Add back FindBatchContainingMessageIndex, was removed in the nitro la…

…test head

staker/stateless_block_validator.go

@@ -112,6 +112,39 @@ func GlobalStatePositionsAtCount(
 	return startPos, GlobalStatePosition{batch, posInBatch + 1}, nil
 }
 
+func FindBatchContainingMessageIndex(
+	tracker InboxTrackerInterface, pos arbutil.MessageIndex, high uint64,
+) (uint64, error) {
+	var low uint64
+	// Iteration preconditions:
+	// - high >= low
+	// - msgCount(low - 1) <= pos implies low <= target
+	// - msgCount(high) > pos implies high >= target
+	// Therefore, if low == high, then low == high == target
+	for high > low {
+		// Due to integer rounding, mid >= low && mid < high
+		mid := (low + high) / 2
+		count, err := tracker.GetBatchMessageCount(mid)
+		if err != nil {
+			return 0, err
+		}
+		if count < pos {
+			// Must narrow as mid >= low, therefore mid + 1 > low, therefore newLow > oldLow
+			// Keeps low precondition as msgCount(mid) < pos
+			low = mid + 1
+		} else if count == pos {
+			return mid + 1, nil
+		} else if count == pos+1 || mid == low { // implied: count > pos
+			return mid, nil
+		} else { // implied: count > pos + 1
+			// Must narrow as mid < high, therefore newHigh < lowHigh
+			// Keeps high precondition as msgCount(mid) > pos
+			high = mid
+		}
+	}
+	return low, nil
+}
+
 type ValidationEntryStage uint32
 
 const (