@CompSciFact

Daily tweets about computer science and related stuff @JohnDCook.

JohnDCook.com
Joined November 2010
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The halting problem: You cannot write a program that can tell whether an arbitrary program will complete or run forever.
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'Methods and object can no longer be separated.' -- Werner Heisenberg
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Converting between quaternions and rotation matrices johndcook.com/blog/2025/05/0…

Converting between quaternions and rotation matrices

Equations and Python code for going back and forth between quaternion and matrix representations of rotations.

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“Cryptography is a mixture of mathematics and muddle, and without the muddle the mathematics can be used against you.” — Ian Cassels
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How the eMudrha certificate debacle might have happened johndcook.com/blog/2026/10/2…

How the eMudrha certificate debacle might have happened

How might eMudrha generated seven tiny ECC keys

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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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