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    * available at http://www.eclipse.org/org/documents/edl-v10.php
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  package org.eclipse.jgit.internal.storage.reftable;
  
  import java.io.IOException;
  import java.util.List;
  import java.util.PriorityQueue;
  
  import org.eclipse.jgit.lib.AnyObjectId;
  import org.eclipse.jgit.lib.Ref;
  import org.eclipse.jgit.lib.ReflogEntry;
  
  /**
   * Merges multiple reference tables together.
   * <p>
   * A {@link org.eclipse.jgit.internal.storage.reftable.MergedReftable}
   * merge-joins multiple
   * {@link org.eclipse.jgit.internal.storage.reftable.ReftableReader} on the fly.
   * Tables higher/later in the stack shadow lower/earlier tables, hiding
   * references that been updated/replaced.
   * <p>
   * By default deleted references are skipped and not returned to the caller.
   * {@link #setIncludeDeletes(boolean)} can be used to modify this behavior if
   * the caller needs to preserve deletions during partial compaction.
   * <p>
   * A {@code MergedReftable} is not thread-safe.
   */
  public class MergedReftable extends Reftable {
  	private final Reftable[] tables;
  
  	/**
  	 * Initialize a merged table reader.
  	 * <p>
  	 * The tables in {@code tableStack} will be closed when this
  	 * {@code MergedReftable} is closed.
  	 *
  	 * @param tableStack
  	 *            stack of tables to read from. The base of the stack is at
  	 *            index 0, the most recent should be at the top of the stack at
  	 *            {@code tableStack.size() - 1}. The top of the stack (higher
  	 *            index) shadows the base of the stack (lower index).
  	 */
  	public MergedReftable(List<Reftable> tableStack) {
  		tables = tableStack.toArray(new Reftable[0]);
  
  		// Tables must expose deletes to this instance to correctly
  		// shadow references from lower tables.
  		for (Reftable t : tables) {
  			t.setIncludeDeletes(true);
  		}
  	}
  
  	/** {@inheritDoc} */
  	@Override
  	public RefCursor allRefs() throws IOException {
  		MergedRefCursor m = new MergedRefCursor();
  		for (int i = 0; i < tables.length; i++) {
  			m.add(new RefQueueEntry(tables[i].allRefs(), i));
 		}
 		return m;
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public RefCursor seekRef(String name) throws IOException {
 		MergedRefCursor m = new MergedRefCursor();
 		for (int i = 0; i < tables.length; i++) {
 			m.add(new RefQueueEntry(tables[i].seekRef(name), i));
 		}
 		return m;
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public RefCursor seekRefsWithPrefix(String prefix) throws IOException {
 		MergedRefCursor m = new MergedRefCursor();
 		for (int i = 0; i < tables.length; i++) {
 			m.add(new RefQueueEntry(tables[i].seekRefsWithPrefix(prefix), i));
 		}
 		return m;
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public RefCursor byObjectId(AnyObjectId name) throws IOException {
 		MergedRefCursor m = new MergedRefCursor();
 		for (int i = 0; i < tables.length; i++) {
 			m.add(new RefQueueEntry(tables[i].byObjectId(name), i));
 		}
 		return m;
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public LogCursor allLogs() throws IOException {
 		MergedLogCursor m = new MergedLogCursor();
 		for (int i = 0; i < tables.length; i++) {
 			m.add(new LogQueueEntry(tables[i].allLogs(), i));
 		}
 		return m;
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public LogCursor seekLog(String refName, long updateIdx)
 			throws IOException {
 		MergedLogCursor m = new MergedLogCursor();
 		for (int i = 0; i < tables.length; i++) {
 			m.add(new LogQueueEntry(tables[i].seekLog(refName, updateIdx), i));
 		}
 		return m;
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public void close() throws IOException {
 		for (Reftable t : tables) {
 			t.close();
 		}
 	}
 
 	int queueSize() {
 		return Math.max(1, tables.length);
 	}
 
 	private class MergedRefCursor extends RefCursor {
 		private final PriorityQueue<RefQueueEntry> queue;
 		private RefQueueEntry head;
 		private Ref ref;
 
 		MergedRefCursor() {
 			queue = new PriorityQueue<>(queueSize(), RefQueueEntry::compare);
 		}
 
 		void add(RefQueueEntry t) throws IOException {
 			// Common case is many iterations over the same RefQueueEntry
 			// for the bottom of the stack (scanning all refs). Its almost
 			// always less than the top of the queue. Avoid the queue's
 			// O(log N) insertion and removal costs for this common case.
 			if (!t.rc.next()) {
 				t.rc.close();
 			} else if (head == null) {
 				RefQueueEntry p = queue.peek();
 				if (p == null || RefQueueEntry.compare(t, p) < 0) {
 					head = t;
 				} else {
 					head = queue.poll();
 					queue.add(t);
 				}
 			} else if (RefQueueEntry.compare(t, head) > 0) {
 				queue.add(t);
 			} else {
 				queue.add(head);
 				head = t;
 			}
 		}
 
 		@Override
 		public boolean next() throws IOException {
 			for (;;) {
 				RefQueueEntry t = poll();
 				if (t == null) {
 					return false;
 				}
 
 				ref = t.rc.getRef();
 				boolean include = includeDeletes || !t.rc.wasDeleted();
 				add(t);
 				skipShadowedRefs(ref.getName());
 				if (include) {
 					return true;
 				}
 			}
 		}
 
 		private RefQueueEntry poll() {
 			RefQueueEntry e = head;
 			if (e != null) {
 				head = null;
 				return e;
 			}
 			return queue.poll();
 		}
 
 		private void skipShadowedRefs(String name) throws IOException {
 			for (;;) {
 				RefQueueEntry t = head != null ? head : queue.peek();
 				if (t != null && name.equals(t.name())) {
 					add(poll());
 				} else {
 					break;
 				}
 			}
 		}
 
 		@Override
 		public Ref getRef() {
 			return ref;
 		}
 
 		@Override
 		public void close() {
 			if (head != null) {
 				head.rc.close();
 				head = null;
 			}
 			while (!queue.isEmpty()) {
 				queue.remove().rc.close();
 			}
 		}
 	}
 
 	private static class RefQueueEntry {
 		static int compare(RefQueueEntry a, RefQueueEntry b) {
 			int cmp = a.name().compareTo(b.name());
 			if (cmp == 0) {
 				// higher updateIndex shadows lower updateIndex.
 				cmp = Long.signum(b.updateIndex() - a.updateIndex());
 			}
 			if (cmp == 0) {
 				// higher index shadows lower index, so higher index first.
 				cmp = b.stackIdx - a.stackIdx;
 			}
 			return cmp;
 		}
 
 		final RefCursor rc;
 		final int stackIdx;
 
 		RefQueueEntry(RefCursor rc, int stackIdx) {
 			this.rc = rc;
 			this.stackIdx = stackIdx;
 		}
 
 		String name() {
 			return rc.getRef().getName();
 		}
 
 		long updateIndex() {
 			return rc.getRef().getUpdateIndex();
 		}
 	}
 
 	private class MergedLogCursor extends LogCursor {
 		private final PriorityQueue<LogQueueEntry> queue;
 		private String refName;
 		private long updateIndex;
 		private ReflogEntry entry;
 
 		MergedLogCursor() {
 			queue = new PriorityQueue<>(queueSize(), LogQueueEntry::compare);
 		}
 
 		void add(LogQueueEntry t) throws IOException {
 			if (t.lc.next()) {
 				queue.add(t);
 			} else {
 				t.lc.close();
 			}
 		}
 
 		@Override
 		public boolean next() throws IOException {
 			for (;;) {
 				LogQueueEntry t = queue.poll();
 				if (t == null) {
 					return false;
 				}
 
 				refName = t.lc.getRefName();
 				updateIndex = t.lc.getUpdateIndex();
 				entry = t.lc.getReflogEntry();
 				boolean include = includeDeletes || entry != null;
 				skipShadowed(refName, updateIndex);
 				add(t);
 				if (include) {
 					return true;
 				}
 			}
 		}
 
 		private void skipShadowed(String name, long index) throws IOException {
 			for (;;) {
 				LogQueueEntry t = queue.peek();
 				if (t != null && name.equals(t.name()) && index == t.index()) {
 					add(queue.remove());
 				} else {
 					break;
 				}
 			}
 		}
 
 		@Override
 		public String getRefName() {
 			return refName;
 		}
 
 		@Override
 		public long getUpdateIndex() {
 			return updateIndex;
 		}
 
 		@Override
 		public ReflogEntry getReflogEntry() {
 			return entry;
 		}
 
 		@Override
 		public void close() {
 			while (!queue.isEmpty()) {
 				queue.remove().lc.close();
 			}
 		}
 	}
 
 	private static class LogQueueEntry {
 		static int compare(LogQueueEntry a, LogQueueEntry b) {
 			int cmp = a.name().compareTo(b.name());
 			if (cmp == 0) {
 				// higher update index sorts first.
 				cmp = Long.signum(b.index() - a.index());
 			}
 			if (cmp == 0) {
 				// higher index comes first.
 				cmp = b.stackIdx - a.stackIdx;
 			}
 			return cmp;
 		}
 
 		final LogCursor lc;
 		final int stackIdx;
 
 		LogQueueEntry(LogCursor lc, int stackIdx) {
 			this.lc = lc;
 			this.stackIdx = stackIdx;
 		}
 
 		String name() {
 			return lc.getRefName();
 		}
 
 		long index() {
 			return lc.getUpdateIndex();
 		}
 	}
 }