java/com/google/gerrit/server/util/IdGenerator.java

// Copyright (C) 2009 The Android Open Source Project
//
// Licensed 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 com.google.gerrit.server.util;

import com.google.inject.Inject;
import com.google.inject.Singleton;
import java.util.Random;
import java.util.concurrent.atomic.AtomicInteger;

/** Simple class to produce 4 billion keys randomly distributed. */
@Singleton
public class IdGenerator {

  private final AtomicInteger gen;

  @Inject
  IdGenerator() {
    gen = new AtomicInteger(new Random().nextInt());
  }

  /** Produce the next identifier. */
  public int next() {
    return mix(gen.getAndIncrement());
  }

  private static final int salt = 0x9e3779b9;

  static int mix(int in) {
    return mix(salt, in);
  }

  /** A very simple bit permutation to mask a simple incrementer. */
  public static int mix(int salt, int in) {
    short v0 = hi16(in);
    short v1 = lo16(in);
    v0 += (short) (((v1 << 2) + 0 ^ v1) + (salt ^ (v1 >>> 3)) + 1);
    v1 += (short) (((v0 << 2) + 2 ^ v0) + (salt ^ (v0 >>> 3)) + 3);
    return result(v0, v1);
  }

  /* For testing only. */
  static int unmix(int in) {
    short v0 = hi16(in);
    short v1 = lo16(in);
    v1 -= (short) (((v0 << 2) + 2 ^ v0) + (salt ^ (v0 >>> 3)) + 3);
    v0 -= (short) (((v1 << 2) + 0 ^ v1) + (salt ^ (v1 >>> 3)) + 1);
    return result(v0, v1);
  }

  private static short hi16(int in) {
    return (short)
        ( //
        ((in >>> 24 & 0xff))
            | //
            ((in >>> 16 & 0xff) << 8) //
        );
  }

  private static short lo16(int in) {
    return (short)
        ( //
        ((in >>> 8 & 0xff))
            | //
            ((in & 0xff) << 8) //
        );
  }

  private static int result(short v0, short v1) {
    return ((v0 & 0xff) << 24)
        | //
        (((v0 >>> 8) & 0xff) << 16)
        | //
        ((v1 & 0xff) << 8)
        | //
        ((v1 >>> 8) & 0xff);
  }
}