Productionizing a Sliding-Window String Algorithm

Context: You have implemented a correct sliding-window algorithm for a classic string task (e.g., longest substring without repeating characters). You now need to evolve it into production-ready code used both as a library function and as a service.

Requirements:

1. Define a clear API boundary for both a library function and a service endpoint. Specify inputs/outputs, error handling, and how "character" is defined (bytes, code points, or grapheme clusters).
2. Validate inputs and character encodings. Specify accepted encodings, normalization behavior, and how invalid sequences are handled.
3. Handle very long inputs and memory limits. Describe streaming/iterative approaches, memory caps, and failure/degradation modes.
4. Ensure thread safety in a shared service. Address shared state, object pooling, and per-request isolation.
5. Add logging and metrics (latency, error rates, input sizes). Include tracing and sampling.
6. Design tests: unit, property-based, fuzz, concurrency, performance/soak, and regression tests.
7. Document complexity guarantees and practical performance characteristics.
8. Consider internationalization (Unicode) and define the unit of iteration; address normalization and grapheme segmentation.
9. Address security and resource-exhaustion risks (input limits, timeouts, decompression bombs, PII in logs).
10. Outline deployment, rollback, monitoring, and alerting plans with SLOs.