http://www.eclipse.org/jdt: Java Development Tools Core (Eclipse Foundation) 
Copyright (c) 2015, 2016 Google, Inc and others.
This program and the accompanying materials
are made available under the terms of the Eclipse Public License 2.0
which accompanies this distribution, and is available at
https://www.eclipse.org/legal/epl-2.0/
SPDX-License-Identifier: EPL-2.0
Contributors:
  Stefan Xenos (Google) - Initial implementation

/*******************************************************************************
 * Copyright (c) 2015, 2016 Google, Inc and others.
 *
 * This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License 2.0
 * which accompanies this distribution, and is available at
 * https://www.eclipse.org/legal/epl-2.0/
 *
 * SPDX-License-Identifier: EPL-2.0
 *
 * Contributors:
 *   Stefan Xenos (Google) - Initial implementation
 *******************************************************************************/
package org.eclipse.jdt.internal.core.nd;

import java.io.File;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.function.Supplier;

import org.eclipse.core.runtime.IProgressMonitor;
import org.eclipse.core.runtime.OperationCanceledException;
import org.eclipse.jdt.internal.core.nd.db.ChunkCache;
import org.eclipse.jdt.internal.core.nd.db.Database;
import org.eclipse.jdt.internal.core.nd.db.IndexException;


Network Database for storing semantic information.

/**
 * Network Database for storing semantic information.
 */
public final class Nd {
	private static final int CANCELLATION_CHECK_INTERVAL = 500;
	private static final int BLOCKED_WRITE_LOCK_OUTPUT_INTERVAL = 30000;
	private static final int LONG_WRITE_LOCK_REPORT_THRESHOLD = 1000;
	private static final int LONG_READ_LOCK_WAIT_REPORT_THRESHOLD = 1000;

	
Controls the number of pages that are allowed to be dirty before a
flush will occur. Specified as a ratio of the total cache size. For
example, a ration of 0.5 would mean that a flush is forced if half
of the cache is dirty.

/**
	 * Controls the number of pages that are allowed to be dirty before a
	 * flush will occur. Specified as a ratio of the total cache size. For
	 * example, a ration of 0.5 would mean that a flush is forced if half
	 * of the cache is dirty.
	 */
	private static final double MAX_DIRTY_CACHE_RATIO = 0.25;
	public static boolean sDEBUG_LOCKS= false;
	public static boolean DEBUG_DUPLICATE_DELETIONS = false;

	private final int currentVersion;
	private final int maxVersion;
	private final int minVersion;

	
Stores data that has been stored via setData. Synchronize on cookies before accessing. 
/**
	 * Stores data that has been stored via {@link #setData}. Synchronize on {@link #cookies} before accessing.
	 */
	private final Map<Class<?>, Object> cookies = new HashMap<>();

	public static int version(int major, int minor) {
		return (major << 16) + minor;
	}

	
Returns the version that shall be used when creating new databases.

/**
	 * Returns the version that shall be used when creating new databases.
	 */
	public int getDefaultVersion() {
		return this.currentVersion;
	}

	public boolean isSupportedVersion(int vers) {
		return vers >= this.minVersion && vers <= this.maxVersion;
	}

	public int getMinSupportedVersion() {
		return this.minVersion;
	}

	public int getMaxSupportedVersion() {
		return this.maxVersion;
	}

	public static String versionString(int version) {
		final int major= version >> 16;
		final int minor= version & 0xffff;
		return "" + major + '.' + minor; //$NON-NLS-1$
	}

	// Local caches
	protected Database db;
	private File fPath;
	private final HashMap<Object, Object> fResultCache = new HashMap<>();

	private final NdNodeTypeRegistry<NdNode> fNodeTypeRegistry;
	private HashMap<Long, Object> pendingDeletions = new HashMap<>();

	private IReader fReader = new IReader() {
		@Override
		public void close() {
			releaseReadLock();
		}
	};

	
This long is incremented every time a change is written to the database. Can be used to determine if the database
has changed.

/**
	 * This long is incremented every time a change is written to the database. Can be used to determine if the database
	 * has changed.
	 */
	private long fWriteNumber;

	public Nd(File dbPath, NdNodeTypeRegistry<NdNode> nodeTypes, int minVersion, int maxVersion,
			int currentVersion) throws IndexException {
		this(dbPath, ChunkCache.getSharedInstance(), nodeTypes, minVersion, maxVersion, currentVersion);
	}

	public Nd(File dbPath, ChunkCache chunkCache, NdNodeTypeRegistry<NdNode> nodeTypes, int minVersion,
			int maxVersion, int currentVersion) throws IndexException {
		this.currentVersion = currentVersion;
		this.maxVersion = maxVersion;
		this.minVersion = minVersion;
		this.fNodeTypeRegistry = nodeTypes;
		loadDatabase(dbPath, chunkCache);
		if (sDEBUG_LOCKS) {
			this.fLockDebugging = new HashMap<>();
			System.out.println("Debugging database Locks"); //$NON-NLS-1$
		}
	}

	public File getPath() {
		return this.fPath;
	}

	public long getWriteNumber() {
		return this.fWriteNumber;
	}

	public void scheduleDeletion(long addressOfNodeToDelete) {
		if (this.pendingDeletions.containsKey(addressOfNodeToDelete)) {
			logDoubleDeletion(addressOfNodeToDelete);
			return;
		}

		Object data = Boolean.TRUE;
		if (DEBUG_DUPLICATE_DELETIONS) {
			data = new RuntimeException();
		}
		this.pendingDeletions.put(addressOfNodeToDelete, data);
	}

	protected void logDoubleDeletion(long addressOfNodeToDelete) {
		// Sometimes an object can be scheduled for deletion twice, if it is created and then discarded shortly
		// afterward during indexing. This may indicate an inefficiency in the indexer but is not necessarily
		// a bug.
		// If you're debugging issues related to duplicate deletions, set DEBUG_DUPLICATE_DELETIONS to true
		Package.log("Database object queued for deletion twice", new RuntimeException()); //$NON-NLS-1$
		Object earlierData = this.pendingDeletions.get(addressOfNodeToDelete);
		if (earlierData instanceof RuntimeException) {
			RuntimeException exception = (RuntimeException) earlierData;

			Package.log("Data associated with earlier deletion stack was:", exception); //$NON-NLS-1$
		}
	}

	
Synchronously processes all pending deletions

/**
	 * Synchronously processes all pending deletions
	 */
	public void processDeletions() {
		while (!this.pendingDeletions.isEmpty()) {
			long next = this.pendingDeletions.keySet().iterator().next();

			deleteIfUnreferenced(next);

			this.pendingDeletions.remove(next);
		}
	}

	
Inserts a cookie that can be later retrieved via getData(String). 

/**
	 * Inserts a cookie that can be later retrieved via getData(String). 
	 */
	public <T> void setData(Class<T> key, T value) {
		synchronized (this.cookies) {
			this.cookies.put(key, value);
		}
	}

	
Returns a cookie that was previously attached using setData(Class<Object>, Object). If no such cookie exists, it is computed using the given function and remembered for later. The function may return null. If it does, this method will also return null and no cookie will be stored. 
/**
	 * Returns a cookie that was previously attached using {@link #setData(Class, Object)}. If no such cookie
	 * exists, it is computed using the given function and remembered for later. The function may return null.
	 * If it does, this method will also return null and no cookie will be stored.
	 */
	@SuppressWarnings("unchecked")
	public <T> T getData(Class<T> key, Supplier<T> defaultValue) {
		T result;
		synchronized (this.cookies) {
			result = (T) this.cookies.get(key);
		}

		if (result == null) {
			result = defaultValue.get();

			if (result != null) {
				synchronized (this.cookies) {
					T newResult = (T) this.cookies.get(key);
					if (newResult == null) {
						this.cookies.put(key, result);
					} else {
						result = newResult;
					}
				}
			}
		}

		return result;
	}

	
Returns whether this Nd can never be written to. Writable subclasses should return false. 
/**
	 * Returns whether this {@link Nd} can never be written to. Writable subclasses should return false.
	 */
	protected boolean isPermanentlyReadOnly() {
		return false;
	}

	private void loadDatabase(File dbPath, ChunkCache cache) throws IndexException {
		this.fPath= dbPath;

		clearCaches();
		this.db = new Database(this.fPath, cache, getDefaultVersion(), isPermanentlyReadOnly());
		this.db.setExclusiveLock();
		if (!isSupportedVersion()) {
			Package.logInfo("Index database uses the unsupported version " + this.db.getVersion() //$NON-NLS-1$
				+ ". Deleting and recreating."); //$NON-NLS-1$
			this.db.close();
			this.fPath.delete();
			this.db = new Database(this.fPath, cache, getDefaultVersion(), isPermanentlyReadOnly());
			this.db.setExclusiveLock();
		}
		this.db.giveUpExclusiveLock();
		this.fWriteNumber = this.db.getLong(Database.WRITE_NUMBER_OFFSET);
		this.db.setLocked(this.lockCount != 0);
	}

	public Database getDB() {
		return this.db;
	}

	// Read-write lock rules. Readers don't conflict with other readers,
	// Writers conflict with readers, and everyone conflicts with writers.
	private final Object mutex = new Object();
	private int lockCount;
	private int waitingReaders;
	private long lastWriteAccess= 0;
	//private long lastReadAccess= 0;
	private long timeWriteLockAcquired;
	private Thread writeLockOwner;

	public IReader acquireReadLock() {
		try {
			long t = sDEBUG_LOCKS ? System.nanoTime() : 0;
			synchronized (this.mutex) {
				++this.waitingReaders;
				try {
					while (this.lockCount < 0)
						this.mutex.wait();
				} finally {
					--this.waitingReaders;
				}
				++this.lockCount;
				this.db.setLocked(true);

				if (sDEBUG_LOCKS) {
					t = (System.nanoTime() - t) / 1000000;
					if (t >= LONG_READ_LOCK_WAIT_REPORT_THRESHOLD) {
						System.out.println("Acquired index read lock after " + t + " ms wait."); //$NON-NLS-1$//$NON-NLS-2$
					}
					incReadLock(this.fLockDebugging);
				}
				return this.fReader;
			}
		} catch (InterruptedException e) {
			throw new OperationCanceledException();
		}
	}

	public void releaseReadLock() {
		synchronized (this.mutex) {
			assert this.lockCount > 0: "No lock to release"; //$NON-NLS-1$
			if (sDEBUG_LOCKS) {
				decReadLock(this.fLockDebugging);
			}

			//this.lastReadAccess= System.currentTimeMillis();
			if (this.lockCount > 0)
				--this.lockCount;
			this.mutex.notifyAll();
			this.db.setLocked(this.lockCount != 0);
		}
		// A lock release probably means that some AST is going away. The result cache has to be
		// cleared since it may contain objects belonging to the AST that is going away. A failure
		// to release an AST object would cause a memory leak since the whole AST would remain
		// pinned to memory.
		// TODO(sprigogin): It would be more efficient to replace the global result cache with
		// separate caches for each AST.
		//clearResultCache();
	}

	
Acquire a write lock on this Nd. Blocks until any existing read/write locks are released. 
Throws:
/**
	 * Acquire a write lock on this {@link Nd}. Blocks until any existing read/write locks are released.
	 * @throws OperationCanceledException
	 * @throws IllegalStateException if this {@link Nd} is not writable
	 */
	public void acquireWriteLock(IProgressMonitor monitor) {
		try {
			acquireWriteLock(0, monitor);
		} catch (InterruptedException e) {
			throw new OperationCanceledException();
		}
	}

	
Acquire a write lock on this Nd, giving up the specified number of read locks first. Blocks until any existing read/write locks are released. 
Throws:
/**
	 * Acquire a write lock on this {@link Nd}, giving up the specified number of read locks first. Blocks
	 * until any existing read/write locks are released.
	 * @throws InterruptedException
	 * @throws IllegalStateException if this {@link Nd} is not writable
	 */
	public void acquireWriteLock(int giveupReadLocks, IProgressMonitor monitor) throws InterruptedException {
		assert !isPermanentlyReadOnly();
		synchronized (this.mutex) {
			if (sDEBUG_LOCKS) {
				incWriteLock(giveupReadLocks);
			}

			if (giveupReadLocks > 0) {
				// give up on read locks
				assert this.lockCount >= giveupReadLocks: "Not enough locks to release"; //$NON-NLS-1$
				if (this.lockCount < giveupReadLocks) {
					giveupReadLocks= this.lockCount;
				}
			} else {
				giveupReadLocks= 0;
			}

			// Let the readers go first
			long start= sDEBUG_LOCKS ? System.currentTimeMillis() : 0;
			while (this.lockCount > giveupReadLocks || this.waitingReaders > 0 || (this.lockCount < 0)) {
				this.mutex.wait(CANCELLATION_CHECK_INTERVAL);
				if (monitor != null && monitor.isCanceled()) {
					throw new OperationCanceledException();
				}
				if (sDEBUG_LOCKS) {
					start = reportBlockedWriteLock(start, giveupReadLocks);
				}
			}
			this.lockCount= -1;
			if (sDEBUG_LOCKS)
				this.timeWriteLockAcquired = System.currentTimeMillis();
			this.db.setExclusiveLock();
			if (this.writeLockOwner != null && this.writeLockOwner != Thread.currentThread()) {
				throw new IllegalStateException("We somehow managed to acquire a write lock while another thread already holds it."); //$NON-NLS-1$
			}
			this.writeLockOwner = Thread.currentThread();
		}
	}

	public final void releaseWriteLock() {
		releaseWriteLock(0, false);
	}

	@SuppressWarnings("nls")
	public void releaseWriteLock(int establishReadLocks, boolean flush) {
		synchronized (this.mutex) {
			Thread current = Thread.currentThread();
			if (current != this.writeLockOwner) {
				throw new IllegalStateException("Index wasn't locked by this thread!!!");
			}
			this.writeLockOwner = null;
		}
		RuntimeException exception = null;
		boolean wasInterrupted = false;
		try {
			// When all locks are released we can clear the result cache.
			if (establishReadLocks == 0) {
				clearResultCache();
			}
			this.db.putLong(Database.WRITE_NUMBER_OFFSET, ++this.fWriteNumber);
			// Process any outstanding deletions now
			processDeletions();
		} catch (RuntimeException e) {
			exception = e;
		} finally {
			this.db.giveUpExclusiveLock();
			assert this.lockCount == -1;
			this.lastWriteAccess = System.currentTimeMillis();
			try {
				releaseWriteLockAndFlush(establishReadLocks, flush);
			} catch (RuntimeException e) {
				if (exception != null) {
					e.addSuppressed(exception);
				}
				throw e;
			}
		}

		if (wasInterrupted) {
			throw new OperationCanceledException();
		}
	}

	private void releaseWriteLockAndFlush(int establishReadLocks, boolean flush) throws AssertionError {
		int dirtyPages = this.getDB().getDirtyChunkCount();

		// If there are too many dirty pages, force a flush now.
		int totalCacheSize = (int) (this.db.getCache().getMaxSize() / Database.CHUNK_SIZE);
		if (dirtyPages > totalCacheSize * MAX_DIRTY_CACHE_RATIO) {
			flush = true;
		}

		int initialReadLocks = flush ? establishReadLocks + 1 : establishReadLocks;
		// Convert this write lock to a read lock while we flush the page cache to disk. That will prevent
		// other writers from dirtying more pages during the flush but will allow reads to proceed.
		synchronized (this.mutex) {
			if (sDEBUG_LOCKS) {
				long timeHeld = this.lastWriteAccess - this.timeWriteLockAcquired;
				if (timeHeld >= LONG_WRITE_LOCK_REPORT_THRESHOLD) {
					System.out.println("Index write lock held for " + timeHeld + " ms");  //$NON-NLS-1$//$NON-NLS-2$
				}
				decWriteLock(initialReadLocks);
			}

			if (this.lockCount < 0) {
				this.lockCount = initialReadLocks;
			}
			this.mutex.notifyAll();
			this.db.setLocked(initialReadLocks != 0);
		}

		if (flush) {
			try {
				this.db.flush();
			} finally {
				releaseReadLock();
			}
		}
	}

	public boolean hasWaitingReaders() {
		synchronized (this.mutex) {
			return this.waitingReaders > 0;
		}
	}

	public long getLastWriteAccess() {
		return this.lastWriteAccess;
	}

	public boolean isSupportedVersion() throws IndexException {
		final int version = this.db.getVersion();
		return version >= this.minVersion && version <= this.maxVersion;
	}

	public void close() throws IndexException {
		this.db.close();
		clearCaches();
	}

	private void clearCaches() {
//		fileIndex= null;
//		tagIndex = null;
//		indexOfDefectiveFiles= null;
//		indexOfFiledWithUnresolvedIncludes= null;
//		fLinkageIDCache.clear();
		clearResultCache();
	}

	public void clearResultCache() {
		synchronized (this.fResultCache) {
			this.fResultCache.clear();
		}
	}

	public Object getCachedResult(Object key) {
		synchronized (this.fResultCache) {
			return this.fResultCache.get(key);
		}
	}

	public void putCachedResult(Object key, Object result) {
		putCachedResult(key, result, true);
	}

	public Object putCachedResult(Object key, Object result, boolean replace) {
		synchronized (this.fResultCache) {
			Object old= this.fResultCache.put(key, result);
			if (old != null && !replace) {
				this.fResultCache.put(key, old);
				return old;
			}
			return result;
		}
	}

	public void removeCachedResult(Object key) {
		synchronized (this.fResultCache) {
			this.fResultCache.remove(key);
		}
	}

	// For debugging lock issues
	static class DebugLockInfo {
		int fReadLocks;
		int fWriteLocks;
		List<StackTraceElement[]> fTraces= new ArrayList<>();

		public int addTrace() {
			this.fTraces.add(Thread.currentThread().getStackTrace());
			return this.fTraces.size();
		}

		@SuppressWarnings("nls")
		public void write(String threadName) {
			System.out.println("Thread: '" + threadName + "': " + this.fReadLocks + " readlocks, " + this.fWriteLocks + " writelocks");
			for (StackTraceElement[] trace : this.fTraces) {
				System.out.println("  Stacktrace:");
				for (StackTraceElement ste : trace) {
					System.out.println("    " + ste);
				}
			}
		}

		public void inc(DebugLockInfo val) {
			this.fReadLocks+= val.fReadLocks;
			this.fWriteLocks+= val.fWriteLocks;
			this.fTraces.addAll(val.fTraces);
		}
	}

	// For debugging lock issues
	private Map<Thread, DebugLockInfo> fLockDebugging;

	// For debugging lock issues
	private static DebugLockInfo getLockInfo(Map<Thread, DebugLockInfo> lockDebugging) {
		assert sDEBUG_LOCKS;

		Thread key = Thread.currentThread();
		DebugLockInfo result= lockDebugging.get(key);
		if (result == null) {
			result= new DebugLockInfo();
			lockDebugging.put(key, result);
		}
		return result;
	}

	// For debugging lock issues
	static void incReadLock(Map<Thread, DebugLockInfo> lockDebugging) {
		DebugLockInfo info = getLockInfo(lockDebugging);
		info.fReadLocks++;
		if (info.addTrace() > 10) {
			outputReadLocks(lockDebugging);
		}
	}

	// For debugging lock issues
	@SuppressWarnings("nls")
	static void decReadLock(Map<Thread, DebugLockInfo> lockDebugging) throws AssertionError {
		DebugLockInfo info = getLockInfo(lockDebugging);
		if (info.fReadLocks <= 0) {
			outputReadLocks(lockDebugging);
			throw new AssertionError("Superfluous releaseReadLock");
		}
		if (info.fWriteLocks != 0) {
			outputReadLocks(lockDebugging);
			throw new AssertionError("Releasing readlock while holding write lock");
		}
		if (--info.fReadLocks == 0) {
			lockDebugging.remove(Thread.currentThread());
		} else {
			info.addTrace();
		}
	}

	// For debugging lock issues
	@SuppressWarnings("nls")
	private void incWriteLock(int giveupReadLocks) throws AssertionError {
		DebugLockInfo info = getLockInfo(this.fLockDebugging);
		if (info.fReadLocks != giveupReadLocks) {
			outputReadLocks(this.fLockDebugging);
			throw new AssertionError("write lock with " + giveupReadLocks + " readlocks, expected " + info.fReadLocks);
		}
		if (info.fWriteLocks != 0)
			throw new AssertionError("Duplicate write lock");
		info.fWriteLocks++;
	}

	// For debugging lock issues
	private void decWriteLock(int establishReadLocks) throws AssertionError {
		DebugLockInfo info = getLockInfo(this.fLockDebugging);
		if (info.fReadLocks != establishReadLocks)
			throw new AssertionError("release write lock with " + establishReadLocks + " readlocks, expected " + info.fReadLocks); //$NON-NLS-1$ //$NON-NLS-2$
		if (info.fWriteLocks != 1)
			throw new AssertionError("Wrong release write lock"); //$NON-NLS-1$
		info.fWriteLocks= 0;
		if (info.fReadLocks == 0) {
			this.fLockDebugging.remove(Thread.currentThread());
		}
	}

	// For debugging lock issues
	@SuppressWarnings("nls")
	private long reportBlockedWriteLock(long start, int giveupReadLocks) {
		long now= System.currentTimeMillis();
		if (now >= start + BLOCKED_WRITE_LOCK_OUTPUT_INTERVAL) {
			System.out.println();
			System.out.println("Blocked writeLock");
			System.out.println("  lockcount= " + this.lockCount + ", giveupReadLocks=" + giveupReadLocks + ", waitingReaders=" + this.waitingReaders);
			outputReadLocks(this.fLockDebugging);
			start= now;
		}
		return start;
	}

	// For debugging lock issues
	@SuppressWarnings("nls")
	private static void outputReadLocks(Map<Thread, DebugLockInfo> lockDebugging) {
		System.out.println("---------------------  Lock Debugging -------------------------");
		for (Thread th: lockDebugging.keySet()) {
			DebugLockInfo info = lockDebugging.get(th);
			info.write(th.getName());
		}
		System.out.println("---------------------------------------------------------------");
	}

	// For debugging lock issues
	public void adjustThreadForReadLock(Map<Thread, DebugLockInfo> lockDebugging) {
		for (Thread th : lockDebugging.keySet()) {
			DebugLockInfo val= lockDebugging.get(th);
			if (val.fReadLocks > 0) {
				DebugLockInfo myval= this.fLockDebugging.get(th);
				if (myval == null) {
					myval= new DebugLockInfo();
					this.fLockDebugging.put(th, myval);
				}
				myval.inc(val);
				for (int i = 0; i < val.fReadLocks; i++) {
					decReadLock(this.fLockDebugging);
				}
			}
		}
	}

    public NdNode getNode(long address, short nodeType) throws IndexException {
    	return this.fNodeTypeRegistry.createNode(this, address, nodeType);
    }

    public <T extends NdNode> ITypeFactory<T> getTypeFactory(short nodeType) {
    	return this.fNodeTypeRegistry.getTypeFactory(nodeType);
    }

	
Returns the type ID for the given class

/**
	 * Returns the type ID for the given class
	 */
	public short getNodeType(Class<?> toQuery) {
		return this.fNodeTypeRegistry.getTypeForClass(toQuery);
	}

	private void deleteIfUnreferenced(long address) {
		if (address == 0) {
			return;
		}
		short nodeType = NdNode.NODE_TYPE.get(this, address);

		// Look up the type
		ITypeFactory<? extends NdNode> factory1 = getTypeFactory(nodeType);

		if (factory1.isReadyForDeletion(this, address)) {
			// Call its destructor
			factory1.destruct(this, address);

			// Free up its memory
			getDB().free(address, (short)(Database.POOL_FIRST_NODE_TYPE + nodeType));
		}
	}

	public void delete(long address) {
		if (address == 0) {
			return;
		}
		short nodeType = NdNode.NODE_TYPE.get(this, address);

		// Look up the type
		ITypeFactory<? extends NdNode> factory1 = getTypeFactory(nodeType);

		// Call its destructor
		factory1.destruct(this, address);

		// Free up its memory
		getDB().free(address, (short)(Database.POOL_FIRST_NODE_TYPE + nodeType));

		// If this node was in the list of pending deletions, remove it since it's now been deleted
		if (this.pendingDeletions.containsKey(address)) {
			logDoubleDeletion(address);
			this.pendingDeletions.remove(address);
		}
	}

	public NdNodeTypeRegistry<NdNode> getTypeRegistry() {
		return this.fNodeTypeRegistry;
	}

	public void clear(IProgressMonitor monitor) {
		this.pendingDeletions.clear();
		getDB().clear(getDefaultVersion());
	}

	public boolean isValidAddress(long address) {
		return address > 0 && address < (long) getDB().getChunkCount() * Database.CHUNK_SIZE;
	}

	
Creates a IndexExceptionBuilder object that collects information about database corruption after it is detected. 
/**
	 * Creates a {@link IndexExceptionBuilder} object that collects information about database corruption after it is 
	 * detected.
	 */
	public IndexExceptionBuilder describeProblem() {
		return this.db.describeProblem();
	}
}