Penumbra Tech
[Projects—]

Live, exercisable evidence.

Each project below is either shipped as a real product you can install, or running right now and you can click into it. A few require sign-in (free, no spam) — that's because they save data to the database, which only makes sense per-user.

16 projects

Products

Shipped, downloadable software — Steam releases and installable Windows apps under the Penumbra family of studios.

3 projects

Client work

Paid engagements with named outside clients.

2 projects
Client work · brand + full-stack

Yūki's Sacred Space

Yūki does distance reiki, spirit guidance, and death doula work, which is about as non-technical as a business gets. The whole job was making it read as legit, and the trust all comes from the engineering. Five free printable CBT workbooks (one file, branded on screen and printer-friendly with real lines to write on, 988 crisis lines on the heavy ones). The same Node + MySQL + nginx stack I'd give a software client. Her Square checkout left exactly where it already worked. And the witchy background (aura, kanji watermark, spiders on threads) built with reduced-motion and checked at real screen sizes instead of just thrown on top. Live at yukis.space.

Node + ExpressMySQLnginx + PM2AccessibilitySEOPrintable CBT workbooks
Live site (yukis.space) link inside the case study
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Infrastructure & SaaS

The plumbing — load testing, payments, deploys, SaaS rescue patterns.

3 projects

Web apps

Full-stack web apps with auth, MySQL, and the seams that always need work.

3 projects

Study tools

Two free, open-source study apps I built and gave away — no signup, no backend, fully offline, progress saved in your browser. A beginner on-ramp into discrete math, and its advanced follow-up for a theory-of-computation course. Launch either in the browser or read the source.

2 projects
① Start here
Free · Open source · Offline · No signup

Toddler to Turing

Discrete math from scratch — from “what does ∀ even mean?” to understanding a Turing machine.

A gentle, gamified on-ramp into the discrete-math foundations of computer science, for anyone the notation has ever scared off. Eight gated modules — Symbols & Notation, Logic, Sets, Functions & Relations, Proofs & Induction, Strings & Languages, Graphs, and Machines & Algorithms — with 146 auto-graded questions, spaced repetition, plain-English “explain like I’m five” walkthroughs, Venn / graph / state-machine diagrams, a printable cheat sheet, and a timed quiz. Fully offline; progress saves in your browser. It ends by handing you off to the Theory of Computation review app.

Vanilla JSKaTeX8 gated modules146 questionsSpaced repetitionOffline-first
② Follow-up
Free · Open source · Offline · Sipser 3rd ed.

Theory of Computation Review

An 859-question spaced-repetition review app for a Theory of Computation course (Sipser, 3rd ed.).

The advanced follow-up: comprehensive final-exam prep covering Sipser Chapters 0–8 plus exam checkpoints — regular & context-free languages, Turing machines, decidability, reducibility, and time & space complexity. Auto-graded questions on a custom Leitner + streak + mastery-gated scheduler, real-world discussion questions, DFA / NFA / TM state-diagram practice, a timed mock-exam mode, and a reference cheat sheet. Built to prove I can reason about computability and complexity, not just stitch libraries. A themed build also runs on this site at /toc/.

Vanilla JSKaTeX859 questionsMastery-gated SRSMock-exam modeDFA / NFA / TM diagrams

Computer science & tools

Pure-craft work: zero-dependency, single-file browser tools built from scratch.

1 project
Single-file web app · Open source

Penumbra Music Visualizer

I wanted a music visualizer that survived bad wifi at a party — no install, no cookies, no signup, no cloud dependency, just one HTML file you double-click. Drag a song onto it and nine reactive visualizers light up (three Penumbra-branded, six conventional spectrum treatments). From-scratch BPM detection via onset-envelope autocorrelation, musical-key detection via Krumhansl-Schmuckler chroma — both cached in localStorage so each song is analyzed once. Tab-audio capture lets it visualize YouTube Music or Spotify; built-in canvas+audio recorder exports a .webm. Zero dependencies, no build step, ~52KB of one index.html.

Single-file HTMLWeb Audio APICanvas 2DFFT / BPM / Key detectionlocalStorage cacheMediaRecorder
Open source · MIT · Download index.html, double-click
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Source & documentation

Where to read the actual code and the public API.

2 projects

See something close to your problem?

Most engagements start with a 30-minute call. Quick sketch of the problem, honest answer on whether I'm the right person for it, and a rough shape of the work.