fix: groupBy to correctly remove groups when a key is completely removed (#70)

* fix: index compaction for reduce

* changeset

* repo of bug

* fix groupby bug

* changeset

Author: samwillis

.changeset/odd-garlics-chew.md

@@ -0,0 +1,5 @@
+---
+'@electric-sql/d2mini': patch
+---
+
+fix a bug where groupBy would not remove a group if it's key was completely removed from the stream

packages/d2mini/src/operators/groupBy.ts

@@ -87,6 +87,17 @@ export function groupBy<
     // Then reduce to compute aggregates
     const reduced = withKeysAndValues.pipe(
       reduce((values) => {
+        // Calculate total multiplicity to check if the group should exist
+        let totalMultiplicity = 0
+        for (const [_, multiplicity] of values) {
+          totalMultiplicity += multiplicity
+        }
+
+        // If total multiplicity is 0 or negative, the group should be removed completely
+        if (totalMultiplicity <= 0) {
+          return []
+        }
+
         const result: Record<string, unknown> = {}
 
         // Get the original key from first value in group