replace JenkinsHash w/ MurmurHash. its hash distribution is just as g…

…ood, and it's faster

src/org/apache/cassandra/utils/MurmurHash.java

@@ -0,0 +1,77 @@
+/**
+ * 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.cassandra.utils;
+
+/**
+ * This is a very fast, non-cryptographic hash suitable for general hash-based
+ * lookup.  See http://murmurhash.googlepages.com/ for more details.
+ * 
+ * <p>The C version of MurmurHash 2.0 found at that site was ported
+ * to Java by Andrzej Bialecki (ab at getopt org).</p>
+ */
+public class MurmurHash {  
+  public int hash(byte[] data, int length, int seed) {
+    int m = 0x5bd1e995;
+    int r = 24;
+
+    int h = seed ^ length;
+
+    int len_4 = length >> 2;
+
+    for (int i = 0; i < len_4; i++) {
+      int i_4 = i << 2;
+      int k = data[i_4 + 3];
+      k = k << 8;
+      k = k | (data[i_4 + 2] & 0xff);
+      k = k << 8;
+      k = k | (data[i_4 + 1] & 0xff);
+      k = k << 8;
+      k = k | (data[i_4 + 0] & 0xff);
+      k *= m;
+      k ^= k >>> r;
+      k *= m;
+      h *= m;
+      h ^= k;
+    }
+
+    // avoid calculating modulo
+    int len_m = len_4 << 2;
+    int left = length - len_m;
+
+    if (left != 0) {
+      if (left >= 3) {
+        h ^= (int) data[length - 3] << 16;
+      }
+      if (left >= 2) {
+        h ^= (int) data[length - 2] << 8;
+      }
+      if (left >= 1) {
+        h ^= (int) data[length - 1];
+      }
+
+      h *= m;
+    }
+
+    h ^= h >>> 13;
+    h *= m;
+    h ^= h >>> 15;
+
+    return h;
+  }
+}