/*
    * Copyright (C) 2019, Google LLC and others
    *
    * This program and the accompanying materials are made available under the
    * terms of the Eclipse Distribution License v. 1.0 which is available at
    * https://www.eclipse.org/org/documents/edl-v10.php.
    *
    * SPDX-License-Identifier: BSD-3-Clause
    */
  package org.eclipse.jgit.internal.revwalk;
  
  import java.io.IOException;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Optional;
  import java.util.stream.Stream;
  
  import org.eclipse.jgit.errors.IncorrectObjectTypeException;
  import org.eclipse.jgit.errors.MissingObjectException;
  import org.eclipse.jgit.lib.BitmapIndex;
  import org.eclipse.jgit.lib.BitmapIndex.Bitmap;
  import org.eclipse.jgit.lib.BitmapIndex.BitmapBuilder;
  import org.eclipse.jgit.lib.Constants;
  import org.eclipse.jgit.revwalk.ReachabilityChecker;
  import org.eclipse.jgit.revwalk.RevCommit;
  import org.eclipse.jgit.revwalk.RevFlag;
  import org.eclipse.jgit.revwalk.RevSort;
  import org.eclipse.jgit.revwalk.RevWalk;
  import org.eclipse.jgit.revwalk.filter.RevFilter;
  
  /**
   * Checks the reachability using bitmaps.
   */
  public class BitmappedReachabilityChecker implements ReachabilityChecker {
  
  	private final RevWalk walk;
  
  	/**
  	 * @param walk
  	 *            walk on the repository to get or create the bitmaps for the
  	 *            commits. It must have bitmaps.
  	 * @throws AssertionError
  	 *             runtime exception if walk is over a repository without
  	 *             bitmaps
  	 * @throws IOException
  	 *             if the index or the object reader cannot be opened.
  	 */
  	public BitmappedReachabilityChecker(RevWalk walk)
  			throws IOException {
  		this.walk = walk;
  		if (walk.getObjectReader().getBitmapIndex() == null) {
  			throw new AssertionError(
  					"Trying to use bitmapped reachability check " //$NON-NLS-1$
  							+ "on a repository without bitmaps"); //$NON-NLS-1$
  		}
  	}
  
  	/**
  	 * Check all targets are reachable from the starters.
  	 * <p>
  	 * In this implementation, it is recommended to put the most popular
  	 * starters (e.g. refs/heads tips) at the beginning.
  	 */
  	@Override
  	public Optional<RevCommit> areAllReachable(Collection<RevCommit> targets,
  			Stream<RevCommit> starters) throws MissingObjectException,
  			IncorrectObjectTypeException, IOException {
  
  		List<RevCommit> remainingTargets = new ArrayList<>(targets);
  
  		walk.reset();
  		walk.sort(RevSort.TOPO);
  
  		// Filter emits only commits that are unreachable from previously
  		// visited commits. Internally it keeps a bitmap of everything
  		// reachable so far, which we use to discard reachable targets.
  		BitmapIndex repoBitmaps = walk.getObjectReader().getBitmapIndex();
  		ReachedFilter reachedFilter = new ReachedFilter(repoBitmaps);
  		walk.setRevFilter(reachedFilter);
  
  		Iterator<RevCommit> startersIter = starters.iterator();
  		while (startersIter.hasNext()) {
  			walk.markStart(startersIter.next());
  			while (walk.next() != null) {
  				remainingTargets.removeIf(reachedFilter::isReachable);
  
  				if (remainingTargets.isEmpty()) {
  					return Optional.empty();
  				}
  			}
  			walk.reset();
  		}
  
  		return Optional.of(remainingTargets.get(0));
  	}
  
  	/**
 	 * This filter emits commits that were not bitmap-reachable from anything
 	 * visited before. Or in other words, commits that add something (themselves
 	 * or their bitmap) to the "reached" bitmap.
 	 *
 	 * Current progress can be queried via {@link #isReachable(RevCommit)}.
 	 */
 	private static class ReachedFilter extends RevFilter {
 
 		private final BitmapIndex repoBitmaps;
 		private final BitmapBuilder reached;
 
 		/**
 		 * Create a filter that emits only previously unreachable commits.
 		 *
 		 * @param repoBitmaps
 		 *            bitmap index of the repo
 		 */
 		public ReachedFilter(BitmapIndex repoBitmaps) {
 			this.repoBitmaps = repoBitmaps;
 			this.reached = repoBitmaps.newBitmapBuilder();
 		}
 
 		/** {@inheritDoc} */
 		@Override
 		public final boolean include(RevWalk walker, RevCommit cmit) {
 			Bitmap commitBitmap;
 
 			if (reached.contains(cmit)) {
 				// already seen or included
 				dontFollow(cmit);
 				return false;
 			}
 
 			if ((commitBitmap = repoBitmaps.getBitmap(cmit)) != null) {
 				reached.or(commitBitmap);
 				// Emit the commit because there are new contents in the bitmap
 				// but don't follow parents (they are already in the bitmap)
 				dontFollow(cmit);
 				return true;
 			}
 
 			// No bitmaps, keep going
 			reached.addObject(cmit, Constants.OBJ_COMMIT);
 			return true;
 		}
 
 		private static final void dontFollow(RevCommit cmit) {
 			for (RevCommit p : cmit.getParents()) {
 				p.add(RevFlag.SEEN);
 			}
 		}
 
 		/** {@inheritDoc} */
 		@Override
 		public final RevFilter clone() {
 			throw new UnsupportedOperationException();
 		}
 
 		/** {@inheritDoc} */
 		@Override
 		public final boolean requiresCommitBody() {
 			return false;
 		}
 
 		boolean isReachable(RevCommit commit) {
 			return reached.contains(commit);
 		}
 	}
 }