Some WIP work on prefix comparison.

core/src/main/java/org/apache/spark/util/collection/unsafe/sort/PrefixComparator.java

@@ -17,10 +17,13 @@
 
 package org.apache.spark.util.collection.unsafe.sort;
 
+import org.apache.spark.annotation.Private;
+
 /**
  * Compares 8-byte key prefixes in prefix sort. Subclasses may implement type-specific
  * comparisons, such as lexicographic comparison for strings.
  */
+@Private
 public abstract class PrefixComparator {
   public abstract int compare(long prefix1, long prefix2);
 }

core/src/main/java/org/apache/spark/util/collection/unsafe/sort/PrefixComparators.java

@@ -0,0 +1,47 @@
+/*
+ * 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.spark.util.collection.unsafe.sort;
+
+import org.apache.spark.annotation.Private;
+
+@Private
+public class PrefixComparators {
+  private PrefixComparators() {}
+
+  public static final IntPrefixComparator INTEGER = new IntPrefixComparator();
+
+  static final class IntPrefixComparator extends PrefixComparator {
+    @Override
+    public int compare(long aPrefix, long bPrefix) {
+      int a = (int) aPrefix;
+      int b = (int) bPrefix;
+      return (a < b) ? -1 : (a > b) ? 1 : 0;
+    }
+
+    public long computePrefix(int value) {
+      return value & 0xffffffffL;
+    }
+  }
+
+  static final class LongPrefixComparator extends PrefixComparator {
+    @Override
+    public int compare(long a, long b) {
+      return (a < b) ? -1 : (a > b) ? 1 : 0;
+    }
+  }
+}

core/src/test/java/org/apache/spark/util/collection/unsafe/sort/PrefixComparatorsSuite.java

@@ -0,0 +1,45 @@
+/*
+ * 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.spark.util.collection.unsafe.sort;
+
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+public class PrefixComparatorsSuite {
+
+  private static int genericComparison(Comparable a, Comparable b) {
+    return a.compareTo(b);
+  }
+
+  @Test
+  public void intPrefixComparator() {
+    int[] testData = new int[] { 0, Integer.MIN_VALUE, Integer.MAX_VALUE, 0, 1, 2, -1, -2, 1024};
+    for (int a : testData) {
+      for (int b : testData) {
+        long aPrefix = PrefixComparators.INTEGER.computePrefix(a);
+        long bPrefix = PrefixComparators.INTEGER.computePrefix(b);
+        assertEquals(
+          "Wrong prefix comparison results for a=" + a + " b=" + b,
+          genericComparison(a, b),
+          PrefixComparators.INTEGER.compare(aPrefix, bPrefix));
+
+      }
+    }
+  }
+
+}

sql/core/src/main/scala/org/apache/spark/sql/execution/basicOperators.scala

@@ -18,7 +18,6 @@
 package org.apache.spark.sql.execution
 
 import org.apache.spark.sql.types.StructType
-import org.apache.spark.util.collection.unsafe.sort.PrefixComparator
 import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.rdd.{RDD, ShuffledRDD}
 import org.apache.spark.shuffle.sort.SortShuffleManager
@@ -28,6 +27,7 @@ import org.apache.spark.sql.catalyst.expressions._
 import org.apache.spark.sql.Row
 import org.apache.spark.sql.catalyst.plans.physical._
 import org.apache.spark.util.collection.ExternalSorter
+import org.apache.spark.util.collection.unsafe.sort.PrefixComparator
 import org.apache.spark.util.{CompletionIterator, MutablePair}
 import org.apache.spark.{HashPartitioner, SparkEnv}