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Fintech / Payments

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

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Jul 21, 2026Fintech / Payments

I Built My Own Bill-Splitting App — Payments Is My Day Job, and I Wanted One Without Ads

I'm a Japanese software engineer in the Bay Area. I spent years doing payments at PayPal and now do payments at Atlas, a fintech startup. Ledgers, balances, reconciliation, rounding errors —...

personal-toolspaymentsalgorithmsfirebase

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Jul 20, 2026Fintech / Payments

Inside a Contactless Payment—from NFC to Settlement

When a card or phone approaches a terminal, NFC carries payment credentials and transaction-specific cryptographic data. The terminal sends an authorization request through the merchant side...

fintechpaymentscontactless-paymentsnfc

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Jun 28, 2026Fintech / Payments

What Payment Systems Taught Me About State Management

Working on payment systems teaches you how important state management really is. It is easy to think of payments as a simple success or failure flow. A user clicks a button. The payment succ...

paymentsstate-managementreliabilitysystem-design

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