/*
    * Copyright (C) 2008-2011, Google Inc.
    * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org>
    * 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
   *   written permission.
   *
   * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
   * CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
   * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
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   * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
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  package org.eclipse.jgit.internal.storage.dfs;
  
  import java.io.IOException;
  import java.util.concurrent.atomic.AtomicLong;
  import java.util.concurrent.atomic.AtomicReferenceArray;
  import java.util.concurrent.locks.ReentrantLock;
  
  import org.eclipse.jgit.annotations.Nullable;
  import org.eclipse.jgit.internal.JGitText;
  
  /**
   * Caches slices of a {@link BlockBasedFile} in memory for faster read access.
   * <p>
   * The DfsBlockCache serves as a Java based "buffer cache", loading segments of
   * a BlockBasedFile into the JVM heap prior to use. As JGit often wants to do
   * reads of only tiny slices of a file, the DfsBlockCache tries to smooth out
   * these tiny reads into larger block-sized IO operations.
   * <p>
   * Whenever a cache miss occurs, loading is invoked by exactly one thread for
   * the given <code>(DfsPackKey,position)</code> key tuple. This is ensured by an
   * array of locks, with the tuple hashed to a lock instance.
   * <p>
   * Its too expensive during object access to be accurate with a least recently
   * used (LRU) algorithm. Strictly ordering every read is a lot of overhead that
   * typically doesn't yield a corresponding benefit to the application. This
   * cache implements a clock replacement algorithm, giving each block one chance
   * to have been accessed during a sweep of the cache to save itself from
   * eviction.
   * <p>
   * Entities created by the cache are held under hard references, preventing the
   * Java VM from clearing anything. Blocks are discarded by the replacement
   * algorithm when adding a new block would cause the cache to exceed its
   * configured maximum size.
   * <p>
   * The key tuple is passed through to methods as a pair of parameters rather
   * than as a single Object, thus reducing the transient memory allocations of
   * callers. It is more efficient to avoid the allocation, as we can't be 100%
   * sure that a JIT would be able to stack-allocate a key tuple.
   * <p>
   * The internal hash table does not expand at runtime, instead it is fixed in
   * size at cache creation time. The internal lock table used to gate load
   * invocations is also fixed in size.
   */
  public final class DfsBlockCache {
  	private static volatile DfsBlockCache cache;
  
  	static {
  		reconfigure(new DfsBlockCacheConfig());
  	}
  
  	/**
  	 * Modify the configuration of the window cache.
  	 * <p>
  	 * The new configuration is applied immediately, and the existing cache is
  	 * cleared.
 	 *
 	 * @param cfg
 	 *            the new window cache configuration.
 	 * @throws IllegalArgumentException
 	 *             the cache configuration contains one or more invalid
 	 *             settings, usually too low of a limit.
 	 */
 	public static void reconfigure(DfsBlockCacheConfig cfg) {
 		cache = new DfsBlockCache(cfg);
 	}
 
 	/** @return the currently active DfsBlockCache. */
 	public static DfsBlockCache getInstance() {
 		return cache;
 	}
 
 	/** Number of entries in {@link #table}. */
 	private final int tableSize;
 
 	/** Hash bucket directory; entries are chained below. */
 	private final AtomicReferenceArray<HashEntry> table;
 
 	/** Locks to prevent concurrent loads for same (PackFile,position). */
 	private final ReentrantLock[] loadLocks;
 
 	/** Maximum number of bytes the cache should hold. */
 	private final long maxBytes;
 
 	/** Pack files smaller than this size can be copied through the cache. */
 	private final long maxStreamThroughCache;
 
 	/**
 	 * Suggested block size to read from pack files in.
 	 * <p>
 	 * If a pack file does not have a native block size, this size will be used.
 	 * <p>
 	 * If a pack file has a native size, a whole multiple of the native size
 	 * will be used until it matches this size.
 	 * <p>
 	 * The value for blockSize must be a power of 2.
 	 */
 	private final int blockSize;
 
 	/** As {@link #blockSize} is a power of 2, bits to shift for a / blockSize. */
 	private final int blockSizeShift;
 
 	/** Number of times a block was found in the cache. */
 	private final AtomicLong statHit;
 
 	/** Number of times a block was not found, and had to be loaded. */
 	private final AtomicLong statMiss;
 
 	/** Number of blocks evicted due to cache being full. */
 	private volatile long statEvict;
 
 	/** Protects the clock and its related data. */
 	private final ReentrantLock clockLock;
 
 	/** Current position of the clock. */
 	private Ref clockHand;
 
 	/** Number of bytes currently loaded in the cache. */
 	private volatile long liveBytes;
 
 	@SuppressWarnings("unchecked")
 	private DfsBlockCache(final DfsBlockCacheConfig cfg) {
 		tableSize = tableSize(cfg);
 		if (tableSize < 1)
 			throw new IllegalArgumentException(JGitText.get().tSizeMustBeGreaterOrEqual1);
 
 		table = new AtomicReferenceArray<>(tableSize);
 		loadLocks = new ReentrantLock[cfg.getConcurrencyLevel()];
 		for (int i = 0; i < loadLocks.length; i++)
 			loadLocks[i] = new ReentrantLock(true /* fair */);
 
 		maxBytes = cfg.getBlockLimit();
 		maxStreamThroughCache = (long) (maxBytes * cfg.getStreamRatio());
 		blockSize = cfg.getBlockSize();
 		blockSizeShift = Integer.numberOfTrailingZeros(blockSize);
 
 		clockLock = new ReentrantLock(true /* fair */);
 		String none = ""; //$NON-NLS-1$
 		clockHand = new Ref<>(
 				DfsStreamKey.of(new DfsRepositoryDescription(none), none),
 				-1, 0, null);
 		clockHand.next = clockHand;
 
 		statHit = new AtomicLong();
 		statMiss = new AtomicLong();
 	}
 
 	boolean shouldCopyThroughCache(long length) {
 		return length <= maxStreamThroughCache;
 	}
 
 	/** @return total number of bytes in the cache. */
 	public long getCurrentSize() {
 		return liveBytes;
 	}
 
 	/** @return 0..100, defining how full the cache is. */
 	public long getFillPercentage() {
 		return getCurrentSize() * 100 / maxBytes;
 	}
 
 	/** @return number of requests for items in the cache. */
 	public long getHitCount() {
 		return statHit.get();
 	}
 
 	/** @return number of requests for items not in the cache. */
 	public long getMissCount() {
 		return statMiss.get();
 	}
 
 	/** @return total number of requests (hit + miss). */
 	public long getTotalRequestCount() {
 		return getHitCount() + getMissCount();
 	}
 
 	/** @return 0..100, defining number of cache hits. */
 	public long getHitRatio() {
 		long hits = statHit.get();
 		long miss = statMiss.get();
 		long total = hits + miss;
 		if (total == 0)
 			return 0;
 		return hits * 100 / total;
 	}
 
 	/** @return number of evictions performed due to cache being full. */
 	public long getEvictions() {
 		return statEvict;
 	}
 
 	private int hash(int packHash, long off) {
 		return packHash + (int) (off >>> blockSizeShift);
 	}
 
 	int getBlockSize() {
 		return blockSize;
 	}
 
 	private static int tableSize(final DfsBlockCacheConfig cfg) {
 		final int wsz = cfg.getBlockSize();
 		final long limit = cfg.getBlockLimit();
 		if (wsz <= 0)
 			throw new IllegalArgumentException(JGitText.get().invalidWindowSize);
 		if (limit < wsz)
 			throw new IllegalArgumentException(JGitText.get().windowSizeMustBeLesserThanLimit);
 		return (int) Math.min(5 * (limit / wsz) / 2, Integer.MAX_VALUE);
 	}
 
 	/**
 	 * Lookup a cached object, creating and loading it if it doesn't exist.
 	 *
 	 * @param file
 	 *            the pack that "contains" the cached object.
 	 * @param position
 	 *            offset within <code>pack</code> of the object.
 	 * @param ctx
 	 *            current thread's reader.
 	 * @param fileChannel
 	 *            optional channel to read {@code pack}.
 	 * @return the object reference.
 	 * @throws IOException
 	 *             the reference was not in the cache and could not be loaded.
 	 */
 	DfsBlock getOrLoad(BlockBasedFile file, long position, DfsReader ctx,
 			@Nullable ReadableChannel fileChannel) throws IOException {
 		final long requestedPosition = position;
 		position = file.alignToBlock(position);
 
 		DfsStreamKey key = file.key;
 		int slot = slot(key, position);
 		HashEntry e1 = table.get(slot);
 		DfsBlock v = scan(e1, key, position);
 		if (v != null && v.contains(key, requestedPosition)) {
 			ctx.stats.blockCacheHit++;
 			statHit.incrementAndGet();
 			return v;
 		}
 
 		reserveSpace(blockSize);
 		ReentrantLock regionLock = lockFor(key, position);
 		regionLock.lock();
 		try {
 			HashEntry e2 = table.get(slot);
 			if (e2 != e1) {
 				v = scan(e2, key, position);
 				if (v != null) {
 					ctx.stats.blockCacheHit++;
 					statHit.incrementAndGet();
 					creditSpace(blockSize);
 					return v;
 				}
 			}
 
 			statMiss.incrementAndGet();
 			boolean credit = true;
 			try {
 				v = file.readOneBlock(requestedPosition, ctx, fileChannel);
 				credit = false;
 			} finally {
 				if (credit)
 					creditSpace(blockSize);
 			}
 			if (position != v.start) {
 				// The file discovered its blockSize and adjusted.
 				position = v.start;
 				slot = slot(key, position);
 				e2 = table.get(slot);
 			}
 
 			Ref<DfsBlock> ref = new Ref<>(key, position, v.size(), v);
 			ref.hot = true;
 			for (;;) {
 				HashEntry n = new HashEntry(clean(e2), ref);
 				if (table.compareAndSet(slot, e2, n))
 					break;
 				e2 = table.get(slot);
 			}
 			addToClock(ref, blockSize - v.size());
 		} finally {
 			regionLock.unlock();
 		}
 
 		// If the block size changed from the default, it is possible the block
 		// that was loaded is the wrong block for the requested position.
 		if (v.contains(file.key, requestedPosition))
 			return v;
 		return getOrLoad(file, requestedPosition, ctx, fileChannel);
 	}
 
 	@SuppressWarnings("unchecked")
 	private void reserveSpace(int reserve) {
 		clockLock.lock();
 		try {
 			long live = liveBytes + reserve;
 			if (maxBytes < live) {
 				Ref prev = clockHand;
 				Ref hand = clockHand.next;
 				do {
 					if (hand.hot) {
 						// Value was recently touched. Clear
 						// hot and give it another chance.
 						hand.hot = false;
 						prev = hand;
 						hand = hand.next;
 						continue;
 					} else if (prev == hand)
 						break;
 
 					// No recent access since last scan, kill
 					// value and remove from clock.
 					Ref dead = hand;
 					hand = hand.next;
 					prev.next = hand;
 					dead.next = null;
 					dead.value = null;
 					live -= dead.size;
 					statEvict++;
 				} while (maxBytes < live);
 				clockHand = prev;
 			}
 			liveBytes = live;
 		} finally {
 			clockLock.unlock();
 		}
 	}
 
 	private void creditSpace(int credit) {
 		clockLock.lock();
 		liveBytes -= credit;
 		clockLock.unlock();
 	}
 
 	@SuppressWarnings("unchecked")
 	private void addToClock(Ref ref, int credit) {
 		clockLock.lock();
 		try {
 			if (credit != 0)
 				liveBytes -= credit;
 			Ref ptr = clockHand;
 			ref.next = ptr.next;
 			ptr.next = ref;
 			clockHand = ref;
 		} finally {
 			clockLock.unlock();
 		}
 	}
 
 	void put(DfsBlock v) {
 		put(v.stream, v.start, v.size(), v);
 	}
 
 	<T> Ref<T> putRef(DfsStreamKey key, long size, T v) {
 		return put(key, 0, (int) Math.min(size, Integer.MAX_VALUE), v);
 	}
 
 	<T> Ref<T> put(DfsStreamKey key, long pos, int size, T v) {
 		int slot = slot(key, pos);
 		HashEntry e1 = table.get(slot);
 		Ref<T> ref = scanRef(e1, key, pos);
 		if (ref != null)
 			return ref;
 
 		reserveSpace(size);
 		ReentrantLock regionLock = lockFor(key, pos);
 		regionLock.lock();
 		try {
 			HashEntry e2 = table.get(slot);
 			if (e2 != e1) {
 				ref = scanRef(e2, key, pos);
 				if (ref != null) {
 					creditSpace(size);
 					return ref;
 				}
 			}
 
 			ref = new Ref<>(key, pos, size, v);
 			ref.hot = true;
 			for (;;) {
 				HashEntry n = new HashEntry(clean(e2), ref);
 				if (table.compareAndSet(slot, e2, n))
 					break;
 				e2 = table.get(slot);
 			}
 			addToClock(ref, 0);
 		} finally {
 			regionLock.unlock();
 		}
 		return ref;
 	}
 
 	boolean contains(DfsStreamKey key, long position) {
 		return scan(table.get(slot(key, position)), key, position) != null;
 	}
 
 	@SuppressWarnings("unchecked")
 	<T> T get(DfsStreamKey key, long position) {
 		T val = (T) scan(table.get(slot(key, position)), key, position);
 		if (val == null)
 			statMiss.incrementAndGet();
 		else
 			statHit.incrementAndGet();
 		return val;
 	}
 
 	private <T> T scan(HashEntry n, DfsStreamKey key, long position) {
 		Ref<T> r = scanRef(n, key, position);
 		return r != null ? r.get() : null;
 	}
 
 	<T> Ref<T> getRef(DfsStreamKey key) {
 		Ref<T> r = scanRef(table.get(slot(key, 0)), key, 0);
 		if (r != null)
 			statHit.incrementAndGet();
 		else
 			statMiss.incrementAndGet();
 		return r;
 	}
 
 	@SuppressWarnings("unchecked")
 	private <T> Ref<T> scanRef(HashEntry n, DfsStreamKey key, long position) {
 		for (; n != null; n = n.next) {
 			Ref<T> r = n.ref;
 			if (r.position == position && r.key.equals(key))
 				return r.get() != null ? r : null;
 		}
 		return null;
 	}
 
 	private int slot(DfsStreamKey key, long position) {
 		return (hash(key.hash, position) >>> 1) % tableSize;
 	}
 
 	private ReentrantLock lockFor(DfsStreamKey key, long position) {
 		return loadLocks[(hash(key.hash, position) >>> 1) % loadLocks.length];
 	}
 
 	private static HashEntry clean(HashEntry top) {
 		while (top != null && top.ref.next == null)
 			top = top.next;
 		if (top == null)
 			return null;
 		HashEntry n = clean(top.next);
 		return n == top.next ? top : new HashEntry(n, top.ref);
 	}
 
 	private static final class HashEntry {
 		/** Next entry in the hash table's chain list. */
 		final HashEntry next;
 
 		/** The referenced object. */
 		final Ref ref;
 
 		HashEntry(HashEntry n, Ref r) {
 			next = n;
 			ref = r;
 		}
 	}
 
 	static final class Ref<T> {
 		final DfsStreamKey key;
 		final long position;
 		final int size;
 		volatile T value;
 		Ref next;
 		volatile boolean hot;
 
 		Ref(DfsStreamKey key, long position, int size, T v) {
 			this.key = key;
 			this.position = position;
 			this.size = size;
 			this.value = v;
 		}
 
 		T get() {
 			T v = value;
 			if (v != null)
 				hot = true;
 			return v;
 		}
 
 		boolean has() {
 			return value != null;
 		}
 	}
 }