/*
    * Copyright (C) 2012, 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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   * without modification, are permitted provided that the following
   * conditions are met:
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   *
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  package org.eclipse.jgit.internal.storage.pack;
  
  import static org.eclipse.jgit.internal.storage.file.PackBitmapIndex.FLAG_REUSE;
  
  import java.io.IOException;
  import java.util.ArrayList;
  import java.util.Collection;
  import java.util.Collections;
  import java.util.Comparator;
  import java.util.HashSet;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Set;
  
  import com.googlecode.javaewah.EWAHCompressedBitmap;
  
  import org.eclipse.jgit.errors.IncorrectObjectTypeException;
  import org.eclipse.jgit.errors.MissingObjectException;
  import org.eclipse.jgit.internal.JGitText;
  import org.eclipse.jgit.internal.storage.file.BitmapIndexImpl;
  import org.eclipse.jgit.internal.storage.file.PackBitmapIndex;
  import org.eclipse.jgit.internal.storage.file.PackBitmapIndexBuilder;
  import org.eclipse.jgit.internal.storage.file.PackBitmapIndexRemapper;
  import org.eclipse.jgit.lib.AnyObjectId;
  import org.eclipse.jgit.lib.Constants;
  import org.eclipse.jgit.lib.ObjectId;
  import org.eclipse.jgit.lib.ObjectReader;
  import org.eclipse.jgit.lib.ProgressMonitor;
  import org.eclipse.jgit.lib.BitmapIndex.BitmapBuilder;
  import org.eclipse.jgit.revwalk.ObjectWalk;
  import org.eclipse.jgit.revwalk.RevCommit;
  import org.eclipse.jgit.revwalk.RevObject;
  import org.eclipse.jgit.revwalk.RevWalk;
  import org.eclipse.jgit.util.BlockList;
  
  /** Helper class for the PackWriter to select commits for pack index bitmaps. */
  class PackWriterBitmapPreparer {
  
  	private static final Comparator<BitmapBuilder> BUILDER_BY_CARDINALITY_DSC =
  			new Comparator<BitmapBuilder>() {
  		public int compare(BitmapBuilder a, BitmapBuilder b) {
  			return Integer.signum(b.cardinality() - a.cardinality());
  		}
  	};
  
  	private final ObjectReader reader;
  	private final ProgressMonitor pm;
  	private final Set<? extends ObjectId> want;
  	private final PackBitmapIndexBuilder writeBitmaps;
  	private final BitmapIndexImpl commitBitmapIndex;
  	private final PackBitmapIndexRemapper bitmapRemapper;
  	private final BitmapIndexImpl bitmapIndex;
  	private final int minCommits = 100;
  	private final int maxCommits = 5000;
  
  	PackWriterBitmapPreparer(ObjectReader reader,
 			PackBitmapIndexBuilder writeBitmaps, ProgressMonitor pm,
 			Set<? extends ObjectId> want) throws IOException {
 		this.reader = reader;
 		this.writeBitmaps = writeBitmaps;
 		this.pm = pm;
 		this.want = want;
 		this.commitBitmapIndex = new BitmapIndexImpl(writeBitmaps);
 		this.bitmapRemapper = PackBitmapIndexRemapper.newPackBitmapIndex(
 				reader.getBitmapIndex(), writeBitmaps);
 		this.bitmapIndex = new BitmapIndexImpl(bitmapRemapper);
 	}
 
 	Collection<BitmapCommit> doCommitSelection(int expectedNumCommits)
 			throws MissingObjectException, IncorrectObjectTypeException,
 			IOException {
 		pm.beginTask(JGitText.get().selectingCommits, ProgressMonitor.UNKNOWN);
 		RevWalk rw = new RevWalk(reader);
 		WalkResult result = findPaths(rw, expectedNumCommits);
 		pm.endTask();
 
 		int totCommits = result.commitsByOldest.length - result.commitStartPos;
 		BlockList<BitmapCommit> selections = new BlockList<BitmapCommit>(
 				totCommits / minCommits + 1);
 		for (BitmapCommit reuse : result.reuse)
 			selections.add(reuse);
 
 		if (totCommits == 0) {
 			for (AnyObjectId id : result.peeledWant)
 				selections.add(new BitmapCommit(id, false, 0));
 			return selections;
 		}
 
 		pm.beginTask(JGitText.get().selectingCommits, totCommits);
 
 		for (BitmapBuilder bitmapableCommits : result.paths) {
 			int cardinality = bitmapableCommits.cardinality();
 
 			List<List<BitmapCommit>> running = new ArrayList<
 					List<BitmapCommit>>();
 
 			// Insert bitmaps at the offsets suggested by the
 			// nextSelectionDistance() heuristic.
 			int index = -1;
 			int nextIn = nextSelectionDistance(0, cardinality);
 			int nextFlg = nextIn == maxCommits ? PackBitmapIndex.FLAG_REUSE : 0;
 			boolean mustPick = nextIn == 0;
 			for (RevCommit c : result) {
 				if (!bitmapableCommits.contains(c))
 					continue;
 
 				index++;
 				nextIn--;
 				pm.update(1);
 
 				// Always pick the items in want and prefer merge commits.
 				if (result.peeledWant.remove(c)) {
 					if (nextIn > 0)
 						nextFlg = 0;
 				} else if (!mustPick && ((nextIn > 0)
 						|| (c.getParentCount() <= 1 && nextIn > -minCommits))) {
 					continue;
 				}
 
 				int flags = nextFlg;
 				nextIn = nextSelectionDistance(index, cardinality);
 				nextFlg = nextIn == maxCommits ? PackBitmapIndex.FLAG_REUSE : 0;
 				mustPick = nextIn == 0;
 
 				BitmapBuilder fullBitmap = commitBitmapIndex.newBitmapBuilder();
 				rw.reset();
 				rw.markStart(c);
 				for (AnyObjectId objectId : result.reuse)
 					rw.markUninteresting(rw.parseCommit(objectId));
 				rw.setRevFilter(
 						PackWriterBitmapWalker.newRevFilter(null, fullBitmap));
 
 				while (rw.next() != null) {
 					// Work is done in the RevFilter.
 				}
 
 				List<List<BitmapCommit>> matches = new ArrayList<
 						List<BitmapCommit>>();
 				for (List<BitmapCommit> list : running) {
 					BitmapCommit last = list.get(list.size() - 1);
 					if (fullBitmap.contains(last))
 						matches.add(list);
 				}
 
 				List<BitmapCommit> match;
 				if (matches.isEmpty()) {
 					match = new ArrayList<BitmapCommit>();
 					running.add(match);
 				} else {
 					match = matches.get(0);
 					// Append to longest
 					for (List<BitmapCommit> list : matches) {
 						if (list.size() > match.size())
 							match = list;
 					}
 				}
 				match.add(new BitmapCommit(c, !match.isEmpty(), flags));
 				writeBitmaps.addBitmap(c, fullBitmap, 0);
 			}
 
 			for (List<BitmapCommit> list : running)
 				selections.addAll(list);
 		}
 		writeBitmaps.clearBitmaps(); // Remove the temporary commit bitmaps.
 
 		// Add the remaining peeledWant
 		for (AnyObjectId remainingWant : result.peeledWant)
 			selections.add(new BitmapCommit(remainingWant, false, 0));
 
 		pm.endTask();
 		return selections;
 	}
 
 	private WalkResult findPaths(RevWalk rw, int expectedNumCommits)
 			throws MissingObjectException, IOException {
 		BitmapBuilder reuseBitmap = commitBitmapIndex.newBitmapBuilder();
 		List<BitmapCommit> reuse = new ArrayList<BitmapCommit>();
 		for (PackBitmapIndexRemapper.Entry entry : bitmapRemapper) {
 			if ((entry.getFlags() & FLAG_REUSE) != FLAG_REUSE)
 				continue;
 
 			RevObject ro = rw.peel(rw.parseAny(entry));
 			if (ro instanceof RevCommit) {
 				RevCommit rc = (RevCommit) ro;
 				reuse.add(new BitmapCommit(rc, false, entry.getFlags()));
 				rw.markUninteresting(rc);
 
 				EWAHCompressedBitmap bitmap = bitmapRemapper.ofObjectType(
 						bitmapRemapper.getBitmap(rc), Constants.OBJ_COMMIT);
 				writeBitmaps.addBitmap(rc, bitmap, 0);
 				reuseBitmap.add(rc, Constants.OBJ_COMMIT);
 			}
 		}
 		writeBitmaps.clearBitmaps(); // Remove temporary bitmaps
 
 		// Do a RevWalk by commit time descending. Keep track of all the paths
 		// from the wants.
 		List<BitmapBuilder> paths = new ArrayList<BitmapBuilder>(want.size());
 		Set<RevCommit> peeledWant = new HashSet<RevCommit>(want.size());
 		for (AnyObjectId objectId : want) {
 			RevObject ro = rw.peel(rw.parseAny(objectId));
 			if (ro instanceof RevCommit && !reuseBitmap.contains(ro)) {
 				RevCommit rc = (RevCommit) ro;
 				peeledWant.add(rc);
 				rw.markStart(rc);
 
 				BitmapBuilder bitmap = commitBitmapIndex.newBitmapBuilder();
 				bitmap.or(reuseBitmap);
 				bitmap.add(rc, Constants.OBJ_COMMIT);
 				paths.add(bitmap);
 			}
 		}
 
 		// Update the paths from the wants and create a list of commits in
 		// reverse iteration order.
 		RevCommit[] commits = new RevCommit[expectedNumCommits];
 		int pos = commits.length;
 		RevCommit rc;
 		while ((rc = rw.next()) != null) {
 			commits[--pos] = rc;
 			for (BitmapBuilder path : paths) {
 				if (path.contains(rc)) {
 					for (RevCommit c : rc.getParents())
 						path.add(c, Constants.OBJ_COMMIT);
 				}
 			}
 
 			pm.update(1);
 		}
 
 		// Remove the reused bitmaps from the paths
 		if (!reuse.isEmpty())
 			for (BitmapBuilder bitmap : paths)
 				bitmap.andNot(reuseBitmap);
 
 		// Sort the paths
 		List<BitmapBuilder> distinctPaths = new ArrayList<BitmapBuilder>(paths.size());
 		while (!paths.isEmpty()) {
 			Collections.sort(paths, BUILDER_BY_CARDINALITY_DSC);
 			BitmapBuilder largest = paths.remove(0);
 			distinctPaths.add(largest);
 
 			// Update the remaining paths, by removing the objects from
 			// the path that was just added.
 			for (int i = paths.size() - 1; i >= 0; i--)
 				paths.get(i).andNot(largest);
 		}
 
 		return new WalkResult(peeledWant, commits, pos, distinctPaths, reuse);
 	}
 
 	private int nextSelectionDistance(int idx, int cardinality) {
 		if (idx > cardinality)
 			throw new IllegalArgumentException();
 		int idxFromStart = cardinality - idx;
 		int mustRegionEnd = 100;
 		if (idxFromStart <= mustRegionEnd)
 			return 0;
 
 		// Commits more toward the start will have more bitmaps.
 		int minRegionEnd = 20000;
 		if (idxFromStart <= minRegionEnd)
 			return Math.min(idxFromStart - mustRegionEnd, minCommits);
 
 		// Commits more toward the end will have fewer.
 		int next = Math.min(idxFromStart - minRegionEnd, maxCommits);
 		return Math.max(next, minCommits);
 	}
 
 	PackWriterBitmapWalker newBitmapWalker() {
 		return new PackWriterBitmapWalker(
 				new ObjectWalk(reader), bitmapIndex, null);
 	}
 
 	static final class BitmapCommit extends ObjectId {
 		private final boolean reuseWalker;
 		private final int flags;
 
 		private BitmapCommit(
 				AnyObjectId objectId, boolean reuseWalker, int flags) {
 			super(objectId);
 			this.reuseWalker = reuseWalker;
 			this.flags = flags;
 		}
 
 		boolean isReuseWalker() {
 			return reuseWalker;
 		}
 
 		int getFlags() {
 			return flags;
 		}
 	}
 
 	private static final class WalkResult implements Iterable<RevCommit> {
 		private final Set<? extends ObjectId> peeledWant;
 		private final RevCommit[] commitsByOldest;
 		private final int commitStartPos;
 		private final List<BitmapBuilder> paths;
 		private final Iterable<BitmapCommit> reuse;
 
 		private WalkResult(Set<? extends ObjectId> peeledWant,
 				RevCommit[] commitsByOldest, int commitStartPos,
 				List<BitmapBuilder> paths, Iterable<BitmapCommit> reuse) {
 			this.peeledWant = peeledWant;
 			this.commitsByOldest = commitsByOldest;
 			this.commitStartPos = commitStartPos;
 			this.paths = paths;
 			this.reuse = reuse;
 		}
 
 		public Iterator<RevCommit> iterator() {
 			return new Iterator<RevCommit>() {
 				int pos = commitStartPos;
 
 				public boolean hasNext() {
 					return pos < commitsByOldest.length;
 				}
 
 				public RevCommit next() {
 					return commitsByOldest[pos++];
 				}
 
 				public void remove() {
 					throw new UnsupportedOperationException();
 				}
 			};
 		}
 	}
 }