Advent of Code - because I'm an idiot


I wish our kids believed us when we tell them that we have the same programming troubles as they do. We stare at code for hours not seeing problems that could be a simple as passing the wrong value. We spend an inordinate amount of time trying to see the problem and then realize that we just forgot something silly.

At this point, it's common for us CS teachers to tell each other "I figured out the problem I was having – it's that I was an idiot."

Last time, I talked about Advent of Code and how I was somewhat behind the curve. In part because I started late and in part because I've been using it as an excuse to learn some Clojure.

Here's how I burned over two days on Day 6.

The problem is pretty straight forward. You have a 1,000 x 1,000 grid representing a light board and you have to repeatedly do operations on smaller rectangles on that grid. Turning individual lights on and off, if you would.

If I were doing this in a language like C, I'd probably just take a big block of memory and loop through the instructions but since I was working in Clojure, I thought I'd try to do something in more of the functional mode, that is, with immutable data types.

After a while, I was getting the feeling that it was going to be too slow so I shifted to just modifying a big block of memory.

Try as I could, I couldn't get it to work. I was sure my algorithm was fine - it wasn't a hard problem, but I couldn't get the right answer.

After a while, I started looking at existing solutions and they were basically the same as mine.

I was stumped.

Two days after the start, I figured it out.

No matter how hard you try, if your problem specifies the data in the form row1,col1 row2,col2, you're not going to get the right answer if you decide that the input is in the form row1,row2 col1,col2.


Ok, after that the problem was pretty trivial.

Hopefully I'll have some time to do problem 7 and beyond this weekend.

If there are any Clojure people out there, I'm putting my solutions up on GitHub - I'd love some feedback on how to get more idiomatic.


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