/*
    * 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 {@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. */
  	private static final int INITIAL_DIRECTORY = 1024;
  
  	/** 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.
  	 * <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. */
  	int size;
  
  	/** The map doubles in capacity when {@link #size} reaches this target. */
  	private int grow;
  
  	/** Number of low bits used to form the index into {@link #directory}. */
  	int bits;
  
  	/** Low bit mask to index into {@link #directory}, {@code 2^bits-1}. */
  	private int mask;
  
  	/**
  	 * 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.
  	 */
  	public void clear() {
  		size = 0;
  
  		for (V[] tbl : directory) {
  			if (tbl == null)
  				break;
  			Arrays.fill(tbl, null);
  		}
  	}
  
  	/**
  	 * 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}
 	 * <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.
 	 * <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.
 	 * <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.
 	 *
 	 * @return number of objects in this map.
 	 */
 	public int size() {
 		return size;
 	}
 
 	/**
 	 * Whether this map is empty
 	 *
 	 * @return true if {@link #size()} is 0.
 	 */
 	public boolean isEmpty() {
 		return size == 0;
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public Iterator<V> iterator() {
 		return new Iterator<>() {
 			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 {@link ObjectIdOwnerMap}. */
 	public abstract static class Entry extends ObjectId {
 		transient Entry next;
 
 		/**
 		 * Initialize this entry with a specific ObjectId.
 		 *
 		 * @param id
 		 *            the id the entry represents.
 		 */
 		public Entry(AnyObjectId id) {
 			super(id);
 		}
 	}
 }