api: helper for more efficient hash reduction

spectator-api/src/main/java/com/netflix/spectator/impl/Hash64.java

@@ -414,4 +414,22 @@ public long computeAndReset() {
     reset();
     return h;
   }
+
+  /**
+   * Helper to efficiently reduce hash from to range of {@code [0, n)}. Based on
+   * <a href="https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/">fast
+   * alternative to modulo reduction</a> post.
+   */
+  public static int reduce(long hash, int n) {
+    return reduce((int) hash, n);
+  }
+
+  /**
+   * Helper to efficiently reduce hash from to range of {@code [0, n)}. Based on
+   * <a href="https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/">fast
+   * alternative to modulo reduction</a> post.
+   */
+  public static int reduce(int hash, int n) {
+    return (int) (((hash & 0xFFFFFFFFL) * (n & 0xFFFFFFFFL)) >>> 32);
+  }
 }

spectator-api/src/test/java/com/netflix/spectator/impl/Hash64Test.java

@@ -272,4 +272,19 @@ public void hashDoubles() {
       hashes.add(h);
     }
   }
+
+  @Test
+  public void reduce() {
+    int n = 10;
+    int[] counts = new int[n];
+    Random r = new Random();
+    for (int i = 0; i < 100_000; ++i) {
+      long hash = r.nextLong();
+      ++counts[Hash64.reduce(hash, n)];
+    }
+    for (int count : counts) {
+      int delta = Math.abs(10_000 - count);
+      Assertions.assertTrue(delta < 1000);
+    }
+  }
 }