Overview
Calculator Pro is a satirical, production-quality full-stack web application with a deadpan premise: **every mathematical operation is a paid microtransaction**. Addition and subtraction are free. Multiplication costs ₹49. Division costs ₹79. The equals button (`=`) costs ₹2,999.
Behind the comedic concept is an authentic, full-stack architecture built to explore modern payment workflows, resilient authentication, and parser theory.
Key Features
**Pure Expression Engine**: Built with a custom tokenizer and Shunting-Yard parser (zero `eval`). Operates as a pure engine gated at the boundaries—throwing a strict `FeatureLockedError` the moment an un-purchased operator is evaluated.**Razorpay Payment Integration**: Full end-to-end payment gateway lifecycle with server-side signature verification using constant-time HMAC comparison, order creation, and idempotent webhook handlers.**Auth.js (NextAuth v5)**: OAuth authentication supporting GitHub and Google sign-ins, persisting unlocked features and calculation histories per user across sessions.**Database Architecture**: PostgreSQL hosted on Neon, managed through Prisma ORM with strict schema migrations and seeding scripts.**Satirical Polish**: Clean, modern UI styled with Tailwind CSS v4, CSS custom-property theme toggling, Framer Motion unlock animations, and test-card simulation modes.Architecture & Tech Stack
**Framework**: Next.js 15 (App Router, Server Actions), React 19, TypeScript**Styling**: Tailwind CSS v4, Radix UI primitives, Lucide icons**Database**: PostgreSQL (Neon Serverless) via Prisma ORM**Authentication**: Auth.js / NextAuth v5 (Google + GitHub)**Payments**: Razorpay Node SDK with server-side HMAC validation**Testing**: Vitest + React Testing Library unit test suite for arithmetic engine and purchase gates 