/*
    * 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
    *
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  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.ArrayList;
  import java.util.Collection;
  import java.util.Iterator;
  import java.util.Map;
  import java.util.concurrent.ConcurrentHashMap;
  
  import org.eclipse.jgit.errors.RepositoryNotFoundException;
  import org.eclipse.jgit.internal.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);
 		}
 	}
 
 	/**
 	 * Remove a repository from the cache.
 	 * <p>
 	 * Removes a repository from the cache, if it is still registered here,
 	 * permitting it to close.
 	 *
 	 * @param location
 	 *            location of the repository to remove.
 	 * @since 4.1
 	 */
 	public static void unregister(Key location) {
 		cache.unregisterRepository(location);
 	}
 
 	/**
 	 * @return the locations of all repositories registered in the cache.
 	 * @since 4.1
 	 */
 	public static Collection<Key> getRegisteredKeys() {
 		return cache.getKeys();
 	}
 
 	/** 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();
 	}
 
 	@SuppressWarnings("resource")
 	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 Collection<Key> getKeys() {
 		return new ArrayList<Key>(cacheMap.keySet());
 	}
 
 	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() //$NON-NLS-1$
 					&& fs.resolve(dir, "refs").exists() //$NON-NLS-1$
 					&& 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)); //$NON-NLS-1$
 		}
 
 		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;
 		}
 	}
 }