
	}

	private Lock lockFor(final Key location) {
		return openLocks[(location.hashCode() >>> 1) % openLocks.length];
	}

	private static class Lock {
		// Used only for its monitor.
	}

	/**
	 * Abstract hash key for {@link RepositoryCache} entries.
	 * <p>
	 * A Key instance should be lightweight, and implement hashCode() and
	 * equals() such that two Key instances are equal if they represent the same
	 * Repository location.
	 */
	public static interface Key {
		/**
		 * Called by {@link RepositoryCache#open(Key)} if it doesn't exist yet.
		 * <p>
		 * If a repository does not exist yet in the cache, the cache will call
		 * this method to acquire a handle to it.
		 *
		 * @param mustExist
		 *            true if the repository must exist in order to be opened;
		 *            false if a new non-existent repository is permitted to be
		 *            created (the caller is responsible for calling create).
		 * @return the new repository instance.
		 * @throws IOException
		 *             the repository could not be read (likely its core.version
		 *             property is not supported).
		 * @throws RepositoryNotFoundException
		 *             There is no repository at the given location, only thrown
		 *             if {@code mustExist} is true.
		 */
		Repository open(boolean mustExist) throws IOException,
				RepositoryNotFoundException;
	}

	/** Location of a Repository, using the standard java.io.File API. */
	public static class FileKey implements Key {
		/**
		 * Obtain a pointer to an exact location on disk.
		 * <p>
		 * No guessing is performed, the given location is exactly the GIT_DIR
		 * directory of the repository.
		 *
		 * @param directory
		 *            location where the repository database is.
		 * @param fs
		 *            the file system abstraction which will be necessary to
		 *            perform certain file system operations.
		 * @return a key for the given directory.
		 * @see #lenient(File, FS)
		 */
		public static FileKey exact(final File directory, FS fs) {
			return new FileKey(directory, fs);
		}

		/**
		 * Obtain a pointer to a location on disk.
		 * <p>
		 * The method performs some basic guessing to locate the repository.
		 * Searched paths are:
		 * <ol>
		 * <li>{@code directory} // assume exact match</li>
		 * <li>{@code directory} + "/.git" // assume working directory</li>
		 * <li>{@code directory} + ".git" // assume bare</li>
		 * </ol>
		 *
		 * @param directory
		 *            location where the repository database might be.
		 * @param fs
		 *            the file system abstraction which will be necessary to
		 *            perform certain file system operations.
		 * @return a key for the given directory.
		 * @see #exact(File, FS)
		 */
		public static FileKey lenient(final File directory, FS fs) {
			final File gitdir = resolve(directory, fs);
			return new FileKey(gitdir != null ? gitdir : directory, fs);
		}

		private final File path;
		private final FS fs;

		/**
		 * @param directory
		 *            exact location of the repository.
		 * @param fs
		 *            the file system abstraction which will be necessary to
		 *            perform certain file system operations.
		 */
		protected FileKey(final File directory, FS fs) {
			path = canonical(directory);
			this.fs = fs;
		}

		private static File canonical(final File path) {
			try {
				return path.getCanonicalFile();
			} catch (IOException e) {
				return path.getAbsoluteFile();
			}
		}

		/** @return location supplied to the constructor. */
		public final File getFile() {
			return path;
		}

		public Repository open(final boolean mustExist) throws IOException {
			if (mustExist && !isGitRepository(path, fs))
				throw new RepositoryNotFoundException(path);
			return new FileRepository(path);
		}

		@Override
		public int hashCode() {
			return path.hashCode();
		}

		@Override
		public boolean equals(final Object o) {
			return o instanceof FileKey && path.equals(((FileKey) o).path);
		}

		@Override
		public String toString() {
			return path.toString();
		}

		/**
		 * Guess if a directory contains a Git repository.
		 * <p>
		 * This method guesses by looking for the existence of some key files
		 * and directories.
		 *
		 * @param dir
		 *            the location of the directory to examine.
		 * @param fs
		 *            the file system abstraction which will be necessary to
		 *            perform certain file system operations.
		 * @return true if the directory "looks like" a Git repository; false if
		 *         it doesn't look enough like a Git directory to really be a
		 *         Git directory.
		 */
		public static boolean isGitRepository(final File dir, FS fs) {
			return fs.resolve(dir, "objects").exists()
					&& fs.resolve(dir, "refs").exists()
					&& isValidHead(new File(dir, Constants.HEAD));
		}

		private static boolean isValidHead(final File head) {
			final String ref = readFirstLine(head);
			return ref != null
					&& (ref.startsWith("ref: refs/") || ObjectId.isId(ref));
		}

		private static String readFirstLine(final File head) {
			try {
				final byte[] buf = IO.readFully(head, 4096);
				int n = buf.length;
				if (n == 0)
					return null;
				if (buf[n - 1] == '\n')
					n--;
				return RawParseUtils.decode(buf, 0, n);
			} catch (IOException e) {
				return null;
			}
		}

		/**
		 * Guess the proper path for a Git repository.
		 * <p>
		 * The method performs some basic guessing to locate the repository.
		 * Searched paths are:
		 * <ol>
		 * <li>{@code directory} // assume exact match</li>
		 * <li>{@code directory} + "/.git" // assume working directory</li>
		 * <li>{@code directory} + ".git" // assume bare</li>
		 * </ol>
		 *
		 * @param directory
		 *            location to guess from. Several permutations are tried.
		 * @param fs
		 *            the file system abstraction which will be necessary to
		 *            perform certain file system operations.
		 * @return the actual directory location if a better match is found;
		 *         null if there is no suitable match.
		 */
		public static File resolve(final File directory, FS fs) {
			if (isGitRepository(directory, fs))
				return directory;
			if (isGitRepository(new File(directory, Constants.DOT_GIT), fs))
				return new File(directory, Constants.DOT_GIT);

			final String name = directory.getName();
			final File parent = directory.getParentFile();
			if (isGitRepository(new File(parent, name + Constants.DOT_GIT_EXT), fs))
				return new File(parent, name + Constants.DOT_GIT_EXT);
			return null;
		}
	}
}