Design document editor with undo/redo and batching

Design a document-editing layer that supports applying edits and undo. Implement an API with apply(Operation op), undo(), and getText(). Then add transactional batching: beginBatch(), apply(...), commitBatch(), where undo() reverts an entire committed batch atomically. Explain how to optimize batch undo for very large batches (e.g., millions of small operations) in time and memory. Finally, implement redo() that works with both single edits and committed batches, and clarify how redo is invalidated after new edits. Describe data structures, complexity, and edge cases such as overlapping range edits and cursor bookkeeping.