/*
 * Copyright (C) 2011, Google Inc. 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.lib;

import java.util.Arrays;
import java.util.Iterator;
import java.util.NoSuchElementException;

Fast, efficient map for ObjectId subclasses in only one map. 
 To use this map type, applications must have their entry value type extend from Entry, which itself extends from ObjectId. 

Object instances may only be stored in ONE ObjectIdOwnerMap. This
restriction exists because the map stores internal map state within each
object instance. If an instance is be placed in another ObjectIdOwnerMap it
could corrupt one or both map's internal state.
 If an object instance must be in more than one map, applications may use ObjectIdOwnerMap for one of the maps, and ObjectIdSubclassMap for the other map(s). It is encouraged to use ObjectIdOwnerMap for the map that is accessed most often, as this implementation runs faster than the more general ObjectIdSubclassMap implementation. 
Type parameters:
<V> – 
           type of subclass of ObjectId that will be stored in the map./**
 * Fast, efficient map for {@link org.eclipse.jgit.lib.ObjectId} subclasses in
 * only one map.
 * <p>
 * To use this map type, applications must have their entry value type extend
 * from {@link org.eclipse.jgit.lib.ObjectIdOwnerMap.Entry}, which itself
 * extends from ObjectId.
 * <p>
 * Object instances may only be stored in <b>ONE</b> ObjectIdOwnerMap. This
 * restriction exists because the map stores internal map state within each
 * object instance. If an instance is be placed in another ObjectIdOwnerMap it
 * could corrupt one or both map's internal state.
 * <p>
 * If an object instance must be in more than one map, applications may use
 * ObjectIdOwnerMap for one of the maps, and
 * {@link org.eclipse.jgit.lib.ObjectIdSubclassMap} for the other map(s). It is
 * encouraged to use ObjectIdOwnerMap for the map that is accessed most often,
 * as this implementation runs faster than the more general ObjectIdSubclassMap
 * implementation.
 *
 * @param <V>
 *            type of subclass of ObjectId that will be stored in the map.
 */
public class ObjectIdOwnerMap<V extends ObjectIdOwnerMap.Entry>
		implements Iterable<V>, ObjectIdSet {
	
Size of the initial directory, will grow as necessary. /** Size of the initial directory, will grow as necessary. */
	private static final int INITIAL_DIRECTORY = 1024;

	
Number of bits in a segment's index. Segments are 2^11 in size. /** Number of bits in a segment's index. Segments are 2^11 in size. */
	private static final int SEGMENT_BITS = 11;

	private static final int SEGMENT_SHIFT = 32 - SEGMENT_BITS;

	
Top level directory of the segments.
 The low ObjectIdOwnerMap<V>.bits of the SHA-1 are used to select the segment from this directory. Each segment is constant sized at 2^SEGMENT_BITS. /**
	 * Top level directory of the segments.
	 * <p>
	 * The low {@link #bits} of the SHA-1 are used to select the segment from
	 * this directory. Each segment is constant sized at 2^SEGMENT_BITS.
	 */
	V[][] directory;

	
Total number of objects in this map. /** Total number of objects in this map. */
	int size;

	
The map doubles in capacity when ObjectIdOwnerMap<V>.size reaches this target. /** The map doubles in capacity when {@link #size} reaches this target. */
	private int grow;

	
Number of low bits used to form the index into ObjectIdOwnerMap<V>.directory. /** Number of low bits used to form the index into {@link #directory}. */
	int bits;

	
Low bit mask to index into ObjectIdOwnerMap<V>.directory, 2^bits-1. /** Low bit mask to index into {@link #directory}, {@code 2^bits-1}. */
	private int mask;

	
Create an empty map.
/**
	 * Create an empty map.
	 */
	@SuppressWarnings("unchecked")
	public ObjectIdOwnerMap() {
		bits = 0;
		mask = 0;
		grow = computeGrowAt(bits);

		directory = (V[][]) new Entry[INITIAL_DIRECTORY][];
		directory[0] = newSegment();
	}

	
Remove all entries from this map.
/**
	 * Remove all entries from this map.
	 */
	public void clear() {
		size = 0;

		for (V[] tbl : directory) {
			if (tbl == null)
				break;
			Arrays.fill(tbl, null);
		}
	}

	
Lookup an existing mapping.
Params:
toFind – 
           the object identifier to find.
Returns:
the instance mapped to toFind, or null if no mapping exists./**
	 * Lookup an existing mapping.
	 *
	 * @param toFind
	 *            the object identifier to find.
	 * @return the instance mapped to toFind, or null if no mapping exists.
	 */
	@SuppressWarnings("unchecked")
	public V get(AnyObjectId toFind) {
		if (toFind == null) {
			return null;
		}
		int h = toFind.w1;
		V obj = directory[h & mask][h >>> SEGMENT_SHIFT];
		for (; obj != null; obj = (V) obj.next)
			if (equals(obj, toFind))
				return obj;
		return null;
	}

	
{@inheritDoc}

Returns true if this map contains the specified object.
/**
	 * {@inheritDoc}
	 * <p>
	 * Returns true if this map contains the specified object.
	 */
	@Override
	public boolean contains(AnyObjectId toFind) {
		return get(toFind) != null;
	}

	
Store an object for future lookup.

An existing mapping for must not be in this map. Callers must first call get(AnyObjectId) to verify there is no current mapping prior to adding a new mapping, or use addIfAbsent(Entry). 
Params:
newValue – 
           the object to store./**
	 * Store an object for future lookup.
	 * <p>
	 * An existing mapping for <b>must not</b> be in this map. Callers must
	 * first call {@link #get(AnyObjectId)} to verify there is no current
	 * mapping prior to adding a new mapping, or use {@link #addIfAbsent(Entry)}.
	 *
	 * @param newValue
	 *            the object to store.
	 */
	public <Q extends V> void add(Q newValue) {
		if (++size == grow)
			grow();

		int h = newValue.w1;
		V[] table = directory[h & mask];
		h >>>= SEGMENT_SHIFT;

		newValue.next = table[h];
		table[h] = newValue;
	}

	
Store an object for future lookup.
 Stores newValue, but only if there is not already an object for the same object name. Callers can tell if the value is new by checking the return value with reference equality: 
V obj = ...;
boolean wasNew = map.addIfAbsent(obj) == obj;

Params:
newValue – 
           the object to store.
Returns:
newValue if stored, or the prior value already stored and that would have been returned had the caller used get(newValue) first./**
	 * Store an object for future lookup.
	 * <p>
	 * Stores {@code newValue}, but only if there is not already an object for
	 * the same object name. Callers can tell if the value is new by checking
	 * the return value with reference equality:
	 *
	 * <pre>
	 * V obj = ...;
	 * boolean wasNew = map.addIfAbsent(obj) == obj;
	 * </pre>
	 *
	 * @param newValue
	 *            the object to store.
	 * @return {@code newValue} if stored, or the prior value already stored and
	 *         that would have been returned had the caller used
	 *         {@code get(newValue)} first.
	 */
	@SuppressWarnings("unchecked")
	public <Q extends V> V addIfAbsent(Q newValue) {
		int h = newValue.w1;
		V[] table = directory[h & mask];
		h >>>= SEGMENT_SHIFT;

		for (V obj = table[h]; obj != null; obj = (V) obj.next)
			if (equals(obj, newValue))
				return obj;

		newValue.next = table[h];
		table[h] = newValue;

		if (++size == grow)
			grow();
		return newValue;
	}

	
Get number of objects in this map.
Returns:
number of objects in this map./**
	 * Get number of objects in this map.
	 *
	 * @return number of objects in this map.
	 */
	public int size() {
		return size;
	}

	
Whether this map is empty
Returns:
true if size() is 0./**
	 * Whether this map is empty
	 *
	 * @return true if {@link #size()} is 0.
	 */
	public boolean isEmpty() {
		return size == 0;
	}

	
{@inheritDoc} /** {@inheritDoc} */
	@Override
	public Iterator<V> iterator() {
		return new Iterator<V>() {
			private int found;
			private int dirIdx;
			private int tblIdx;
			private V next;

			@Override
			public boolean hasNext() {
				return found < size;
			}

			@Override
			public V next() {
				if (next != null)
					return found(next);

				for (;;) {
					V[] table = directory[dirIdx];
					if (tblIdx == table.length) {
						if (++dirIdx >= (1 << bits))
							throw new NoSuchElementException();
						table = directory[dirIdx];
						tblIdx = 0;
					}

					while (tblIdx < table.length) {
						V v = table[tblIdx++];
						if (v != null)
							return found(v);
					}
				}
			}

			@SuppressWarnings("unchecked")
			private V found(V v) {
				found++;
				next = (V) v.next;
				return v;
			}

			@Override
			public void remove() {
				throw new UnsupportedOperationException();
			}
		};
	}

	@SuppressWarnings("unchecked")
	private void grow() {
		final int oldDirLen = 1 << bits;
		final int s = 1 << bits;

		bits++;
		mask = (1 << bits) - 1;
		grow = computeGrowAt(bits);

		// Quadruple the directory if it needs to expand. Expanding the
		// directory is expensive because it generates garbage, so try
		// to avoid doing it often.
		//
		final int newDirLen = 1 << bits;
		if (directory.length < newDirLen) {
			V[][] newDir = (V[][]) new Entry[newDirLen << 1][];
			System.arraycopy(directory, 0, newDir, 0, oldDirLen);
			directory = newDir;
		}

		// For every bucket of every old segment, split the chain between
		// the old segment and the new segment's corresponding bucket. To
		// select between them use the lowest bit that was just added into
		// the mask above. This causes the table to double in capacity.
		//
		for (int dirIdx = 0; dirIdx < oldDirLen; dirIdx++) {
			final V[] oldTable = directory[dirIdx];
			final V[] newTable = newSegment();

			for (int i = 0; i < oldTable.length; i++) {
				V chain0 = null;
				V chain1 = null;
				V next;

				for (V obj = oldTable[i]; obj != null; obj = next) {
					next = (V) obj.next;

					if ((obj.w1 & s) == 0) {
						obj.next = chain0;
						chain0 = obj;
					} else {
						obj.next = chain1;
						chain1 = obj;
					}
				}

				oldTable[i] = chain0;
				newTable[i] = chain1;
			}

			directory[oldDirLen + dirIdx] = newTable;
		}
	}

	@SuppressWarnings("unchecked")
	private final V[] newSegment() {
		return (V[]) new Entry[1 << SEGMENT_BITS];
	}

	private static final int computeGrowAt(int bits) {
		return 1 << (bits + SEGMENT_BITS);
	}

	private static final boolean equals(AnyObjectId firstObjectId,
			AnyObjectId secondObjectId) {
		return firstObjectId.w2 == secondObjectId.w2
				&& firstObjectId.w3 == secondObjectId.w3
				&& firstObjectId.w4 == secondObjectId.w4
				&& firstObjectId.w5 == secondObjectId.w5
				&& firstObjectId.w1 == secondObjectId.w1;
	}

	
Type of entry stored in the ObjectIdOwnerMap. /** Type of entry stored in the {@link ObjectIdOwnerMap}. */
	public abstract static class Entry extends ObjectId {
		transient Entry next;

		
Initialize this entry with a specific ObjectId.
Params:
id – 
           the id the entry represents./**
		 * Initialize this entry with a specific ObjectId.
		 *
		 * @param id
		 *            the id the entry represents.
		 */
		public Entry(AnyObjectId id) {
			super(id);
		}
	}
}