/*
    * Copyright (C) 2010, 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
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  package org.eclipse.jgit.lib;
  
  import java.io.IOException;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Set;
  
  import org.eclipse.jgit.annotations.Nullable;
  import org.eclipse.jgit.errors.IncorrectObjectTypeException;
  import org.eclipse.jgit.errors.MissingObjectException;
  import org.eclipse.jgit.internal.storage.pack.ObjectReuseAsIs;
  
  /**
   * Reads an {@link ObjectDatabase} for a single thread.
   * <p>
   * Readers that can support efficient reuse of pack encoded objects should also
   * implement the companion interface {@link ObjectReuseAsIs}.
   */
  public abstract class ObjectReader implements AutoCloseable {
  	/** Type hint indicating the caller doesn't know the type. */
  	public static final int OBJ_ANY = -1;
  
  	/**
  	 * Construct a new reader from the same data.
  	 * <p>
  	 * Applications can use this method to build a new reader from the same data
  	 * source, but for an different thread.
  	 *
  	 * @return a brand new reader, using the same data source.
  	 */
  	public abstract ObjectReader newReader();
  
  	/**
  	 * Obtain a unique abbreviation (prefix) of an object SHA-1.
  	 *
  	 * This method uses a reasonable default for the minimum length. Callers who
  	 * don't care about the minimum length should prefer this method.
  	 *
  	 * The returned abbreviation would expand back to the argument ObjectId when
  	 * passed to {@link #resolve(AbbreviatedObjectId)}, assuming no new objects
  	 * are added to this repository between calls.
  	 *
  	 * @param objectId
  	 *            object identity that needs to be abbreviated.
  	 * @return SHA-1 abbreviation.
  	 * @throws IOException
  	 *             the object store cannot be read.
  	 */
  	public AbbreviatedObjectId abbreviate(AnyObjectId objectId)
  			throws IOException {
  		return abbreviate(objectId, 7);
  	}
  
  	/**
 	 * Obtain a unique abbreviation (prefix) of an object SHA-1.
 	 *
 	 * The returned abbreviation would expand back to the argument ObjectId when
 	 * passed to {@link #resolve(AbbreviatedObjectId)}, assuming no new objects
 	 * are added to this repository between calls.
 	 *
 	 * The default implementation of this method abbreviates the id to the
 	 * minimum length, then resolves it to see if there are multiple results.
 	 * When multiple results are found, the length is extended by 1 and resolve
 	 * is tried again.
 	 *
 	 * @param objectId
 	 *            object identity that needs to be abbreviated.
 	 * @param len
 	 *            minimum length of the abbreviated string. Must be in the range
 	 *            [2, {@value Constants#OBJECT_ID_STRING_LENGTH}].
 	 * @return SHA-1 abbreviation. If no matching objects exist in the
 	 *         repository, the abbreviation will match the minimum length.
 	 * @throws IOException
 	 *             the object store cannot be read.
 	 */
 	public AbbreviatedObjectId abbreviate(AnyObjectId objectId, int len)
 			throws IOException {
 		if (len == Constants.OBJECT_ID_STRING_LENGTH)
 			return AbbreviatedObjectId.fromObjectId(objectId);
 
 		AbbreviatedObjectId abbrev = objectId.abbreviate(len);
 		Collection<ObjectId> matches = resolve(abbrev);
 		while (1 < matches.size() && len < Constants.OBJECT_ID_STRING_LENGTH) {
 			abbrev = objectId.abbreviate(++len);
 			List<ObjectId> n = new ArrayList<ObjectId>(8);
 			for (ObjectId candidate : matches) {
 				if (abbrev.prefixCompare(candidate) == 0)
 					n.add(candidate);
 			}
 			if (1 < n.size())
 				matches = n;
 			else
 				matches = resolve(abbrev);
 		}
 		return abbrev;
 	}
 
 	/**
 	 * Resolve an abbreviated ObjectId to its full form.
 	 *
 	 * This method searches for an ObjectId that begins with the abbreviation,
 	 * and returns at least some matching candidates.
 	 *
 	 * If the returned collection is empty, no objects start with this
 	 * abbreviation. The abbreviation doesn't belong to this repository, or the
 	 * repository lacks the necessary objects to complete it.
 	 *
 	 * If the collection contains exactly one member, the abbreviation is
 	 * (currently) unique within this database. There is a reasonably high
 	 * probability that the returned id is what was previously abbreviated.
 	 *
 	 * If the collection contains 2 or more members, the abbreviation is not
 	 * unique. In this case the implementation is only required to return at
 	 * least 2 candidates to signal the abbreviation has conflicts. User
 	 * friendly implementations should return as many candidates as reasonably
 	 * possible, as the caller may be able to disambiguate further based on
 	 * context. However since databases can be very large (e.g. 10 million
 	 * objects) returning 625,000 candidates for the abbreviation "0" is simply
 	 * unreasonable, so implementors should draw the line at around 256 matches.
 	 *
 	 * @param id
 	 *            abbreviated id to resolve to a complete identity. The
 	 *            abbreviation must have a length of at least 2.
 	 * @return candidates that begin with the abbreviated identity.
 	 * @throws IOException
 	 *             the object store cannot be read.
 	 */
 	public abstract Collection<ObjectId> resolve(AbbreviatedObjectId id)
 			throws IOException;
 
 	/**
 	 * Does the requested object exist in this database?
 	 *
 	 * @param objectId
 	 *            identity of the object to test for existence of.
 	 * @return true if the specified object is stored in this database.
 	 * @throws IOException
 	 *             the object store cannot be accessed.
 	 */
 	public boolean has(AnyObjectId objectId) throws IOException {
 		return has(objectId, OBJ_ANY);
 	}
 
 	/**
 	 * Does the requested object exist in this database?
 	 *
 	 * @param objectId
 	 *            identity of the object to test for existence of.
 	 * @param typeHint
 	 *            hint about the type of object being requested, e.g.
 	 *            {@link Constants#OBJ_BLOB}; {@link #OBJ_ANY} if the object
 	 *            type is not known, or does not matter to the caller.
 	 * @return true if the specified object is stored in this database.
 	 * @throws IncorrectObjectTypeException
 	 *             typeHint was not OBJ_ANY, and the object's actual type does
 	 *             not match typeHint.
 	 * @throws IOException
 	 *             the object store cannot be accessed.
 	 */
 	public boolean has(AnyObjectId objectId, int typeHint) throws IOException {
 		try {
 			open(objectId, typeHint);
 			return true;
 		} catch (MissingObjectException notFound) {
 			return false;
 		}
 	}
 
 	/**
 	 * Open an object from this database.
 	 *
 	 * @param objectId
 	 *            identity of the object to open.
 	 * @return a {@link ObjectLoader} for accessing the object.
 	 * @throws MissingObjectException
 	 *             the object does not exist.
 	 * @throws IOException
 	 *             the object store cannot be accessed.
 	 */
 	public ObjectLoader open(AnyObjectId objectId)
 			throws MissingObjectException, IOException {
 		return open(objectId, OBJ_ANY);
 	}
 
 	/**
 	 * Open an object from this database.
 	 *
 	 * @param objectId
 	 *            identity of the object to open.
 	 * @param typeHint
 	 *            hint about the type of object being requested, e.g.
 	 *            {@link Constants#OBJ_BLOB}; {@link #OBJ_ANY} if the object
 	 *            type is not known, or does not matter to the caller.
 	 * @return a {@link ObjectLoader} for accessing the object.
 	 * @throws MissingObjectException
 	 *             the object does not exist.
 	 * @throws IncorrectObjectTypeException
 	 *             typeHint was not OBJ_ANY, and the object's actual type does
 	 *             not match typeHint.
 	 * @throws IOException
 	 *             the object store cannot be accessed.
 	 */
 	public abstract ObjectLoader open(AnyObjectId objectId, int typeHint)
 			throws MissingObjectException, IncorrectObjectTypeException,
 			IOException;
 
 	/**
 	 * Returns IDs for those commits which should be considered as shallow.
 	 *
 	 * @return IDs of shallow commits
 	 * @throws IOException
 	 */
 	public abstract Set<ObjectId> getShallowCommits() throws IOException;
 
 	/**
 	 * Asynchronous object opening.
 	 *
 	 * @param <T>
 	 *            type of identifier being supplied.
 	 * @param objectIds
 	 *            objects to open from the object store. The supplied collection
 	 *            must not be modified until the queue has finished.
 	 * @param reportMissing
 	 *            if true missing objects are reported by calling failure with a
 	 *            MissingObjectException. This may be more expensive for the
 	 *            implementation to guarantee. If false the implementation may
 	 *            choose to report MissingObjectException, or silently skip over
 	 *            the object with no warning.
 	 * @return queue to read the objects from.
 	 */
 	public <T extends ObjectId> AsyncObjectLoaderQueue<T> open(
 			Iterable<T> objectIds, final boolean reportMissing) {
 		final Iterator<T> idItr = objectIds.iterator();
 		return new AsyncObjectLoaderQueue<T>() {
 			private T cur;
 
 			public boolean next() throws MissingObjectException, IOException {
 				if (idItr.hasNext()) {
 					cur = idItr.next();
 					return true;
 				} else {
 					return false;
 				}
 			}
 
 			public T getCurrent() {
 				return cur;
 			}
 
 			public ObjectId getObjectId() {
 				return cur;
 			}
 
 			public ObjectLoader open() throws IOException {
 				return ObjectReader.this.open(cur, OBJ_ANY);
 			}
 
 			public boolean cancel(boolean mayInterruptIfRunning) {
 				return true;
 			}
 
 			public void release() {
 				// Since we are sequential by default, we don't
 				// have any state to clean up if we terminate early.
 			}
 		};
 	}
 
 	/**
 	 * Get only the size of an object.
 	 * <p>
 	 * The default implementation of this method opens an ObjectLoader.
 	 * Databases are encouraged to override this if a faster access method is
 	 * available to them.
 	 *
 	 * @param objectId
 	 *            identity of the object to open.
 	 * @param typeHint
 	 *            hint about the type of object being requested, e.g.
 	 *            {@link Constants#OBJ_BLOB}; {@link #OBJ_ANY} if the object
 	 *            type is not known, or does not matter to the caller.
 	 * @return size of object in bytes.
 	 * @throws MissingObjectException
 	 *             the object does not exist.
 	 * @throws IncorrectObjectTypeException
 	 *             typeHint was not OBJ_ANY, and the object's actual type does
 	 *             not match typeHint.
 	 * @throws IOException
 	 *             the object store cannot be accessed.
 	 */
 	public long getObjectSize(AnyObjectId objectId, int typeHint)
 			throws MissingObjectException, IncorrectObjectTypeException,
 			IOException {
 		return open(objectId, typeHint).getSize();
 	}
 
 	/**
 	 * Asynchronous object size lookup.
 	 *
 	 * @param <T>
 	 *            type of identifier being supplied.
 	 * @param objectIds
 	 *            objects to get the size of from the object store. The supplied
 	 *            collection must not be modified until the queue has finished.
 	 * @param reportMissing
 	 *            if true missing objects are reported by calling failure with a
 	 *            MissingObjectException. This may be more expensive for the
 	 *            implementation to guarantee. If false the implementation may
 	 *            choose to report MissingObjectException, or silently skip over
 	 *            the object with no warning.
 	 * @return queue to read object sizes from.
 	 */
 	public <T extends ObjectId> AsyncObjectSizeQueue<T> getObjectSize(
 			Iterable<T> objectIds, final boolean reportMissing) {
 		final Iterator<T> idItr = objectIds.iterator();
 		return new AsyncObjectSizeQueue<T>() {
 			private T cur;
 
 			private long sz;
 
 			public boolean next() throws MissingObjectException, IOException {
 				if (idItr.hasNext()) {
 					cur = idItr.next();
 					sz = getObjectSize(cur, OBJ_ANY);
 					return true;
 				} else {
 					return false;
 				}
 			}
 
 			public T getCurrent() {
 				return cur;
 			}
 
 			public ObjectId getObjectId() {
 				return cur;
 			}
 
 			public long getSize() {
 				return sz;
 			}
 
 			public boolean cancel(boolean mayInterruptIfRunning) {
 				return true;
 			}
 
 			public void release() {
 				// Since we are sequential by default, we don't
 				// have any state to clean up if we terminate early.
 			}
 		};
 	}
 
 	/**
 	 * Advise the reader to avoid unreachable objects.
 	 * <p>
 	 * While enabled the reader will skip over anything previously proven to be
 	 * unreachable. This may be dangerous in the face of concurrent writes.
 	 *
 	 * @param avoid
 	 *            true to avoid unreachable objects.
 	 * @since 3.0
 	 */
 	public void setAvoidUnreachableObjects(boolean avoid) {
 		// Do nothing by default.
 	}
 
 	/**
 	 * An index that can be used to speed up ObjectWalks.
 	 *
 	 * @return the index or null if one does not exist.
 	 * @throws IOException
 	 *             when the index fails to load
 	 * @since 3.0
 	 */
 	public BitmapIndex getBitmapIndex() throws IOException {
 		return null;
 	}
 
 	/**
 	 * @return the {@link ObjectInserter} from which this reader was created
 	 *         using {@code inserter.newReader()}, or null if this reader was not
 	 *         created from an inserter.
 	 * @since 4.4
 	 */
 	@Nullable
 	public ObjectInserter getCreatedFromInserter() {
 		return null;
 	}
 
 	/**
 	 * Release any resources used by this reader.
 	 * <p>
 	 * A reader that has been released can be used again, but may need to be
 	 * released after the subsequent usage.
 	 *
 	 * @since 4.0
 	 */
 	@Override
 	public abstract void close();
 
 	/**
 	 * Wraps a delegate ObjectReader.
 	 *
 	 * @since 4.4
 	 */
 	public static abstract class Filter extends ObjectReader {
 		/**
 		 * @return delegate ObjectReader to handle all processing.
 		 * @since 4.4
 		 */
 		protected abstract ObjectReader delegate();
 
 		@Override
 		public ObjectReader newReader() {
 			return delegate().newReader();
 		}
 
 		@Override
 		public AbbreviatedObjectId abbreviate(AnyObjectId objectId)
 				throws IOException {
 			return delegate().abbreviate(objectId);
 		}
 
 		@Override
 		public AbbreviatedObjectId abbreviate(AnyObjectId objectId, int len)
 				throws IOException {
 			return delegate().abbreviate(objectId, len);
 		}
 
 		@Override
 		public Collection<ObjectId> resolve(AbbreviatedObjectId id)
 				throws IOException {
 			return delegate().resolve(id);
 		}
 
 		@Override
 		public boolean has(AnyObjectId objectId) throws IOException {
 			return delegate().has(objectId);
 		}
 
 		@Override
 		public boolean has(AnyObjectId objectId, int typeHint) throws IOException {
 			return delegate().has(objectId, typeHint);
 		}
 
 		@Override
 		public ObjectLoader open(AnyObjectId objectId)
 				throws MissingObjectException, IOException {
 			return delegate().open(objectId);
 		}
 
 		@Override
 		public ObjectLoader open(AnyObjectId objectId, int typeHint)
 				throws MissingObjectException, IncorrectObjectTypeException,
 				IOException {
 			return delegate().open(objectId, typeHint);
 		}
 
 		@Override
 		public Set<ObjectId> getShallowCommits() throws IOException {
 			return delegate().getShallowCommits();
 		}
 
 		@Override
 		public <T extends ObjectId> AsyncObjectLoaderQueue<T> open(
 				Iterable<T> objectIds, boolean reportMissing) {
 			return delegate().open(objectIds, reportMissing);
 		}
 
 		@Override
 		public long getObjectSize(AnyObjectId objectId, int typeHint)
 				throws MissingObjectException, IncorrectObjectTypeException,
 				IOException {
 			return delegate().getObjectSize(objectId, typeHint);
 		}
 
 		@Override
 		public <T extends ObjectId> AsyncObjectSizeQueue<T> getObjectSize(
 				Iterable<T> objectIds, boolean reportMissing) {
 			return delegate().getObjectSize(objectIds, reportMissing);
 		}
 
 		@Override
 		public void setAvoidUnreachableObjects(boolean avoid) {
 			delegate().setAvoidUnreachableObjects(avoid);
 		}
 
 		@Override
 		public BitmapIndex getBitmapIndex() throws IOException {
 			return delegate().getBitmapIndex();
 		}
 
 		@Override
 		@Nullable
 		public ObjectInserter getCreatedFromInserter() {
 			return delegate().getCreatedFromInserter();
 		}
 
 		@Override
 		public void close() {
 			delegate().close();
 		}
 	}
 }