Jimmy He

Billing and concurrency fixes for Cal.diy, a 48k-star open source fork of Cal.com.

• Fixed a billing bug where priced add-on charges were dropped for additional seat bookers on paid seated events; traced it to missing bookingFields and locale propagation into the payment handler and patched createNewSeat.ts (PR #29639, merged).
• Proposed a fix for a race condition where concurrent booking requests could bypass limits, using row-level locks (SELECT … FOR UPDATE) so limits are checked atomically, adding a double-check pattern that filters invalid requests before opening a transaction to avoid lock contention, and refactoring Prisma queries to accept transactional clients while keeping type safety across the monorepo (PR #29646, under review).

Karaoke room booking platform for managing reservations, room availability, pricing surcharges, and real-time updates.

• Rebuilt an early prototype into a deployed room booking web app using FastAPI and PostgreSQL, designed to replace the paper-and-calendar scheduling used at a family member's karaoke business.
• Prevented double-bookings, including when an existing reservation is moved to a different room or time, by designing database validations and resource locking rules that reject overlapping requests.
• Created a database compatibility layer to translate SQL queries, allowing the system to run on lightweight SQLite during local testing and seamlessly scale to cloud PostgreSQL in production.

Full-stack predictive analytics platform combining machine learning classifiers with interactive analytics for customer churn tracking.

• Engineered a full-stack machine learning analytics platform for NUU Mobile, enabling stakeholders to upload datasets and generate real-time churn predictions through a React dashboard.
• Built a concurrent REST API in Flask that decoupled heavy ML training workloads by spawning asynchronous background processes to maintain responsiveness.
• Designed an integration contract enabling Python XGBoost models to execute safely on the backend while the React UI consumed live JSON status updates.

A modular Java RPG engine focused on scalable game architecture, extensibility, and maintainable object-oriented design.

• Designed a modular engine across 40+ classes using Strategy, Factory, and Observer patterns.
• Implemented a finite state machine to coordinate combat and transaction workflows.
• Built data-driven parsers to load entities/configuration without recompiling core logic.