/*
    * Copyright (C) 2009, Google Inc.
    * Copyright (C) 2009, Jonas Fonseca <fonseca@diku.dk>
    * Copyright (C) 2008, Shawn O. Pearce <spearce@spearce.org> 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.revwalk;
  
  import java.text.MessageFormat;
  import java.util.AbstractList;
  
  import org.eclipse.jgit.internal.JGitText;
  
  /**
   * An ordered list of {@link org.eclipse.jgit.revwalk.RevObject} subclasses.
   *
   * @param <E>
   *            type of subclass of RevObject the list is storing.
   */
  public class RevObjectList<E extends RevObject> extends AbstractList<E> {
  	static final int BLOCK_SHIFT = 8;
  
  	static final int BLOCK_SIZE = 1 << BLOCK_SHIFT;
  
  	/**
  	 * Items stored in this list.
  	 * <p>
  	 * If {@link Block#shift} = 0 this block holds the list elements; otherwise
  	 * it holds pointers to other {@link Block} instances which use a shift that
  	 * is {@link #BLOCK_SHIFT} smaller.
  	 */
  	protected Block contents = new Block(0);
  
  	/** Current number of elements in the list. */
  	protected int size = 0;
  
  	/**
  	 * Create an empty object list.
  	 */
  	public RevObjectList() {
  		// Initialized above.
  	}
  
  	/** {@inheritDoc} */
  	@Override
  	public void add(int index, E element) {
  		if (index != size)
  			throw new UnsupportedOperationException(MessageFormat.format(
  					JGitText.get().unsupportedOperationNotAddAtEnd,
  					Integer.valueOf(index)));
  		set(index, element);
  		size++;
  	}
  
  	/** {@inheritDoc} */
  	@Override
  	@SuppressWarnings("unchecked")
  	public E set(int index, E element) {
  		Block s = contents;
  		while (index >> s.shift >= BLOCK_SIZE) {
  			s = new Block(s.shift + BLOCK_SHIFT);
  			s.contents[0] = contents;
  			contents = s;
  		}
  		while (s.shift > 0) {
  			final int i = index >> s.shift;
  			index -= i << s.shift;
  			if (s.contents[i] == null)
  				s.contents[i] = new Block(s.shift - BLOCK_SHIFT);
  			s = (Block) s.contents[i];
  		}
  		final Object old = s.contents[index];
  		s.contents[index] = element;
  		return (E) old;
  	}
  
  	/** {@inheritDoc} */
  	@Override
  	@SuppressWarnings("unchecked")
  	public E get(int index) {
  		Block s = contents;
  		if (index >> s.shift >= 1024)
  			return null;
  		while (s != null && s.shift > 0) {
  			final int i = index >> s.shift;
  			index -= i << s.shift;
  			s = (Block) s.contents[i];
  		}
  		return s != null ? (E) s.contents[index] : null;
  	}
  
  	/** {@inheritDoc} */
  	@Override
 	public int size() {
 		return size;
 	}
 
 	/** {@inheritDoc} */
 	@Override
 	public void clear() {
 		contents = new Block(0);
 		size = 0;
 	}
 
 	/** One level of contents, either an intermediate level or a leaf level. */
 	protected static class Block {
 		final Object[] contents = new Object[BLOCK_SIZE];
 
 		final int shift;
 
 		Block(int s) {
 			shift = s;
 		}
 	}
 }