jgit/org.eclipse.jgit/src/org/eclipse/jgit/internal/storage/pack/DeltaIndexScanner.java

/*
 * Copyright (C) 2010, Google Inc.
 * 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
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
 * ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

package org.eclipse.jgit.internal.storage.pack;

/**
 * Supports {@link DeltaIndex} by performing a partial scan of the content.
 */
class DeltaIndexScanner {
	final int[] table;

	// To save memory the buckets for hash chains are stored in correlated
	// arrays. This permits us to get 3 values per entry, without paying
	// the penalty for an object header on each entry.

	final long[] entries;

	final int[] next;

	final int tableMask;

	private int entryCnt;

	DeltaIndexScanner(byte[] raw, int len) {
		// Clip the length so it falls on a block boundary. We won't
		// bother to scan the final partial block.
		//
		len -= (len % DeltaIndex.BLKSZ);

		final int worstCaseBlockCnt = len / DeltaIndex.BLKSZ;
		if (worstCaseBlockCnt < 1) {
			table = new int[] {};
			tableMask = 0;

			entries = new long[] {};
			next = new int[] {};

		} else {
			table = new int[tableSize(worstCaseBlockCnt)];
			tableMask = table.length - 1;

			// As we insert blocks we preincrement so that 0 is never a
			// valid entry. Therefore we have to allocate one extra space.
			//
			entries = new long[1 + worstCaseBlockCnt];
			next = new int[entries.length];

			scan(raw, len);
		}
	}

	private void scan(byte[] raw, final int end) {
		// We scan the input backwards, and always insert onto the
		// front of the chain. This ensures that chains will have lower
		// offsets at the front of the chain, allowing us to prefer the
		// earlier match rather than the later match.
		//
		int lastHash = 0;
		int ptr = end - DeltaIndex.BLKSZ;
		do {
			final int key = DeltaIndex.hashBlock(raw, ptr);
			final int tIdx = key & tableMask;

			final int head = table[tIdx];
			if (head != 0 && lastHash == key) {
				// Two consecutive blocks have the same content hash,
				// prefer the earlier block because we want to use the
				// longest sequence we can during encoding.
				//
				entries[head] = (((long) key) << 32) | ptr;
			} else {
				final int eIdx = ++entryCnt;
				entries[eIdx] = (((long) key) << 32) | ptr;
				next[eIdx] = head;
				table[tIdx] = eIdx;
			}

			lastHash = key;
			ptr -= DeltaIndex.BLKSZ;
		} while (0 <= ptr);
	}

	private static int tableSize(final int worstCaseBlockCnt) {
		int shift = 32 - Integer.numberOfLeadingZeros(worstCaseBlockCnt);
		int sz = 1 << (shift - 1);
		if (sz < worstCaseBlockCnt)
			sz <<= 1;
		return sz;
	}
}