content: /*
   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements.  See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You under the Apache License, Version 2.0
   * (the "License"); you may not use this file except in compliance with
   * the License.  You may obtain a copy of the License at
   *
   *    http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package org.apache.arrow.memory.util.hash;
  
  import org.apache.arrow.memory.ArrowBuf;
  import org.apache.arrow.memory.util.MemoryUtil;
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  /**
   * A simple hasher that calculates the hash code of integers as is, and does not perform any
   * finalization. So the computation is extremely efficient.
   *
   * <p>This algorithm only provides the most basic semantics for the hash code. That is, if two
   * objects are equal, they must have equal hash code. However, the quality of the produced hash code
   * may not be good. In other words, the generated hash codes are far from being uniformly
   * distributed in the universe.
   *
   * <p>Therefore, this algorithm is suitable only for scenarios where the most basic semantics of the
   * hash code is required (e.g. in scenarios that require fast and proactive data pruning)
   *
   * <p>An object of this class is stateless, so it can be shared between threads.
   */
  public class SimpleHasher implements ArrowBufHasher {
  
    public static SimpleHasher INSTANCE = new SimpleHasher();
  
    protected SimpleHasher() {}
  
    /**
     * Calculates the hash code for a memory region.
     *
     * @param address start address of the memory region.
     * @param length length of the memory region.
     * @return the hash code.
     */
    @Override
    public int hashCode(long address, long length) {
      int hashValue = 0;
      int index = 0;
      while (index + 8 <= length) {
        long longValue = MemoryUtil.getLong(address + index);
        int longHash = getLongHashCode(longValue);
        hashValue = combineHashCode(hashValue, longHash);
        index += 8;
      }
  
      if (index + 4 <= length) {
        int intValue = MemoryUtil.getInt(address + index);
        int intHash = intValue;
        hashValue = combineHashCode(hashValue, intHash);
        index += 4;
      }
  
      while (index < length) {
        byte byteValue = MemoryUtil.getByte(address + index);
        int byteHash = byteValue;
        hashValue = combineHashCode(hashValue, byteHash);
        index += 1;
      }
  
      return finalizeHashCode(hashValue);
    }
  
    /**
     * Calculates the hash code for a memory region.
     *
     * @param buf the buffer for the memory region.
     * @param offset offset within the buffer for the memory region.
     * @param length length of the memory region.
     * @return the hash code.
     */
    @Override
    public int hashCode(ArrowBuf buf, long offset, long length) {
      buf.checkBytes(offset, offset + length);
      return hashCode(buf.memoryAddress() + offset, length);
    }
  
    protected int combineHashCode(int currentHashCode, int newHashCode) {
      return currentHashCode * 37 + newHashCode;
    }
  
    protected int getLongHashCode(long longValue) {
      return Long.hashCode(longValue);
    }
  
    protected int finalizeHashCode(int hashCode) {
      return hashCode;
    }
  
    @Override
    public int hashCode() {
      return 123;
    }
  
    @Override
    public boolean equals(@Nullable Object obj) {
      return obj != null && (obj instanceof SimpleHasher);
    }
  }
  