smjn commented on code in PR #17149:
URL: https://github.com/apache/kafka/pull/17149#discussion_r1759247979


##########
share-coordinator/src/main/java/org/apache/kafka/coordinator/share/ShareCoordinatorShard.java:
##########
@@ -537,38 +539,308 @@ private static ShareGroupOffset merge(ShareGroupOffset 
soFar, ShareUpdateValue n
             .setStartOffset(newStartOffset)
             .setLeaderEpoch(newLeaderEpoch)
             .setStateBatches(combineStateBatches(currentBatches, 
newData.stateBatches().stream()
-                .map(PersisterOffsetsStateBatch::from)
-                .collect(Collectors.toCollection(LinkedHashSet::new)), 
newStartOffset))
-            .build();
+                .map(ShareCoordinatorShard::toPersisterStateBatch)
+                .collect(Collectors.toList()), newStartOffset))
+            .build();   
     }
 
     /**
-     * Util method which takes in 2 collections containing {@link 
PersisterOffsetsStateBatch}
+     * Util method which takes in 2 collections containing {@link 
PersisterStateBatch}
      * and the startOffset.
-     * It removes all batches from the 1st collection which have the same 
first and last offset
-     * as the batches in 2nd collection. It then creates a final list of 
batches which contains the
-     * former result and all the batches in the 2nd collection. In set 
notation (A - B) U B (we prefer batches in B
-     * which have same first and last offset in A).
-     * Finally, it removes any batches where the lastOffset < startOffset, if 
the startOffset > -1.
-     * @param currentBatch - collection containing current soft state of 
batches
-     * @param newBatch - collection containing batches in incoming request
+     * This method checks any overlap between current state batches and new 
state batches.
+     * Based on various conditions it creates new non-overlapping records 
preferring new batches.
+     * Finally, it removes any batches where the lastOffset < startOffset, if 
the startOffset > -1 and
+     * merges any contiguous intervals with same state.
+     * @param batchesSoFar - collection containing current soft state of 
batches
+     * @param newBatches - collection containing batches in incoming request
      * @param startOffset - startOffset to consider when removing old batches.
      * @return List containing combined batches
      */
-    private static List<PersisterOffsetsStateBatch> combineStateBatches(
-        Collection<PersisterOffsetsStateBatch> currentBatch,
-        Collection<PersisterOffsetsStateBatch> newBatch,
+    // visibility for testing
+    static List<PersisterStateBatch> combineStateBatches(
+        List<PersisterStateBatch> batchesSoFar,
+        List<PersisterStateBatch> newBatches,
         long startOffset
     ) {
-        currentBatch.removeAll(newBatch);
-        List<PersisterOffsetsStateBatch> batchesToAdd = new 
LinkedList<>(currentBatch);
-        batchesToAdd.addAll(newBatch);
-        // Any batches where the last offset is < the current start offset
-        // are now expired. We should remove them from the persister.
+        List<PersisterStateBatch> combinedList = new 
ArrayList<>(batchesSoFar.size() + newBatches.size());
+        combinedList.addAll(batchesSoFar);
+        combinedList.addAll(newBatches);
+
+        // sort keeping delivery state in mind
+        combinedList.sort(PersisterStateBatch::compareTo);
+
+        return mergeBatches(
+            pruneBatches(
+                combinedList,
+                startOffset
+            )
+        );
+    }
+
+    private static class BatchOverlapState {
+        private final PersisterStateBatch last;
+        private final PersisterStateBatch candidate;
+        private final List<PersisterStateBatch> nonOverlapping;
+        public static final BatchOverlapState SENTINEL = new 
BatchOverlapState(null, null, Collections.emptyList());
+
+        public BatchOverlapState(
+            PersisterStateBatch last,
+            PersisterStateBatch candidate,
+            List<PersisterStateBatch> nonOverlapping
+        ) {
+            this.last = last;
+            this.candidate = candidate;
+            this.nonOverlapping = nonOverlapping;
+        }
+
+        public PersisterStateBatch last() {
+            return last;
+        }
+
+        public PersisterStateBatch candidate() {
+            return candidate;
+        }
+
+        public List<PersisterStateBatch> nonOverlapping() {
+            return nonOverlapping;
+        }
+
+        @Override
+        public boolean equals(Object o) {
+            if (this == o) return true;
+            if (!(o instanceof BatchOverlapState)) return false;
+            BatchOverlapState that = (BatchOverlapState) o;
+            return Objects.equals(last, that.last) && 
Objects.equals(candidate, that.candidate) && Objects.equals(nonOverlapping, 
that.nonOverlapping);
+        }
+
+        @Override
+        public int hashCode() {
+            return Objects.hash(last, candidate, nonOverlapping);
+        }
+    }
+
+    static List<PersisterStateBatch> mergeBatches(List<PersisterStateBatch> 
batches) {
+        if (batches.size() < 2) {
+            return batches;
+        }
+        TreeSet<PersisterStateBatch> sortedBatches = new TreeSet<>(batches);
+        List<PersisterStateBatch> finalBatches = new 
ArrayList<>(batches.size() * 2); // heuristic size
+
+        BatchOverlapState overlapState = getOverlappingState(sortedBatches);
+
+        while (overlapState != BatchOverlapState.SENTINEL) {

Review Comment:
   then we'll make it



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