Added a test illustrating Hash code & segment distribution spec

Conflicts:
	pom.xml

pom.xml

@@ -152,6 +152,13 @@
             <version>1.0</version>
             <scope>provided</scope>
         </dependency>
+
+        <dependency>
+            <groupId>net.openhft</groupId>
+            <artifactId>zero-allocation-hashing</artifactId>
+            <version>0.2</version>
+            <scope>test</scope>
+        </dependency>
     </dependencies>
 
     <build>

src/test/java/net/openhft/chronicle/map/KeySegmentDistributionTest.java

@@ -0,0 +1,90 @@
+/*
+ * Copyright 2015 Higher Frequency Trading
+ *
+ * http://www.higherfrequencytrading.com
+ *
+ * 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 net.openhft.chronicle.map;
+
+import net.openhft.hashing.LongHashFunction;
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.nio.charset.StandardCharsets;
+import java.util.concurrent.ThreadLocalRandom;
+
+public class KeySegmentDistributionTest {
+
+    @Test
+    public void keySegmentDistributionTestOneSegment() {
+        keySegmentDistributionTest(1000, 1);
+    }
+
+    @Test
+    public void keySegmentDistributionTestPowerOfTwoSegments() {
+        keySegmentDistributionTest(1000, 4);
+    }
+
+    @Test
+    public void keySegmentDistributionTestOddSegments() {
+        keySegmentDistributionTest(1000, 5);
+    }
+
+
+    public void keySegmentDistributionTest(int size, int segments) {
+        ChronicleMap<CharSequence, Integer> map = ChronicleMapBuilder
+                .of(CharSequence.class, Integer.class)
+                .actualSegments(segments)
+                // TODO problems with computing proper number of segments/chunks
+                // when I write this without `* 2` I expect not to have ISE "segment is full"...
+                .entries(size * 2)
+                .averageKeySize(10)
+                .create();
+
+        byte[] keyBytes = new byte[10];
+        ThreadLocalRandom random = ThreadLocalRandom.current();
+        for (int i = 0; i < size; i++) {
+            random.nextBytes(keyBytes);
+            String key = new String(keyBytes, StandardCharsets.US_ASCII);
+            long hash = LongHashFunction.murmur_3().hashBytes(StandardCharsets.UTF_8.encode(key));
+            int segmentIndex = (((int) hash) & Integer.MAX_VALUE) % segments;
+            // Put the segment index as a value to the map
+            map.put(key, segmentIndex);
+        }
+        // The following loop checks that internally hash code and segment index is chosen
+        // the same way as we explicitly computed in this test. Since ChMap iteration is segment
+        // by segment, we expect to see the sequence of values 0, 0, ... 0, 1, ..1, 2, ..2, 3, ..3
+        // or opposite -- 3, 3, ... 3, 2, ..2, 1, ..1, 0, ..0
+        int currentSegment = -1;
+        boolean ascendingDirection = false;
+        for (Integer entrySegment : map.values()) {
+            if (currentSegment == -1) {
+                currentSegment = entrySegment;
+                if (currentSegment == 0) {
+                    ascendingDirection = true;
+                }
+            } else {
+                if (ascendingDirection) {
+                    Assert.assertTrue(entrySegment >= currentSegment);
+                    currentSegment = entrySegment;
+                } else {
+                    // descending iteration direction
+                    Assert.assertTrue(entrySegment <= currentSegment);
+                    currentSegment = entrySegment;
+                }
+            }
+        }
+    }
+}