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    * and other copyright owners as documented in the project's IP log.
    *
    * This program and the accompanying materials are made available
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    * 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.IOException;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Collections;
  import java.util.HashMap;
  import java.util.List;
  import java.util.Map;
  
  import org.eclipse.jgit.annotations.NonNull;
  import org.eclipse.jgit.annotations.Nullable;
  
  /**
   * Abstraction of name to {@link ObjectId} mapping.
   * <p>
   * A reference database stores a mapping of reference names to {@link ObjectId}.
   * Every {@link Repository} has a single reference database, mapping names to
   * the tips of the object graph contained by the {@link ObjectDatabase}.
   */
  public abstract class RefDatabase {
  	/**
  	 * Order of prefixes to search when using non-absolute references.
  	 * <p>
  	 * The implementation's {@link #getRef(String)} method must take this search
  	 * space into consideration when locating a reference by name. The first
  	 * entry in the path is always {@code ""}, ensuring that absolute references
  	 * are resolved without further mangling.
  	 */
  	protected static final String[] SEARCH_PATH = { "", //$NON-NLS-1$
  			Constants.R_REFS, //
  			Constants.R_TAGS, //
  			Constants.R_HEADS, //
  			Constants.R_REMOTES //
  	};
  
  	/**
  	 * Maximum number of times a {@link SymbolicRef} can be traversed.
  	 * <p>
  	 * If the reference is nested deeper than this depth, the implementation
  	 * should either fail, or at least claim the reference does not exist.
  	 *
  	 * @since 4.2
  	 */
  	public static final int MAX_SYMBOLIC_REF_DEPTH = 5;
  
  	/** Magic value for {@link #getRefs(String)} to return all references. */
  	public static final String ALL = "";//$NON-NLS-1$
  
  	/**
  	 * Initialize a new reference database at this location.
  	 *
  	 * @throws IOException
  	 *             the database could not be created.
  	 */
  	public abstract void create() throws IOException;
 
 	/** Close any resources held by this database. */
 	public abstract void close();
 
 	/**
 	 * Determine if a proposed reference name overlaps with an existing one.
 	 * <p>
 	 * Reference names use '/' as a component separator, and may be stored in a
 	 * hierarchical storage such as a directory on the local filesystem.
 	 * <p>
 	 * If the reference "refs/heads/foo" exists then "refs/heads/foo/bar" must
 	 * not exist, as a reference cannot have a value and also be a container for
 	 * other references at the same time.
 	 * <p>
 	 * If the reference "refs/heads/foo/bar" exists than the reference
 	 * "refs/heads/foo" cannot exist, for the same reason.
 	 *
 	 * @param name
 	 *            proposed name.
 	 * @return true if the name overlaps with an existing reference; false if
 	 *         using this name right now would be safe.
 	 * @throws IOException
 	 *             the database could not be read to check for conflicts.
 	 * @see #getConflictingNames(String)
 	 */
 	public abstract boolean isNameConflicting(String name) throws IOException;
 
 	/**
 	 * Determine if a proposed reference cannot coexist with existing ones. If
 	 * the passed name already exists, it's not considered a conflict.
 	 *
 	 * @param name
 	 *            proposed name to check for conflicts against
 	 * @return a collection of full names of existing refs which would conflict
 	 *         with the passed ref name; empty collection when there are no
 	 *         conflicts
 	 * @throws IOException
 	 * @since 2.3
 	 * @see #isNameConflicting(String)
 	 */
 	@NonNull
 	public Collection<String> getConflictingNames(String name)
 			throws IOException {
 		Map<String, Ref> allRefs = getRefs(ALL);
 		// Cannot be nested within an existing reference.
 		int lastSlash = name.lastIndexOf('/');
 		while (0 < lastSlash) {
 			String needle = name.substring(0, lastSlash);
 			if (allRefs.containsKey(needle))
 				return Collections.singletonList(needle);
 			lastSlash = name.lastIndexOf('/', lastSlash - 1);
 		}
 
 		List<String> conflicting = new ArrayList<String>();
 		// Cannot be the container of an existing reference.
 		String prefix = name + '/';
 		for (String existing : allRefs.keySet())
 			if (existing.startsWith(prefix))
 				conflicting.add(existing);
 
 		return conflicting;
 	}
 
 	/**
 	 * Create a new update command to create, modify or delete a reference.
 	 *
 	 * @param name
 	 *            the name of the reference.
 	 * @param detach
 	 *            if {@code true} and {@code name} is currently a
 	 *            {@link SymbolicRef}, the update will replace it with an
 	 *            {@link ObjectIdRef}. Otherwise, the update will recursively
 	 *            traverse {@link SymbolicRef}s and operate on the leaf
 	 *            {@link ObjectIdRef}.
 	 * @return a new update for the requested name; never null.
 	 * @throws IOException
 	 *             the reference space cannot be accessed.
 	 */
 	@NonNull
 	public abstract RefUpdate newUpdate(String name, boolean detach)
 			throws IOException;
 
 	/**
 	 * Create a new update command to rename a reference.
 	 *
 	 * @param fromName
 	 *            name of reference to rename from
 	 * @param toName
 	 *            name of reference to rename to
 	 * @return an update command that knows how to rename a branch to another.
 	 * @throws IOException
 	 *             the reference space cannot be accessed.
 	 */
 	@NonNull
 	public abstract RefRename newRename(String fromName, String toName)
 			throws IOException;
 
 	/**
 	 * Create a new batch update to attempt on this database.
 	 * <p>
 	 * The default implementation performs a sequential update of each command.
 	 *
 	 * @return a new batch update object.
 	 */
 	@NonNull
 	public BatchRefUpdate newBatchUpdate() {
 		return new BatchRefUpdate(this);
 	}
 
 	/**
 	 * Whether the database is capable of performing batch updates as atomic
 	 * transactions.
 	 * <p>
 	 * If true, by default {@link BatchRefUpdate} instances will perform updates
 	 * atomically, meaning either all updates will succeed, or all updates will
 	 * fail. It is still possible to turn off this behavior on a per-batch basis
 	 * by calling {@code update.setAtomic(false)}.
 	 * <p>
 	 * If false, {@link BatchRefUpdate} instances will never perform updates
 	 * atomically, and calling {@code update.setAtomic(true)} will cause the
 	 * entire batch to fail with {@code REJECTED_OTHER_REASON}.
 	 * <p>
 	 * This definition of atomicity is stronger than what is provided by
 	 * {@link org.eclipse.jgit.transport.ReceivePack}. {@code ReceivePack} will
 	 * attempt to reject all commands if it knows in advance some commands may
 	 * fail, even if the storage layer does not support atomic transactions. Here,
 	 * atomicity applies even in the case of unforeseeable errors.
 	 *
 	 * @return whether transactions are atomic by default.
 	 * @since 3.6
 	 */
 	public boolean performsAtomicTransactions() {
 		return false;
 	}
 
 	/**
 	 * Read a single reference.
 	 * <p>
 	 * Aside from taking advantage of {@link #SEARCH_PATH}, this method may be
 	 * able to more quickly resolve a single reference name than obtaining the
 	 * complete namespace by {@code getRefs(ALL).get(name)}.
 	 * <p>
 	 * To read a specific reference without using @{link #SEARCH_PATH}, see
 	 * {@link #exactRef(String)}.
 	 *
 	 * @param name
 	 *            the name of the reference. May be a short name which must be
 	 *            searched for using the standard {@link #SEARCH_PATH}.
 	 * @return the reference (if it exists); else {@code null}.
 	 * @throws IOException
 	 *             the reference space cannot be accessed.
 	 */
 	@Nullable
 	public abstract Ref getRef(String name) throws IOException;
 
 	/**
 	 * Read a single reference.
 	 * <p>
 	 * Unlike {@link #getRef}, this method expects an unshortened reference
 	 * name and does not search using the standard {@link #SEARCH_PATH}.
 	 *
 	 * @param name
 	 *             the unabbreviated name of the reference.
 	 * @return the reference (if it exists); else {@code null}.
 	 * @throws IOException
 	 *             the reference space cannot be accessed.
 	 * @since 4.1
 	 */
 	@Nullable
 	public Ref exactRef(String name) throws IOException {
 		Ref ref = getRef(name);
 		if (ref == null || !name.equals(ref.getName())) {
 			return null;
 		}
 		return ref;
 	}
 
 	/**
 	 * Read the specified references.
 	 * <p>
 	 * This method expects a list of unshortened reference names and returns
 	 * a map from reference names to refs.  Any named references that do not
 	 * exist will not be included in the returned map.
 	 *
 	 * @param refs
 	 *             the unabbreviated names of references to look up.
 	 * @return modifiable map describing any refs that exist among the ref
 	 *         ref names supplied. The map can be an unsorted map.
 	 * @throws IOException
 	 *             the reference space cannot be accessed.
 	 * @since 4.1
 	 */
 	@NonNull
 	public Map<String, Ref> exactRef(String... refs) throws IOException {
 		Map<String, Ref> result = new HashMap<>(refs.length);
 		for (String name : refs) {
 			Ref ref = exactRef(name);
 			if (ref != null) {
 				result.put(name, ref);
 			}
 		}
 		return result;
 	}
 
 	/**
 	 * Find the first named reference.
 	 * <p>
 	 * This method expects a list of unshortened reference names and returns
 	 * the first that exists.
 	 *
 	 * @param refs
 	 *             the unabbreviated names of references to look up.
 	 * @return the first named reference that exists (if any); else {@code null}.
 	 * @throws IOException
 	 *             the reference space cannot be accessed.
 	 * @since 4.1
 	 */
 	@Nullable
 	public Ref firstExactRef(String... refs) throws IOException {
 		for (String name : refs) {
 			Ref ref = exactRef(name);
 			if (ref != null) {
 				return ref;
 			}
 		}
 		return null;
 	}
 
 	/**
 	 * Get a section of the reference namespace.
 	 *
 	 * @param prefix
 	 *            prefix to search the namespace with; must end with {@code /}.
 	 *            If the empty string ({@link #ALL}), obtain a complete snapshot
 	 *            of all references.
 	 * @return modifiable map that is a complete snapshot of the current
 	 *         reference namespace, with {@code prefix} removed from the start
 	 *         of each key. The map can be an unsorted map.
 	 * @throws IOException
 	 *             the reference space cannot be accessed.
 	 */
 	@NonNull
 	public abstract Map<String, Ref> getRefs(String prefix) throws IOException;
 
 	/**
 	 * Get the additional reference-like entities from the repository.
 	 * <p>
 	 * The result list includes non-ref items such as MERGE_HEAD and
 	 * FETCH_RESULT cast to be refs. The names of these refs are not returned by
 	 * <code>getRefs(ALL)</code> but are accepted by {@link #getRef(String)}
 	 * and {@link #exactRef(String)}.
 	 *
 	 * @return a list of additional refs
 	 * @throws IOException
 	 *             the reference space cannot be accessed.
 	 */
 	@NonNull
 	public abstract List<Ref> getAdditionalRefs() throws IOException;
 
 	/**
 	 * Peel a possibly unpeeled reference by traversing the annotated tags.
 	 * <p>
 	 * If the reference cannot be peeled (as it does not refer to an annotated
 	 * tag) the peeled id stays null, but {@link Ref#isPeeled()} will be true.
 	 * <p>
 	 * Implementors should check {@link Ref#isPeeled()} before performing any
 	 * additional work effort.
 	 *
 	 * @param ref
 	 *            The reference to peel
 	 * @return {@code ref} if {@code ref.isPeeled()} is true; otherwise a new
 	 *         Ref object representing the same data as Ref, but isPeeled() will
 	 *         be true and getPeeledObjectId() will contain the peeled object
 	 *         (or {@code null}).
 	 * @throws IOException
 	 *             the reference space or object space cannot be accessed.
 	 */
 	@NonNull
 	public abstract Ref peel(Ref ref) throws IOException;
 
 	/**
 	 * Triggers a refresh of all internal data structures.
 	 * <p>
 	 * In case the RefDatabase implementation has internal caches this method
 	 * will trigger that all these caches are cleared.
 	 * <p>
 	 * Implementors should overwrite this method if they use any kind of caches.
 	 */
 	public void refresh() {
 		// nothing
 	}
 
 	/**
 	 * Try to find the specified name in the ref map using {@link #SEARCH_PATH}.
 	 *
 	 * @param map
 	 *            map of refs to search within. Names should be fully qualified,
 	 *            e.g. "refs/heads/master".
 	 * @param name
 	 *            short name of ref to find, e.g. "master" to find
 	 *            "refs/heads/master" in map.
 	 * @return The first ref matching the name, or {@code null} if not found.
 	 * @since 3.4
 	 */
 	@Nullable
 	public static Ref findRef(Map<String, Ref> map, String name) {
 		for (String prefix : SEARCH_PATH) {
 			String fullname = prefix + name;
 			Ref ref = map.get(fullname);
 			if (ref != null)
 				return ref;
 		}
 		return null;
 	}
 }