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    * Copyright (C) 2011, 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.internal.storage.dfs;
  
  import static org.eclipse.jgit.lib.Ref.Storage.NEW;
  
  import java.io.IOException;
  import java.util.Collections;
  import java.util.List;
  import java.util.Map;
  import java.util.concurrent.atomic.AtomicReference;
  
  import org.eclipse.jgit.errors.MissingObjectException;
  import org.eclipse.jgit.lib.ObjectIdRef;
  import org.eclipse.jgit.lib.Ref;
  import org.eclipse.jgit.lib.RefDatabase;
  import org.eclipse.jgit.lib.RefRename;
  import org.eclipse.jgit.lib.RefUpdate;
  import org.eclipse.jgit.lib.SymbolicRef;
  import org.eclipse.jgit.revwalk.RevObject;
  import org.eclipse.jgit.revwalk.RevTag;
  import org.eclipse.jgit.revwalk.RevWalk;
  import org.eclipse.jgit.util.RefList;
  import org.eclipse.jgit.util.RefMap;
  
  /** */
  public abstract class DfsRefDatabase extends RefDatabase {
  	private final DfsRepository repository;
  
  	private final AtomicReference<RefCache> cache;
  
  	/**
  	 * Initialize the reference database for a repository.
  	 *
  	 * @param repository
  	 *            the repository this database instance manages references for.
  	 */
  	protected DfsRefDatabase(DfsRepository repository) {
  		this.repository = repository;
  		this.cache = new AtomicReference<>();
  	}
  
  	/** @return the repository the database holds the references of. */
  	protected DfsRepository getRepository() {
  		return repository;
  	}
  
  	boolean exists() throws IOException {
  		return 0 < read().size();
  	}
  
  	@Override
  	public Ref exactRef(String name) throws IOException {
  		RefCache curr = read();
  		Ref ref = curr.ids.get(name);
  		return ref != null ? resolve(ref, 0, curr.ids) : null;
  	}
  
 	@Override
 	public Ref getRef(String needle) throws IOException {
 		RefCache curr = read();
 		for (String prefix : SEARCH_PATH) {
 			Ref ref = curr.ids.get(prefix + needle);
 			if (ref != null) {
 				ref = resolve(ref, 0, curr.ids);
 				return ref;
 			}
 		}
 		return null;
 	}
 
 	@Override
 	public List<Ref> getAdditionalRefs() {
 		return Collections.emptyList();
 	}
 
 	@Override
 	public Map<String, Ref> getRefs(String prefix) throws IOException {
 		RefCache curr = read();
 		RefList<Ref> packed = RefList.emptyList();
 		RefList<Ref> loose = curr.ids;
 		RefList.Builder<Ref> sym = new RefList.Builder<>(curr.sym.size());
 
 		for (int idx = 0; idx < curr.sym.size(); idx++) {
 			Ref ref = curr.sym.get(idx);
 			String name = ref.getName();
 			ref = resolve(ref, 0, loose);
 			if (ref != null && ref.getObjectId() != null) {
 				sym.add(ref);
 			} else {
 				// A broken symbolic reference, we have to drop it from the
 				// collections the client is about to receive. Should be a
 				// rare occurrence so pay a copy penalty.
 				int toRemove = loose.find(name);
 				if (0 <= toRemove)
 					loose = loose.remove(toRemove);
 			}
 		}
 
 		return new RefMap(prefix, packed, loose, sym.toRefList());
 	}
 
 	private Ref resolve(Ref ref, int depth, RefList<Ref> loose)
 			throws IOException {
 		if (!ref.isSymbolic())
 			return ref;
 
 		Ref dst = ref.getTarget();
 
 		if (MAX_SYMBOLIC_REF_DEPTH <= depth)
 			return null; // claim it doesn't exist
 
 		dst = loose.get(dst.getName());
 		if (dst == null)
 			return ref;
 
 		dst = resolve(dst, depth + 1, loose);
 		if (dst == null)
 			return null;
 		return new SymbolicRef(ref.getName(), dst);
 	}
 
 	@Override
 	public Ref peel(Ref ref) throws IOException {
 		final Ref oldLeaf = ref.getLeaf();
 		if (oldLeaf.isPeeled() || oldLeaf.getObjectId() == null)
 			return ref;
 
 		Ref newLeaf = doPeel(oldLeaf);
 
 		RefCache cur = read();
 		int idx = cur.ids.find(oldLeaf.getName());
 		if (0 <= idx && cur.ids.get(idx) == oldLeaf) {
 			RefList<Ref> newList = cur.ids.set(idx, newLeaf);
 			cache.compareAndSet(cur, new RefCache(newList, cur));
 			cachePeeledState(oldLeaf, newLeaf);
 		}
 
 		return recreate(ref, newLeaf);
 	}
 
 	private Ref doPeel(final Ref leaf) throws MissingObjectException,
 			IOException {
 		try (RevWalk rw = new RevWalk(repository)) {
 			RevObject obj = rw.parseAny(leaf.getObjectId());
 			if (obj instanceof RevTag) {
 				return new ObjectIdRef.PeeledTag(
 						leaf.getStorage(),
 						leaf.getName(),
 						leaf.getObjectId(),
 						rw.peel(obj).copy());
 			} else {
 				return new ObjectIdRef.PeeledNonTag(
 						leaf.getStorage(),
 						leaf.getName(),
 						leaf.getObjectId());
 			}
 		}
 	}
 
 	private static Ref recreate(Ref old, Ref leaf) {
 		if (old.isSymbolic()) {
 			Ref dst = recreate(old.getTarget(), leaf);
 			return new SymbolicRef(old.getName(), dst);
 		}
 		return leaf;
 	}
 
 	@Override
 	public RefUpdate newUpdate(String refName, boolean detach)
 			throws IOException {
 		boolean detachingSymbolicRef = false;
 		Ref ref = exactRef(refName);
 		if (ref == null)
 			ref = new ObjectIdRef.Unpeeled(NEW, refName, null);
 		else
 			detachingSymbolicRef = detach && ref.isSymbolic();
 
 		DfsRefUpdate update = new DfsRefUpdate(this, ref);
 		if (detachingSymbolicRef)
 			update.setDetachingSymbolicRef();
 		return update;
 	}
 
 	@Override
 	public RefRename newRename(String fromName, String toName)
 			throws IOException {
 		RefUpdate src = newUpdate(fromName, true);
 		RefUpdate dst = newUpdate(toName, true);
 		return new DfsRefRename(src, dst);
 	}
 
 	@Override
 	public boolean isNameConflicting(String refName) throws IOException {
 		RefList<Ref> all = read().ids;
 
 		// Cannot be nested within an existing reference.
 		int lastSlash = refName.lastIndexOf('/');
 		while (0 < lastSlash) {
 			String needle = refName.substring(0, lastSlash);
 			if (all.contains(needle))
 				return true;
 			lastSlash = refName.lastIndexOf('/', lastSlash - 1);
 		}
 
 		// Cannot be the container of an existing reference.
 		String prefix = refName + '/';
 		int idx = -(all.find(prefix) + 1);
 		if (idx < all.size() && all.get(idx).getName().startsWith(prefix))
 			return true;
 		return false;
 	}
 
 	@Override
 	public void create() {
 		// Nothing to do.
 	}
 
 	@Override
 	public void refresh() {
 		clearCache();
 	}
 
 	@Override
 	public void close() {
 		clearCache();
 	}
 
 	void clearCache() {
 		cache.set(null);
 	}
 
 	void stored(Ref ref) {
 		RefCache oldCache, newCache;
 		do {
 			oldCache = cache.get();
 			if (oldCache == null)
 				return;
 			newCache = oldCache.put(ref);
 		} while (!cache.compareAndSet(oldCache, newCache));
 	}
 
 	void removed(String refName) {
 		RefCache oldCache, newCache;
 		do {
 			oldCache = cache.get();
 			if (oldCache == null)
 				return;
 			newCache = oldCache.remove(refName);
 		} while (!cache.compareAndSet(oldCache, newCache));
 	}
 
 	private RefCache read() throws IOException {
 		RefCache c = cache.get();
 		if (c == null) {
 			c = scanAllRefs();
 			cache.set(c);
 		}
 		return c;
 	}
 
 	/**
 	 * Read all known references in the repository.
 	 *
 	 * @return all current references of the repository.
 	 * @throws IOException
 	 *             references cannot be accessed.
 	 */
 	protected abstract RefCache scanAllRefs() throws IOException;
 
 	/**
 	 * Compare a reference, and put if it matches.
 	 * <p>
 	 * Two reference match if and only if they satisfy the following:
 	 *
 	 * <ul>
 	 * <li>If one reference is a symbolic ref, the other one should be a symbolic
 	 * ref.
 	 * <li>If both are symbolic refs, the target names should be same.
 	 * <li>If both are object ID refs, the object IDs should be same.
 	 * </ul>
 	 *
 	 * @param oldRef
 	 *            old value to compare to. If the reference is expected to not
 	 *            exist the old value has a storage of
 	 *            {@link org.eclipse.jgit.lib.Ref.Storage#NEW} and an ObjectId
 	 *            value of {@code null}.
 	 * @param newRef
 	 *            new reference to store.
 	 * @return true if the put was successful; false otherwise.
 	 * @throws IOException
 	 *             the reference cannot be put due to a system error.
 	 */
 	protected abstract boolean compareAndPut(Ref oldRef, Ref newRef)
 			throws IOException;
 
 	/**
 	 * Compare a reference, and delete if it matches.
 	 *
 	 * @param oldRef
 	 *            the old reference information that was previously read.
 	 * @return true if the remove was successful; false otherwise.
 	 * @throws IOException
 	 *             the reference could not be removed due to a system error.
 	 */
 	protected abstract boolean compareAndRemove(Ref oldRef) throws IOException;
 
 	/**
 	 * Update the cached peeled state of a reference
 	 * <p>
 	 * The ref database invokes this method after it peels a reference that had
 	 * not been peeled before. This allows the storage to cache the peel state
 	 * of the reference, and if it is actually peelable, the target that it
 	 * peels to, so that on-the-fly peeling doesn't have to happen on the next
 	 * reference read.
 	 *
 	 * @param oldLeaf
 	 *            the old reference.
 	 * @param newLeaf
 	 *            the new reference, with peel information.
 	 */
 	protected void cachePeeledState(Ref oldLeaf, Ref newLeaf) {
 		try {
 			compareAndPut(oldLeaf, newLeaf);
 		} catch (IOException e) {
 			// Ignore an exception during caching.
 		}
 	}
 
 	/** Collection of references managed by this database. */
 	public static class RefCache {
 		final RefList<Ref> ids;
 
 		final RefList<Ref> sym;
 
 		/**
 		 * Initialize a new reference cache.
 		 * <p>
 		 * The two reference lists supplied must be sorted in correct order
 		 * (string compare order) by name.
 		 *
 		 * @param ids
 		 *            references that carry an ObjectId, and all of {@code sym}.
 		 * @param sym
 		 *            references that are symbolic references to others.
 		 */
 		public RefCache(RefList<Ref> ids, RefList<Ref> sym) {
 			this.ids = ids;
 			this.sym = sym;
 		}
 
 		RefCache(RefList<Ref> ids, RefCache old) {
 			this(ids, old.sym);
 		}
 
 		/** @return number of references in this cache. */
 		public int size() {
 			return ids.size();
 		}
 
 		/**
 		 * Find a reference by name.
 		 *
 		 * @param name
 		 *            full name of the reference.
 		 * @return the reference, if it exists, otherwise null.
 		 */
 		public Ref get(String name) {
 			return ids.get(name);
 		}
 
 		/**
 		 * Obtain a modified copy of the cache with a ref stored.
 		 * <p>
 		 * This cache instance is not modified by this method.
 		 *
 		 * @param ref
 		 *            reference to add or replace.
 		 * @return a copy of this cache, with the reference added or replaced.
 		 */
 		public RefCache put(Ref ref) {
 			RefList<Ref> newIds = this.ids.put(ref);
 			RefList<Ref> newSym = this.sym;
 			if (ref.isSymbolic()) {
 				newSym = newSym.put(ref);
 			} else {
 				int p = newSym.find(ref.getName());
 				if (0 <= p)
 					newSym = newSym.remove(p);
 			}
 			return new RefCache(newIds, newSym);
 		}
 
 		/**
 		 * Obtain a modified copy of the cache with the ref removed.
 		 * <p>
 		 * This cache instance is not modified by this method.
 		 *
 		 * @param refName
 		 *            reference to remove, if it exists.
 		 * @return a copy of this cache, with the reference removed.
 		 */
 		public RefCache remove(String refName) {
 			RefList<Ref> newIds = this.ids;
 			int p = newIds.find(refName);
 			if (0 <= p)
 				newIds = newIds.remove(p);
 
 			RefList<Ref> newSym = this.sym;
 			p = newSym.find(refName);
 			if (0 <= p)
 				newSym = newSym.remove(p);
 			return new RefCache(newIds, newSym);
 		}
 	}
 }