/*
    * Copyright (C) 2010, 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.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, 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(int worstCaseBlockCnt) {
  		int shift = 32 - Integer.numberOfLeadingZeros(worstCaseBlockCnt);
  		int sz = 1 << (shift - 1);
  		if (sz < worstCaseBlockCnt)
  			sz <<= 1;
  		return sz;
  	}
  }