/**
 *
 * 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.hadoop.hbase.util;

import static java.lang.Integer.rotateLeft;

import java.io.FileInputStream;
import java.io.IOException;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;

/**
 * Produces 32-bit hash for hash table lookup.
 *
 * <pre>lookup3.c, by Bob Jenkins, May 2006, Public Domain.
 *
 * You can use this free for any purpose.  It's in the public domain.
 * It has no warranty.
 * </pre>
 *
 * @see <a href="http://burtleburtle.net/bob/c/lookup3.c">lookup3.c</a>
 * @see <a href="http://www.ddj.com/184410284">Hash Functions (and how this
 * function compares to others such as CRC, MD?, etc</a>
 * @see <a href="http://burtleburtle.net/bob/hash/doobs.html">Has update on the
 * Dr. Dobbs Article</a>
 */
@InterfaceAudience.Public
@InterfaceStability.Stable
public class JenkinsHash extends Hash {
  private static final int BYTE_MASK = 0xff;

  private static JenkinsHash _instance = new JenkinsHash();

  public static Hash getInstance() {
    return _instance;
  }

  /**
   * taken from  hashlittle() -- hash a variable-length key into a 32-bit value
   *
   * @param key the key (the unaligned variable-length array of bytes)
   * @param nbytes number of bytes to include in hash
   * @param initval can be any integer value
   * @return a 32-bit value.  Every bit of the key affects every bit of the
   * return value.  Two keys differing by one or two bits will have totally
   * different hash values.
   *
   * <p>The best hash table sizes are powers of 2.  There is no need to do mod
   * a prime (mod is sooo slow!).  If you need less than 32 bits, use a bitmask.
   * For example, if you need only 10 bits, do
   * <code>h = (h & hashmask(10));</code>
   * In which case, the hash table should have hashsize(10) elements.
   *
   * <p>If you are hashing n strings byte[][] k, do it like this:
   * for (int i = 0, h = 0; i < n; ++i) h = hash( k[i], h);
   *
   * <p>By Bob Jenkins, 2006.  bob_jenkins@burtleburtle.net.  You may use this
   * code any way you wish, private, educational, or commercial.  It's free.
   *
   * <p>Use for hash table lookup, or anything where one collision in 2^^32 is
   * acceptable.  Do NOT use for cryptographic purposes.
  */
  @Override
  @SuppressWarnings("fallthrough")
  public int hash(byte[] key, int off, int nbytes, int initval) {
    int length = nbytes;
    int a, b, c;
    a = b = c = 0xdeadbeef + length + initval;
    int offset = off;
    for (; length > 12; offset += 12, length -= 12) {
      a += (key[offset] & BYTE_MASK);
      a += ((key[offset + 1] & BYTE_MASK) <<  8);
      a += ((key[offset + 2] & BYTE_MASK) << 16);
      a += ((key[offset + 3] & BYTE_MASK) << 24);
      b += (key[offset + 4] & BYTE_MASK);
      b += ((key[offset + 5] & BYTE_MASK) <<  8);
      b += ((key[offset + 6] & BYTE_MASK) << 16);
      b += ((key[offset + 7] & BYTE_MASK) << 24);
      c += (key[offset + 8] & BYTE_MASK);
      c += ((key[offset + 9] & BYTE_MASK) <<  8);
      c += ((key[offset + 10] & BYTE_MASK) << 16);
      c += ((key[offset + 11] & BYTE_MASK) << 24);

      /*
       * mix -- mix 3 32-bit values reversibly.
       * This is reversible, so any information in (a,b,c) before mix() is
       * still in (a,b,c) after mix().
       *
       * If four pairs of (a,b,c) inputs are run through mix(), or through
       * mix() in reverse, there are at least 32 bits of the output that
       * are sometimes the same for one pair and different for another pair.
       *
       * This was tested for:
       * - pairs that differed by one bit, by two bits, in any combination
       *   of top bits of (a,b,c), or in any combination of bottom bits of
       *   (a,b,c).
       * - "differ" is defined as +, -, ^, or ~^.  For + and -, I transformed
       *   the output delta to a Gray code (a^(a>>1)) so a string of 1's (as
       *    is commonly produced by subtraction) look like a single 1-bit
       *    difference.
       * - the base values were pseudorandom, all zero but one bit set, or
       *   all zero plus a counter that starts at zero.
       *
       * Some k values for my "a-=c; a^=rot(c,k); c+=b;" arrangement that
       * satisfy this are
       *     4  6  8 16 19  4
       *     9 15  3 18 27 15
       *    14  9  3  7 17  3
       * Well, "9 15 3 18 27 15" didn't quite get 32 bits diffing for
       * "differ" defined as + with a one-bit base and a two-bit delta.  I
       * used http://burtleburtle.net/bob/hash/avalanche.html to choose
       * the operations, constants, and arrangements of the variables.
       *
       * This does not achieve avalanche.  There are input bits of (a,b,c)
       * that fail to affect some output bits of (a,b,c), especially of a.
       * The most thoroughly mixed value is c, but it doesn't really even
       * achieve avalanche in c.
       *
       * This allows some parallelism.  Read-after-writes are good at doubling
       * the number of bits affected, so the goal of mixing pulls in the
       * opposite direction as the goal of parallelism.  I did what I could.
       * Rotates seem to cost as much as shifts on every machine I could lay
       * my hands on, and rotates are much kinder to the top and bottom bits,
       * so I used rotates.
       *
       * #define mix(a,b,c) \
       * { \
       *   a -= c;  a ^= rot(c, 4);  c += b; \
       *   b -= a;  b ^= rot(a, 6);  a += c; \
       *   c -= b;  c ^= rot(b, 8);  b += a; \
       *   a -= c;  a ^= rot(c,16);  c += b; \
       *   b -= a;  b ^= rot(a,19);  a += c; \
       *   c -= b;  c ^= rot(b, 4);  b += a; \
       * }
       *
       * mix(a,b,c);
       */
      a -= c; a ^= rotateLeft(c, 4); c += b;
      b -= a; b ^= rotateLeft(a, 6); a += c;
      c -= b; c ^= rotateLeft(b, 8); b += a;
      a -= c; a ^= rotateLeft(c, 16); c += b;
      b -= a; b ^= rotateLeft(a, 19); a += c;
      c -= b; c ^= rotateLeft(b, 4); b += a;
    }

    //-------------------------------- last block: affect all 32 bits of (c)
    switch (length) {                   // all the case statements fall through
    case 12:
    	c += ((key[offset + 11] & BYTE_MASK) << 24);
    case 11:
      c += ((key[offset + 10] & BYTE_MASK) << 16);
    case 10:
      c += ((key[offset + 9] & BYTE_MASK) <<  8);
    case  9:
      c += (key[offset + 8] & BYTE_MASK);
    case  8:
      b += ((key[offset + 7] & BYTE_MASK) << 24);
    case  7:
      b += ((key[offset + 6] & BYTE_MASK) << 16);
    case  6:
      b += ((key[offset + 5] & BYTE_MASK) <<  8);
    case  5:
      b += (key[offset + 4] & BYTE_MASK);
    case  4:
      a += ((key[offset + 3] & BYTE_MASK) << 24);
    case  3:
      a += ((key[offset + 2] & BYTE_MASK) << 16);
    case  2:
      a += ((key[offset + 1] & BYTE_MASK) <<  8);
    case  1:
      //noinspection PointlessArithmeticExpression
      a += (key[offset + 0] & BYTE_MASK);
      break;
    case  0:
      return c;
    }
    /*
     * final -- final mixing of 3 32-bit values (a,b,c) into c
     *
     * Pairs of (a,b,c) values differing in only a few bits will usually
     * produce values of c that look totally different.  This was tested for
     * - pairs that differed by one bit, by two bits, in any combination
     *   of top bits of (a,b,c), or in any combination of bottom bits of
     *   (a,b,c).
     *
     * - "differ" is defined as +, -, ^, or ~^.  For + and -, I transformed
     *   the output delta to a Gray code (a^(a>>1)) so a string of 1's (as
     *   is commonly produced by subtraction) look like a single 1-bit
     *   difference.
     *
     * - the base values were pseudorandom, all zero but one bit set, or
     *   all zero plus a counter that starts at zero.
     *
     * These constants passed:
     *   14 11 25 16 4 14 24
     *   12 14 25 16 4 14 24
     * and these came close:
     *    4  8 15 26 3 22 24
     *   10  8 15 26 3 22 24
     *   11  8 15 26 3 22 24
     *
     * #define final(a,b,c) \
     * {
     *   c ^= b; c -= rot(b,14); \
     *   a ^= c; a -= rot(c,11); \
     *   b ^= a; b -= rot(a,25); \
     *   c ^= b; c -= rot(b,16); \
     *   a ^= c; a -= rot(c,4);  \
     *   b ^= a; b -= rot(a,14); \
     *   c ^= b; c -= rot(b,24); \
     * }
     *
     */
    c ^= b; c -= rotateLeft(b, 14);
    a ^= c; a -= rotateLeft(c, 11);
    b ^= a; b -= rotateLeft(a, 25);
    c ^= b; c -= rotateLeft(b, 16);
    a ^= c; a -= rotateLeft(c, 4);
    b ^= a; b -= rotateLeft(a, 14);
    c ^= b; c -= rotateLeft(b, 24);
    return c;
  }

  /**
   * Compute the hash of the specified file
   * @param args name of file to compute hash of.
   * @throws IOException e
   */
  public static void main(String[] args) throws IOException {
    if (args.length != 1) {
      System.err.println("Usage: JenkinsHash filename");
      System.exit(-1);
    }
    FileInputStream in = new FileInputStream(args[0]);
    byte[] bytes = new byte[512];
    int value = 0;
    JenkinsHash hash = new JenkinsHash();
    for (int length = in.read(bytes); length > 0; length = in.read(bytes)) {
      value = hash.hash(bytes, length, value);
    }
    System.out.println(Math.abs(value));
  }
}
