fix: Rank missing field values lower than extant

src/aggregation/metric/mod.rs

@@ -91,7 +91,7 @@ pub struct TopHitsVecEntry {
     /// The document id, composed of segment local `DocId` and segment ordinal.
     pub id: DocAddress,
     /// The sort values of the document, depending on the sort criteria in the request.
-    pub sort: Vec<u64>,
+    pub sort: Vec<Option<u64>>,
 }
 
 /// The top_hits metric aggregation results a list of top hits by sort criteria.

src/aggregation/metric/top_hits.rs

@@ -110,16 +110,23 @@ impl TopHitsAggregation {
 #[derive(Clone, Serialize, Deserialize, Debug)]
 struct ComparableDocFeature {
     /// Stores any u64-mappable feature.
-    value: u64,
+    value: Option<u64>,
     /// Sort order for the doc feature
     order: Order,
 }
 
 impl Ord for ComparableDocFeature {
     fn cmp(&self, other: &Self) -> std::cmp::Ordering {
-        match self.order {
-            Order::Asc => self.value.cmp(&other.value),
-            Order::Desc => other.value.cmp(&self.value),
+        let invert = |cmp: std::cmp::Ordering| match self.order {
+            Order::Asc => cmp,
+            Order::Desc => cmp.reverse(),
+        };
+
+        match (self.value, other.value) {
+            (Some(self_value), Some(other_value)) => invert(self_value.cmp(&other_value)),
+            (Some(_), None) => std::cmp::Ordering::Greater,
+            (None, Some(_)) => std::cmp::Ordering::Less,
+            (None, None) => std::cmp::Ordering::Equal,
         }
     }
 }
@@ -290,10 +297,9 @@ impl SegmentAggregationCollector for SegmentTopHitsCollector {
             .zip(self.inner_collector.req.sort.iter())
             .map(|((c, _), KeyOrder(_, order))| {
                 let order = *order;
-                match c.values_for_doc(doc_id).next() {
-                    Some(value) => ComparableDocFeature { value, order },
-                    // TODO: confirm if this default 0-value is correct
-                    None => ComparableDocFeature { value: 0, order },
+                ComparableDocFeature {
+                    value: c.values_for_doc(doc_id).next(),
+                    order,
                 }
             })
             .collect();
@@ -323,3 +329,54 @@ impl SegmentAggregationCollector for SegmentTopHitsCollector {
         Ok(())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use serde_json::Value;
+
+    use super::{ComparableDoc, ComparableDocFeature, ComparableDocFeatures, Order};
+    use crate::aggregation::agg_req::Aggregations;
+    use crate::aggregation::agg_result::AggregationResults;
+    use crate::aggregation::bucket::tests::get_test_index_from_docs;
+    use crate::aggregation::tests::get_test_index_from_values;
+    use crate::aggregation::AggregationCollector;
+    use crate::query::AllQuery;
+
+    fn invert_order(cmp_feature: ComparableDocFeature) -> ComparableDocFeature {
+        let ComparableDocFeature { value, order } = cmp_feature;
+        let order = match order {
+            Order::Asc => Order::Desc,
+            Order::Desc => Order::Asc,
+        };
+        ComparableDocFeature { value, order }
+    }
+
+    #[test]
+    fn test_comparable_doc_feature() -> crate::Result<()> {
+        let small = ComparableDocFeature {
+            value: Some(1),
+            order: Order::Asc,
+        };
+        let big = ComparableDocFeature {
+            value: Some(2),
+            order: Order::Asc,
+        };
+        let none = ComparableDocFeature {
+            value: None,
+            order: Order::Asc,
+        };
+
+        assert!(small < big);
+        assert!(none < small);
+        assert!(none < big);
+
+        let small = invert_order(small);
+        let big = invert_order(big);
+        let none = invert_order(none);
+
+        assert!(small > big);
+        assert!(none < small);
+        assert!(none < big);
+
+        Ok(())
+    }