Make sort order enum writable.

Related to #15178

core/src/main/java/org/elasticsearch/search/sort/SortOrder.java

@@ -19,12 +19,19 @@
 
 package org.elasticsearch.search.sort;
 
+import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.common.io.stream.Writeable;
+
+import java.io.IOException;
+import java.util.Locale;
+
 /**
  * A sorting order.
  *
  *
  */
-public enum SortOrder {
+public enum SortOrder implements Writeable<SortOrder> {
     /**
      * Ascending order.
      */
@@ -42,5 +49,30 @@ public enum SortOrder {
         public String toString() {
             return "desc";
         }
+    };
+    
+    public static final SortOrder DEFAULT = DESC;
+    private static final SortOrder PROTOTYPE = DEFAULT;
+
+    @Override
+    public SortOrder readFrom(StreamInput in) throws IOException {
+        int ordinal = in.readVInt();
+        if (ordinal < 0 || ordinal >= values().length) {
+            throw new IOException("Unknown SortOrder ordinal [" + ordinal + "]");
+        }
+        return values()[ordinal];
+    }
+
+    public static SortOrder readOrderFrom(StreamInput in) throws IOException {
+        return PROTOTYPE.readFrom(in);
+    }
+
+    @Override
+    public void writeTo(StreamOutput out) throws IOException {
+        out.writeVInt(this.ordinal());
+    }
+
+    public static SortOrder fromString(String op) {
+        return valueOf(op.toUpperCase(Locale.ROOT));
     }
 }

core/src/test/java/org/elasticsearch/search/sort/SortOrderTests.java

@@ -0,0 +1,52 @@
+/*
+ * Licensed to Elasticsearch under one or more contributor
+ * license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright
+ * ownership. Elasticsearch 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.elasticsearch.search.sort;
+
+import org.elasticsearch.common.io.stream.BytesStreamOutput;
+import org.elasticsearch.common.io.stream.StreamInput;
+import org.elasticsearch.test.ESTestCase;
+
+import static org.hamcrest.Matchers.equalTo;
+
+public class SortOrderTests extends ESTestCase {
+
+    /** Check that ordinals remain stable as we rely on them for serialisation. */
+    public void testDistanceUnitNames() {
+        assertEquals(0, SortOrder.ASC.ordinal());
+        assertEquals(1, SortOrder.DESC.ordinal());
+    }
+
+    public void testReadWrite() throws Exception {
+        for (SortOrder unit : SortOrder.values()) {
+          try (BytesStreamOutput out = new BytesStreamOutput()) {
+              unit.writeTo(out);
+              try (StreamInput in = StreamInput.wrap(out.bytes())) {
+                  assertThat("Roundtrip serialisation failed.", SortOrder.readOrderFrom(in), equalTo(unit));
+              }
+          }
+        }
+    }
+
+    public void testFromString() {
+        for (SortOrder unit : SortOrder.values()) {
+            assertThat("Roundtrip string parsing failed.", SortOrder.fromString(unit.toString()), equalTo(unit));
+        }
+    }
+}