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Notes on product engineering, fintech systems, learning, and the decisions behind the work.

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Jul 23, 2026Fintech / Payments

Never Store Money in a Float — Building Payments Where 0.1 + 0.2 Isn't 0.3 (TS / Java / Python / C / Go)

Open your browser's dev tools, a Python REPL, whatever you have handy, and type this: It doesn't equal 0.3. It's not a bug. You get the same result in nearly every language. Payments is my d...

paymentsfloating-pointdecimalrounding

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Jul 21, 2026Ai Engineering

Adding a Japanese Voice Chat AI to My Portfolio with VOICEVOX × Next.js

I added a voice chat feature to my portfolio site (meetyudai.com): you talk into the microphone, and it answers back in a natural Japanese voice. The speech synthesis is powered by VOICEVOX,...

voicevoxttsnextjscloud-run

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Jul 21, 2026Ai Engineering

I Built a Personal Learning Pipeline Where AI Writes Me a Study Article Every Day

I'm a Japanese software engineer working in the Bay Area. After several years in payments at PayPal, I moved to Atlas, a fintech startup, where I still spend my days close to money movement,...

aiopenaiautomationfirebase

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Jul 21, 2026Ai Engineering

I Built My Own NotebookLM-Style Article-to-Podcast Pipeline

The first time I heard a NotebookLM audio overview — two AI hosts having a genuinely listenable conversation about a document I'd uploaded — my reaction wasn't "wow, magic." It was "wait, I...

aittsopenaiautomation

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Jul 21, 2026Applied Algorithms

Spaced Repetition, Flashcards, and Auto-Generated Quizzes: Turning Learning Science into Code

I used to finish a technical article, nod along with every paragraph, and be unable to explain a single claim from it two weeks later. Not roughly unable — completely unable, as if the readi...

spaced-repetitionalgorithmslearningai

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