/*
* 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
*
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* 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.
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* names of its contributors may be used to endorse or promote
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*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
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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;
}
}