COSESAI

Luhn Checksum Field Guide: When One mod-10 Walk Beats Eight Different Identifier Pipelines

By xiezixing 2026年10月11日

A field guide to the four-step Luhn algorithm, the eight identifier families that depend on it, and the three failure modes that silently ship past unit tests.

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UPC/EAN Barcode Validator Field Guide: When the Mod-10 Catches the Bad One

By xiezixing 2026年8月16日

A field guide to the mod-10 weighted checksum baked into the last digit of every GTIN-8, GTIN-12, GTIN-13, and GTIN-14 barcode on every shelf.

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CUSIP Validator Field Guide: The Mod-10 Check That Quietly Catches Bad Securities IDs

By xiezixing 2026年8月16日

Every North American stock, bond, and ETF ticker rides on a nine-character code, and the…

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Mod-11 Checksum Calculator Field Guide: ISBN-10, fodselsnummer, and the X check digit

By xiezixing 2026年8月13日

The Mod-11 check digit is the quiet workhorse behind ISBN-10, ISBT-128 blood bags, Norwegian fødselsnummer,…

CRC32 Checksum tool — IEEE 802.3 32-bit fingerprint for ZIP, PNG, gzip, and Ethernet integrity

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The 32-bit Fingerprint Your Computer Uses a Billion Times a Day: A CRC32 Field Guide

By xiezixing 2026年7月14日

CRC32 is not a hash. It is the cheapest integrity check in computing, and the reason ZIP, PNG, gzip, and Ethernet all know when something went wrong.

Why every national ID has a hidden self-checking code you never see

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Why Every National ID Number Has a Hidden Self-Checking Code You Never See

By xiezixing 2026年6月8日

What an ID is really for Somewhere in the back of a Chinese government building…