Scripting: Make _score in groovy scripts comparable

closes #8828
closes #9094

src/main/java/org/elasticsearch/script/ScoreAccessor.java

@@ -30,7 +30,7 @@ import java.io.IOException;
  * The provided {@link DocLookup} is used to retrieve the score
  * for the current document.
  */
-public final class ScoreAccessor extends Number {
+public final class ScoreAccessor extends Number implements Comparable<Number> {
 
     Scorer scorer;
 
@@ -65,4 +65,9 @@ public final class ScoreAccessor extends Number {
     public double doubleValue() {
         return score();
     }
+
+    @Override
+    public int compareTo(Number o) {
+        return Float.compare(this.score(), o.floatValue());
+    }
 }

src/test/java/org/elasticsearch/script/GroovyScriptTests.java

@@ -117,20 +117,33 @@ public class GroovyScriptTests extends ElasticsearchIntegrationTest {
         client().prepareIndex("test", "doc", "3").setSource("foo", "dog spiders that can eat a dog", "bar", 3).get();
         refresh();
 
-        // _score access
-        SearchResponse resp = client().prepareSearch("test").setQuery(functionScoreQuery(matchQuery("foo", "dog"))
-                .add(scriptFunction("_score", "groovy"))
-                .boostMode(CombineFunction.REPLACE)).get();
+        // doc[] access
+        SearchResponse resp = client().prepareSearch("test").setQuery(functionScoreQuery(matchAllQuery())
+            .add(scriptFunction("doc['bar'].value", "groovy"))
+            .boostMode(CombineFunction.REPLACE)).get();
 
         assertNoFailures(resp);
-        assertSearchHits(resp, "3", "1");
+        assertOrderedSearchHits(resp, "3", "2", "1");
+    }
+    
+    public void testScoreAccess() {
+        client().prepareIndex("test", "doc", "1").setSource("foo", "quick brow fox jumped over the lazy dog", "bar", 1).get();
+        client().prepareIndex("test", "doc", "2").setSource("foo", "fast jumping spiders", "bar", 2).get();
+        client().prepareIndex("test", "doc", "3").setSource("foo", "dog spiders that can eat a dog", "bar", 3).get();
+        refresh();
 
-        // doc[] access
-        resp = client().prepareSearch("test").setQuery(functionScoreQuery(matchAllQuery())
-                .add(scriptFunction("doc['bar'].value", "groovy"))
-                .boostMode(CombineFunction.REPLACE)).get();
+        // _score can be accessed
+        SearchResponse resp = client().prepareSearch("test").setQuery(functionScoreQuery(matchQuery("foo", "dog"))
+            .add(scriptFunction("_score", "groovy"))
+            .boostMode(CombineFunction.REPLACE)).get();
+        assertNoFailures(resp);
+        assertSearchHits(resp, "3", "1");
 
+        // _score is comparable
+        resp = client().prepareSearch("test").setQuery(functionScoreQuery(matchQuery("foo", "dog"))
+            .add(scriptFunction("_score > 0 ? _score : 0", "groovy"))
+            .boostMode(CombineFunction.REPLACE)).get();
         assertNoFailures(resp);
-        assertOrderedSearchHits(resp, "3", "2", "1");
+        assertSearchHits(resp, "3", "1");
     }
 }