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Viewing changes to contrib/NGit/NGit.Storage.Pack/DeltaIndexScanner.cs

  • Committer: Bazaar Package Importer
  • Author(s): Jo Shields
  • Date: 2011-06-27 17:03:13 UTC
  • mto: (1.8.1 upstream)
  • mto: This revision was merged to the branch mainline in revision 54.
  • Revision ID: james.westby@ubuntu.com-20110627170313-6cvz3s19x6e9hqe9
Import upstream version 2.5.92+dfsg

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contrib/NGit/NGit.Storage.Pack/DeltaIndexScanner.cs
 
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/*
 
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This code is derived from jgit (http://eclipse.org/jgit).
 
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Copyright owners are documented in jgit's IP log.
 
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This program and the accompanying materials are made available
 
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under the terms of the Eclipse Distribution License v1.0 which
 
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accompanies this distribution, is reproduced below, and is
 
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available at http://www.eclipse.org/org/documents/edl-v10.php
 
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All rights reserved.
 
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Redistribution and use in source and binary forms, with or
 
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without modification, are permitted provided that the following
 
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conditions are met:
 
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- Redistributions of source code must retain the above copyright
 
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  notice, this list of conditions and the following disclaimer.
 
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- Redistributions in binary form must reproduce the above
 
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  copyright notice, this list of conditions and the following
 
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  disclaimer in the documentation and/or other materials provided
 
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  with the distribution.
 
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- Neither the name of the Eclipse Foundation, Inc. nor the
 
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  names of its contributors may be used to endorse or promote
 
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  products derived from this software without specific prior
 
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  written permission.
 
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THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND
 
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CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES,
 
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INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 
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OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 
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ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 
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CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 
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SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
 
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NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 
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LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 
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CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 
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STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 
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ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
 
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ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
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*/
 
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using NGit.Storage.Pack;
 
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using Sharpen;
 
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namespace NGit.Storage.Pack
 
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{
 
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        /// <summary>
 
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        /// Supports
 
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        /// <see cref="DeltaIndex">DeltaIndex</see>
 
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        /// by performing a partial scan of the content.
 
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        /// </summary>
 
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        internal class DeltaIndexScanner
 
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        {
 
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                internal readonly int[] table;
 
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                internal readonly long[] entries;
 
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                internal readonly int[] next;
 
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                internal readonly int tableMask;
 
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                private int entryCnt;
 
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                internal DeltaIndexScanner(byte[] raw, int len)
 
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                {
 
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                        // To save memory the buckets for hash chains are stored in correlated
 
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                        // arrays. This permits us to get 3 values per entry, without paying
 
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                        // the penalty for an object header on each entry.
 
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                        // Clip the length so it falls on a block boundary. We won't
 
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                        // bother to scan the final partial block.
 
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                        //
 
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                        len -= (len % DeltaIndex.BLKSZ);
 
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                        int worstCaseBlockCnt = len / DeltaIndex.BLKSZ;
 
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                        if (worstCaseBlockCnt < 1)
 
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                        {
 
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                                table = new int[] {  };
 
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                                tableMask = 0;
 
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                                entries = new long[] {  };
 
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                                next = new int[] {  };
 
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                        }
 
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                        else
 
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                        {
 
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                                table = new int[TableSize(worstCaseBlockCnt)];
 
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                                tableMask = table.Length - 1;
 
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                                // As we insert blocks we preincrement so that 0 is never a
 
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                                // valid entry. Therefore we have to allocate one extra space.
 
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                                //
 
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                                entries = new long[1 + worstCaseBlockCnt];
 
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                                next = new int[entries.Length];
 
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                                Scan(raw, len);
 
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                        }
 
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                }
 
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                private void Scan(byte[] raw, int end)
 
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                {
 
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                        // We scan the input backwards, and always insert onto the
 
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                        // front of the chain. This ensures that chains will have lower
 
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                        // offsets at the front of the chain, allowing us to prefer the
 
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                        // earlier match rather than the later match.
 
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                        //
 
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                        int lastHash = 0;
 
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                        int ptr = end - DeltaIndex.BLKSZ;
 
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                        do
 
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                        {
 
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                                int key = DeltaIndex.HashBlock(raw, ptr);
 
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                                int tIdx = key & tableMask;
 
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                                int head = table[tIdx];
 
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                                if (head != 0 && lastHash == key)
 
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                                {
 
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                                        // Two consecutive blocks have the same content hash,
 
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                                        // prefer the earlier block because we want to use the
 
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                                        // longest sequence we can during encoding.
 
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                                        //
 
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                                        entries[head] = (((long)key) << 32) | ptr;
 
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                                }
 
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                                else
 
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                                {
 
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                                        int eIdx = ++entryCnt;
 
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                                        entries[eIdx] = (((long)key) << 32) | ptr;
 
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                                        next[eIdx] = head;
 
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                                        table[tIdx] = eIdx;
 
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                                }
 
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                                lastHash = key;
 
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                                ptr -= DeltaIndex.BLKSZ;
 
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                        }
 
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                        while (0 <= ptr);
 
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                }
 
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                private static int TableSize(int worstCaseBlockCnt)
 
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                {
 
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                        int shift = 32 - Sharpen.Extensions.NumberOfLeadingZeros(worstCaseBlockCnt);
 
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                        int sz = 1 << (shift - 1);
 
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                        if (sz < worstCaseBlockCnt)
 
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                        {
 
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                                sz <<= 1;
 
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                        }
 
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                        return sz;
 
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                }
 
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        }
 
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}