Robert A. Uhl

Advent of Code 2018, day 10

Yesterday’s solution was awesome: I read the problem, walked my dog, then sat down at the computer and wrote both solutions perfectly, one after the other. First time I managed to do that! Today took a bit longer. The difficult thing was coming up with a heuristic for knowing when the stars aligned to give a message, without manually reviewing each possible image. My first idea was to somehow calculate entropy across the grid of stars (and find the lowest-entropy frame), but a quick Googling didn’t really yield anything which was likely to help. Read more →

Advent of Code 2018, day 4

All I’m going to say about today’s solutions is read the problem carefully. I missed a key fact, which meant I spent way too much time handling cases I didn’t need to. There’re still some vestiges of that approach in the (working) solutions. Sheesh. Read more →

Advent of Code 2018, day 3

I’ve got today’s solutions up. This problem wasn’t really amenable to using hashes, so I just brute-forced it with a 1,000×1,000-element array (hey, memory is cheap!). FIND-VALID-CLAIMS is O(n²) (due to a O(n) list deletion performed O(n) times), but … CPU is cheap too, and these are puzzles, not production code. That’s a running theme throughout these solutions: I’m not playing for high-performance or highly-elegant solutions to the puzzles, just the correct answers, quickly. Read more →

Advent of Code 2018, day 2

I’ve got my solutions to today’s puzzles up. Believe it or not, a lot of folks dislike LOOP because it’s not terribly Lispy — they’re right, of course, but it is a pretty handy Swiss Army knife to keep in one’s pocket. I use hash tables a lot, but that’s because hash tables are an awesome data structure. My biggest takeaway from working on these puzzles so far is gratitude that Common Lisp is a large enough language that it has hash tables; were I writing in C I’d either be scanning & rescanning & re-rescanning strings, or I’d have to write my own hash table implementation. Read more →

Advent of Code 2018, day 1

I’ve decided to participate in the Advent of Code this year. The idea is that each day two simple puzzles are released, and folks use their favourite programming language to solve them. Folks compete to be the first person to submit a solution (only the first 100 count for points, though). Following Javier Olaechea’s example, I’ll track each day’s solution and post notes here. Sum numbers I decided to just drop the input in as a string. Read more →

In which goods get cheaper faster than inflation

I was recently going through my bookshelf, looking for books to trim, when I came across Mark Bowden’s excellent Black Hawk Down; it happened to have a receipt inside, from November 2008 when I bought it (almost exactly a decade to the day — strange how coïncidences happen sometimes!). I noticed that it was $13.95 back then, which wasn’t terribly cheap in 2008 ($16.32 in 2018 dollars, for a paperback). I was curious how much it’d cost today: turns out that Amazon have it for $13. Read more →

Identity thieves abusing U.S. mail

Brian Krebs reports that the Secret Service has alerted law enforcement that the U.S. Postal Service’s Informed Delivery is being abused by so-called ‘identity thieves.’ From the first time I read about it, it sounded like a neat idea, but rife with problems. Glad that I held off jumping on the bandwagon. Also of note, ‘identity theft’ is a really bad name for the problem: people aren’t stealing identities so much as financial institutions are performing insufficient identity verification. Read more →

Multi-device end-to-end encryption & identity

I had an idea late last night about how to handle multi-device end-to-end encryption and identity. An issue with end-to-end encryption is how to support multiple devices: I may want to read messages on my laptop, my phone, my desktop and my tablet, but I don’t want a central server to be able to read those messages. This can be handled with encryption: when someone sends me a message, he encrypts it for all of my devices. Read more →

How to write a spelling corrector — in Lisp

Back in 2007 Peter Norvig shared a simple spelling corrector (last updated in 2016); I thought that I’d share my translation into Lisp, with some comments. Norvig’s original article explains the actual algorithm he’s using; I’ll focus on stuff specific to my own version.

I’m definitely not a Lisp wizard (although I’ve been using it for many years), so it’s entirely possible that I’ve messed one thing or another up — I’m glad to receive corrections or comments.

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