Fix negative scores returned from multi_match query with `cross_fie…

…lds` (opensearch-project#13829)

Under specific circumstances, when using `cross_fields` scoring on a
`multi_match` query, we can end up with negative scores from the inverse
document frequency calculation in the BM25 formula.

Specifically, the IDF is calculated as:

```
log(1 + (N - n + 0.5) / (n + 0.5))
```

where `N` is the number of documents containing the field and `n` is the
number of documents containing the given term in the field. Obviously,
`n` should always be less than or equal to `N`.

Unfortunately, `cross_fields` makes up a new value for `n` and tries to
use it across all fields.

This change finds the (nonzero) value of `N` for each field and uses that as an
upper bound for the new value of `n`.

Signed-off-by: Michael Froh <froh@amazon.com>

---------

Signed-off-by: Michael Froh <froh@amazon.com>

CHANGELOG.md

@@ -46,6 +46,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fix get field mapping API returns 404 error in mixed cluster with multiple versions ([#13624](https://github.com/opensearch-project/OpenSearch/pull/13624))
 - Allow clearing `remote_store.compatibility_mode` setting ([#13646](https://github.com/opensearch-project/OpenSearch/pull/13646))
 - Fix ReplicaShardBatchAllocator to batch shards without duplicates ([#13710](https://github.com/opensearch-project/OpenSearch/pull/13710))
+- Don't return negative scores from `multi_match` query with `cross_fields` type  ([#13829](https://github.com/opensearch-project/OpenSearch/pull/13829))
 - Pass parent filter to inner hit query ([#13903](https://github.com/opensearch-project/OpenSearch/pull/13903))
 
 ### Security

rest-api-spec/src/main/resources/rest-api-spec/test/search/50_multi_match.yml

@@ -0,0 +1,35 @@
+"Cross fields do not return negative scores":
+  - skip:
+      version: " - 2.99.99"
+      reason: "This fix is in 2.15. Until we do the BWC dance, we need to skip all pre-3.0, though."
+  - do:
+      index:
+        index: test
+        id: 1
+        body: { "color" : "orange red yellow" }
+  - do:
+      index:
+        index: test
+        id: 2
+        body: { "color": "orange red purple", "shape": "red square" }
+  - do:
+      index:
+        index: test
+        id: 3
+        body: { "color" : "orange red yellow purple" }
+  - do:
+      indices.refresh: { }
+  - do:
+      search:
+        index: test
+        body:
+          query:
+            multi_match:
+              query: "red"
+              type: "cross_fields"
+              fields: [ "color", "shape^100"]
+              tie_breaker: 0.1
+          explain: true
+  - match: { hits.total.value: 3 }
+  - match: { hits.hits.0._id: "2" }
+  - gt: { hits.hits.2._score: 0.0 }

server/src/main/java/org/apache/lucene/queries/BlendedTermQuery.java

@@ -120,6 +120,7 @@ protected void blend(final TermStates[] contexts, int maxDoc, IndexReader reader
         }
         int max = 0;
         long minSumTTF = Long.MAX_VALUE;
+        int[] docCounts = new int[contexts.length];
         for (int i = 0; i < contexts.length; i++) {
             TermStates ctx = contexts[i];
             int df = ctx.docFreq();
@@ -133,6 +134,7 @@ protected void blend(final TermStates[] contexts, int maxDoc, IndexReader reader
                 // we need to find out the minimum sumTTF to adjust the statistics
                 // otherwise the statistics don't match
                 minSumTTF = Math.min(minSumTTF, reader.getSumTotalTermFreq(terms[i].field()));
+                docCounts[i] = reader.getDocCount(terms[i].field());
             }
         }
         if (maxDoc > minSumTTF) {
@@ -175,7 +177,11 @@ protected int compare(int i, int j) {
             if (prev > current) {
                 actualDf++;
             }
-            contexts[i] = ctx = adjustDF(reader.getContext(), ctx, Math.min(maxDoc, actualDf));
+            // Per field, we want to guarantee that the adjusted df does not exceed the number of docs with the field.
+            // That is, in the IDF formula (log(1 + (N - n + 0.5) / (n + 0.5))), we need to make sure that n (the
+            // adjusted df) is never bigger than N (the number of docs with the field).
+            int fieldMaxDoc = Math.min(maxDoc, docCounts[i]);
+            contexts[i] = ctx = adjustDF(reader.getContext(), ctx, Math.min(fieldMaxDoc, actualDf));
             prev = current;
             sumTTF += ctx.totalTermFreq();
         }

test/framework/src/main/java/org/opensearch/test/rest/yaml/section/Assertion.java

@@ -37,6 +37,8 @@
 import java.io.IOException;
 import java.util.Map;
 
+import static org.junit.Assert.fail;
+
 /**
  * Base class for executable sections that hold assertions
  */
@@ -79,6 +81,41 @@ protected final Object getActualValue(ClientYamlTestExecutionContext executionCo
         return executionContext.response(field);
     }
 
+    static Object convertActualValue(Object actualValue, Object expectedValue) {
+        if (actualValue == null || expectedValue.getClass().isAssignableFrom(actualValue.getClass())) {
+            return actualValue;
+        }
+        if (actualValue instanceof Number && expectedValue instanceof Number) {
+            if (expectedValue instanceof Float) {
+                return Float.parseFloat(actualValue.toString());
+            } else if (expectedValue instanceof Double) {
+                return Double.parseDouble(actualValue.toString());
+            } else if (expectedValue instanceof Integer) {
+                return Integer.parseInt(actualValue.toString());
+            } else if (expectedValue instanceof Long) {
+                return Long.parseLong(actualValue.toString());
+            }
+        }
+        // Force a class cast exception here, so developers can flesh out the above logic as needed.
+        try {
+            expectedValue.getClass().cast(actualValue);
+        } catch (ClassCastException e) {
+            fail(
+                "Type mismatch: Expected value ("
+                    + expectedValue
+                    + ") has type "
+                    + expectedValue.getClass()
+                    + ". "
+                    + "Actual value ("
+                    + actualValue
+                    + ") has type "
+                    + actualValue.getClass()
+                    + "."
+            );
+        }
+        return actualValue;
+    }
+
     @Override
     public XContentLocation getLocation() {
         return location;

test/framework/src/main/java/org/opensearch/test/rest/yaml/section/GreaterThanAssertion.java

@@ -71,6 +71,7 @@ public GreaterThanAssertion(XContentLocation location, String field, Object expe
     @Override
     protected void doAssert(Object actualValue, Object expectedValue) {
         logger.trace("assert that [{}] is greater than [{}] (field: [{}])", actualValue, expectedValue, getField());
+        actualValue = convertActualValue(actualValue, expectedValue);
         assertThat(
             "value of [" + getField() + "] is not comparable (got [" + safeClass(actualValue) + "])",
             actualValue,

test/framework/src/main/java/org/opensearch/test/rest/yaml/section/GreaterThanEqualToAssertion.java

@@ -72,6 +72,7 @@ public GreaterThanEqualToAssertion(XContentLocation location, String field, Obje
     @Override
     protected void doAssert(Object actualValue, Object expectedValue) {
         logger.trace("assert that [{}] is greater than or equal to [{}] (field: [{}])", actualValue, expectedValue, getField());
+        actualValue = convertActualValue(actualValue, expectedValue);
         assertThat(
             "value of [" + getField() + "] is not comparable (got [" + safeClass(actualValue) + "])",
             actualValue,

test/framework/src/main/java/org/opensearch/test/rest/yaml/section/LessThanAssertion.java

@@ -72,6 +72,7 @@ public LessThanAssertion(XContentLocation location, String field, Object expecte
     @Override
     protected void doAssert(Object actualValue, Object expectedValue) {
         logger.trace("assert that [{}] is less than [{}] (field: [{}])", actualValue, expectedValue, getField());
+        actualValue = convertActualValue(actualValue, expectedValue);
         assertThat(
             "value of [" + getField() + "] is not comparable (got [" + safeClass(actualValue) + "])",
             actualValue,

test/framework/src/main/java/org/opensearch/test/rest/yaml/section/LessThanOrEqualToAssertion.java

@@ -72,6 +72,7 @@ public LessThanOrEqualToAssertion(XContentLocation location, String field, Objec
     @Override
     protected void doAssert(Object actualValue, Object expectedValue) {
         logger.trace("assert that [{}] is less than or equal to [{}] (field: [{}])", actualValue, expectedValue, getField());
+        actualValue = convertActualValue(actualValue, expectedValue);
         assertThat(
             "value of [" + getField() + "] is not comparable (got [" + safeClass(actualValue) + "])",
             actualValue,