Design a Resumable Iterator with Checkpoint and Restore

Design an iterator that can pause and later resume from the same position. At any point, the iterator can produce a compact checkpoint value that represents exactly where it is. That checkpoint may be serialized, stored, or sent elsewhere, and a new iterator built from the same source plus the checkpoint must continue after the last element that was already returned — even if the new iterator is created in an entirely new process. Adopt a test-first workflow: write tests for…