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The 32-bit Fingerprint Your Computer Uses a Billion Times a Day: A CRC32 Field Guide
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.
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Five Rules That Decide Whether Your Measurement Is Honest: The Significant Figures Sandbox
Count, round, and propagate significant figures with a sandbox that grades each digit and walks you through the rules — built for chemistry and physics students who keep getting 0.005030 wrong.
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Seven Characters, One Check Digit: Catch a Bad SEDOL Before It Reaches Your Books
A practical guide to SEDOL-7 format rules, weighted mod-10 validation, and where securities-data pipelines should catch a bad identifier.
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Before You Crawl Your Docs, Inventory the Links in the Markdown
Five link forms, two reference passes, and a five-stage audit walk-through for the Markdown Link Extractor on Elysia Tools.
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Chat Transcript to PDF: How to Turn JSON Conversations into Bubble-Style Documents
A working walkthrough of the Chat Transcript to PDF tool — JSON input shape, five layout switches, three raw-transcript pitfalls, and how it sits among the rest of the document-conversion family.
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The Regex That Hangs Your Server at 3 a.m. — A Pre-Commit Guide
Five catastrophic-backtracking categories, four mechanical rewrites, and a 60-second pre-commit check that beats every ReDoS post-mortem.
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Batch Email Validation: From RFC 5321 Syntax to Real-World Deliverability
A batch email validator parses, normalises, and rates an entire list in one pass — and the gaps it leaves behind are exactly the gaps that matter when you decide which addresses to keep, quarantine, or drop.
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Why R-Squared Lies to You When You Add Predictors
Adjusted R-squared is the correction that penalizes each new variable for not pulling its weight. Use it to compare nested models instead of chasing raw R².
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Test Card Numbers That Actually Pass Luhn: A Credit Card Generator Walkthrough
BIN-correct, brand-correct, check-digit-valid test card numbers for Visa, MasterCard, Amex, Discover, JCB, and China UnionPay — without leaking real PII into your fixtures.
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VAT, GST, Sales Tax: The Calculator That Knows Which Number Is the Price
European VAT, Australian GST, and U.S. sales tax use the same five operations. The conventions are different. A working VAT calculator has to commit to inclusive or exclusive, round at the basket, and keep per-line data for the regulator.