Simon Willison’s Weblog

SQLite Internals: Pages & B-trees (via) Ben Johnson provides a delightfully clear introduction to SQLite internals, describing the binary format used to store rows on disk and how SQLite uses 4KB pages for both row storage and for the b-trees used to look up records. # 27th July 2022, 2:57 pm

Sha256 Algorithm Explained (via) Absolutely beautiful interactive animated explanation by Domingo Martin of the SHA256 hashing algorithm. # 7th February 2022, 7:27 pm

The humble hash aggregate (via) Today I learned that “hash aggregate” is the name for the algorithm where you split a list of tuples on a common key, run an aggregation against each resulting group and combine the results back together again—I’d previously thought if this in terms of map/reduce but hash aggregate is a much older term used widely by SQL engines—I’ve seen it come up in PostgreSQL explain query output (for GROUP BY) before but didn’t know what it meant. # 6th June 2021, 4:03 pm

I was wrong. CRDTs are the future (via) Joseph Gentle has been working on collaborative editors since being a developer on Google Wave back in 2010, later building ShareJS. He’s used Operational Transforms throughout, due to their performance and memory benefits over CRDTs (Conflict-free replicated data types)—but the latest work in that space from Martin Kleppmann and other researchers has seen him finally switch allegiance to these newer algorithms. As a long-time fan of collaborative editing (ever since the Hydra/SubEthaEdit days) I thoroughly enjoyed this as an update on how things have evolved over the past decade. # 28th September 2020, 9:03 pm

Creating Shazam in Java. Using a Fast Fourier Transformation. # 22nd September 2010, 9:39 pm

OpenStreetMap: QuadTiles. Fascinating explanation of a proposal for replacing lat, lon pairs in the OpenStreetMap database with a QuadTile-based addressing system. # 10th September 2009, 3:54 pm

How to Build a Popularity Algorithm You can be Proud of. Filed for future reference. # 6th September 2009, 9:25 pm

Visualising Sorting Algorithms. Aldo Cortesi dislikes animations of sorting algorithms, so he designed a beautiful technique for statically visualising them instead (using Python and Cairo to generate the images). # 14th April 2009, 8:55 am

Neil Fraser: Differential Synchronization. Paper describing a robust method for “keeping two or more copies of the same document synchronized with each other in real-time”, over a variable network connection using clever diff algorithms. # 24th January 2009, 11:57 pm

Animated Sorting Algorithms (via) JavaScript animations of various sorting algorithms, running against four different initial conditions (random, nearly ordered, reversed and few unique). I wish I’d had this during my computer science degree. # 21st October 2008, 12:17 am

Core Techniques and Algorithms in Game Programming. Scarily detailed online book on games programming, including 2D and 3D graphics, AI, multiplayer network code, indoor and outdoor rendering, character animation and much more. UPDATE: Removed the original link, which appeared to be a pirated copy. # 1st May 2008, 12:26 am

Google Maps API gets clickable polylines and polygons. Interesting explanation of how they optimised calculating the distance to the nearest point on a polyline. # 8th September 2007, 12:58 pm