Robert A. Uhl

Advent of Code 2018 update

Normally the end-of-year timeframe is pretty relaxed — in fact, my company announced an office closure next week! I figured that this would be an ideal opportunity to solve some programming puzzles & expand my brain a bit. Sadly, we’ve got an end-of-year project which is going to consume all my time (and which means that I’ll be working next week). I need to devote all of my mental energy to it, which means that I’m going to have to suspend working on Advent of Code puzzles. Read more →

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 →

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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One hundred questions

Here’s a nifty list of 100 interview questions for developers. I can’t say that I can answer them all, but I know most … and will learn the rest. Read more →

One hundred questions

Here’s a nifty list of 100 interview questions for developers. I can’t say that I can answer them all, but I know most … and will learn the rest. Read more →

Running Lisp as a Linux service

One of the truly wonderful things about programming in Common Lisp is that the system is complete interactive: the programmer can manipulate anything at run time, including the language itself. This is a really powerful technique — but how does one preserve the state of the system between reboots? And how does one get an image-based Lisp system to play nice with Linux’s system service model? Well, John Wiegley published a great technique a few years ago which I’ve adapted for Tasting Notes. Read more →