com.google.gerrit.server.util.IdGenerator Maven / Gradle / Ivy
// 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 += ((v1 << 2) + 0 ^ v1) + (salt ^ (v1 >>> 3)) + 1;
v1 += ((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 -= ((v0 << 2) + 2 ^ v0) + (salt ^ (v0 >>> 3)) + 3;
v0 -= ((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);
}
}