Added YAML test

Author: Mikep86

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/RankRRFFeatures.java

@@ -13,6 +13,7 @@
 import java.util.Set;
 
 import static org.elasticsearch.search.retriever.CompoundRetrieverBuilder.INNER_RETRIEVERS_FILTER_SUPPORT;
+import static org.elasticsearch.xpack.rank.linear.MinMaxScoreNormalizer.LINEAR_RETRIEVER_MINMAX_SINGLE_DOC_FIX;
 
 public class RankRRFFeatures implements FeatureSpecification {
 
@@ -25,6 +26,6 @@ public Set<NodeFeature> getFeatures() {
 
     @Override
     public Set<NodeFeature> getTestFeatures() {
-        return Set.of(INNER_RETRIEVERS_FILTER_SUPPORT);
+        return Set.of(INNER_RETRIEVERS_FILTER_SUPPORT, LINEAR_RETRIEVER_MINMAX_SINGLE_DOC_FIX);
     }
 }

x-pack/plugin/rank-rrf/src/main/java/org/elasticsearch/xpack/rank/linear/MinMaxScoreNormalizer.java

@@ -8,11 +8,13 @@
 package org.elasticsearch.xpack.rank.linear;
 
 import org.apache.lucene.search.ScoreDoc;
+import org.elasticsearch.features.NodeFeature;
 
 public class MinMaxScoreNormalizer extends ScoreNormalizer {
-
     public static final MinMaxScoreNormalizer INSTANCE = new MinMaxScoreNormalizer();
 
+    public static final NodeFeature LINEAR_RETRIEVER_MINMAX_SINGLE_DOC_FIX = new NodeFeature("linear_retriever.minmax_single_doc_fix");
+
     public static final String NAME = "minmax";
 
     private static final float EPSILON = 1e-6f;

x-pack/plugin/rank-rrf/src/yamlRestTest/resources/rest-api-spec/test/linear/10_linear_retriever.yml

@@ -1102,3 +1102,36 @@ setup:
               ]
               rank_window_size: -10
   - match: { status: 400 }
+
+---
+"minmax normalization properly handles a single doc result set":
+  - requires:
+      cluster_features: [ "linear_retriever.minmax_single_doc_fix" ]
+      reason: "Fix bug where minmax normalizer would emit unnormalized score when handling a single doc result set"
+
+  - do:
+      search:
+        index: test
+        body:
+          retriever:
+            linear:
+              retrievers: [
+                {
+                  retriever: {
+                    standard: {
+                      query: {
+                        term: {
+                          "keyword": {
+                            "value": "one"
+                          }
+                        }
+                      }
+                    }
+                  },
+                  normalizer: "minmax"
+                }
+              ]
+
+  - match: { hits.total.value: 1 }
+  - match: { hits.hits.0._id: "1" }
+  - match: { hits.hits.0._score: 1.0 }