1 files changed, 89 insertions, 0 deletions
diff --git a/java/com/google/gerrit/server/util/IdGenerator.java b/java/com/google/gerrit/server/util/IdGenerator.java
new file mode 100644
index 0000000000..276df06d66
--- /dev/null
+++ b/java/com/google/gerrit/server/util/IdGenerator.java
@@ -0,0 +1,89 @@
+// 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);
+  }
+}