@CompSciFact

Daily tweets about computer science and related stuff @JohnDCook.

JohnDCook.com
Joined November 2010
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“Most of the problems in writing software come from not knowing what depends on what.” — Stuart Feldman, creator of make (1976)
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'Writing code is often a constant struggle against distraction.' -- Joy of Clojure
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Classic cryptography: The Playfair cipher johndcook.com/blog/2023/02/2…

Playfair cipher

The Playfair cipher, invented by Wheatstone, was the first encryption method to encrypt text two letters at a time. Symmetry makes it easy to use and to break.

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Fitting a regular expression to a list of words johndcook.com/blog/2026/07/1…

Fitting a regular expression to a list of words

Creating a regular expression that matches a given list of words. Improving on brute force. Example compressing all HCPCS codes to a regex.

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Minimum criteria for a language to be Turing complete cs.stackexchange.com/questio…
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Cardinal rule of floating point arithmetic: If x and y agree to n bits, then you can lose up to n bits of precision computing x-y.
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Floor, ceiling, bracket johndcook.com/blog/2021/04/1…

Iverson's floor replaced Gauss's bracket

The notation [x] reigned for nearly two centuries before being replaced by ⌊x⌋. Why the latter is much better and what else we might learn from its inventor.

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'It has long been my personal view that the separation of practical and theoretical work is artificial and injurious.' -- Christopher Strachey
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Algorithms to find which words can be spelled with chemical symbols. From O(n) to O(2^n). johndcook.com/blog/2026/10/1…
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Alan Turing Found alluring Machines whose only fault Was that they would not halt. -- Karl David
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AI-generated ASCII diagrams johndcook.com/blog/2026/08/2…
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NIST curves exonerated Giving the backstory to the recent discovery of how the curves were seeded johndcook.com/blog/2026/10/1…

Cloud of suspicion lifted from NIST curves

NIST published a set of elliptic curves for cryptography in 2000. The seeds for the curves were supplied by the NSA, and cryptographers suspected the agency may have installed backdoors in the...

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'The fastest algorithm can frequently be replaced by one that is almost as fast and much easier to understand.' -- Douglas W. Jones
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C++ templates are Turing-complete matt.might.net/articles/c++-…
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Mathematics for Computer Science cs.princeton.edu/courses/arc… [339 page pdf]
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Eight fallacies of declarative computing johndcook.com/blog/2012/11/3…

8 fallacies of declarative programming

Erik Meijer listed eight fallacies of declarative programming in his keynote address at YOW in Melbourne this morning: Exceptions do not exist. Statistics are precise. Memory is infinite. There are...

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Computer Science retweeted
Anthony Bonato
@Anthony_BonatoOct 14
It's Halloween season again and as a mathematician I'm obliged to post this. I don't make the rules
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“Modern cryptanalysis is possible primarily because people make mistakes.” — Bruce Schneier
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Kerkhoffs' Principle: The security of a cryptosystem should not depend on keeping the cryptographic algorithm secret.
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Privacy implications of hashing database attributes johndcook.com/blog/2023/03/2…

Privacy and Hashing Data | Rainbow Tables | Leaking Information

Hashing data may not adequately protect privacy, especially when applied to attributes rather than keys. Attributes are especially vulnerable to rainbow tables.

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