Implement Batched Undo/Redo Layer

You are implementing a simplified document layer for a design tool. The layer stores properties as a map<string, string>.

Implement a class that supports the following operations:

• beginBatch(): start a batch of edits.
• setProperty(key, value): set or overwrite a property inside the current batch.
• endBatch(): commit the batch as one undoable action.
• undo(): revert the most recently committed batch.
• redo(): reapply the most recently undone batch.

Requirements:

1. A batch may update the same key multiple times.
2. undo() must restore the exact document state from before that batch.
3. redo() must restore the exact document state after that batch.
4. If a property did not exist before the batch, then undo() should remove that property.
5. If undo() has been called and then a new batch is committed, the redo history must be cleared.
6. Follow-up: make the history memory-efficient. Instead of storing every intermediate edit in a batch, compact the batch so that each key appears at most once in history while still preserving enough information for correct undo() and redo().

A useful compact record for one key is:

Change {
 string key;
 string value; // final value after the batch
 string old_value; // value before the batch
 bool has_old_value; // whether the key existed before the batch
}

Explain how you would build and use such a compact history so that repeated updates to the same key inside one batch do not waste memory.