2026年7月12日

Credit Card Generator - Luhn-valid test card numbers for development

Test Card Numbers That Actually Pass Luhn: A Credit Card Generator Walkthrough

By xiezixing 2026年7月12日

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.

VAT, GST, and Sales Tax: Same arithmetic, three different regimes

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VAT, GST, Sales Tax: The Calculator That Knows Which Number Is the Price

By xiezixing 2026年7月12日

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.

API Contract Stress Tester cover poster — every OpenAPI constraint implies its own negation.

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API Contract Stress Tester: Every Constraint Implies Its Own Negation

By xiezixing 2026年7月12日

A boundary-value grammar that walks your OpenAPI document field by field — every documented constraint implies its own negation, and the tool turns that implication graph into a JSON test plan you can replay.

Accessibility Audit Report Generator — Cover Poster

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Accessibility Audit Report Generator: 6 Categories, 4 Severities, 1 Readable PDF

By xiezixing 2026年7月12日

A WCAG audit that ships a PDF a product manager can read in three minutes — 6 categories, 4 severity levels, 3 report sections, 1 shipped artifact.

X.509 Certificate Decoder — read PEM certs in 30 seconds

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X.509 Certificate Decoder: 8 Fields, 4 Traps, 5 Sibling Tools for Reading PEM

By xiezixing 2026年7月12日

Parse PEM certificates into subject, issuer, validity, SANs, fingerprint, and key usage — what openssl x509 prints, rendered without the command line.

GeoJSON to KML converter cover showing coordinate translation workflow

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GeoJSON to KML Converter: The Two Map Formats That Don’t Speak the Same Language

By xiezixing 2026年7月12日

GeoJSON lives in your browser, KML lives in desktop GIS. The translation rules between them are subtle — altitude, ExtendedData, and antimeridian crossings quietly lose fidelity if you get them wrong.