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---
permalink: "/{{ year }}/{{ month }}/{{ day }}/a-lil-advent-of-code"
title: "A Lil Advent of Code"
published_date: "2024-12-20 20:00:00 +0100"
layout: post.liquid
data:
route: blog
excerpt: |
I solved the first 3 days worth of puzzles of this year's Advent of Code in Lil,
a tiny programming language, part of the Decker project.
These are my solutions and brief thoughts on Lil.
---
_This post won't include too much explanation. I wanted a place to at least put my code and that's what I have a blog for._
It's December, so everyone is doing [Advent of Code](https://adventofcode.com/).
I'm not. Or so I told myself.
This week, while on vacation, I had some downtime and played around with [Decker][][^1] and its accompanying programming language [Lil].
Armed with the [tutorial][liltutorial], the [reference card][lilquickref] and the [Lil Terminal][lilt] installed, I set out to learn it and solved three days worth of puzzles from the ongoing Advent of Code 2024[^2].
[Decker]: https://beyondloom.com/decker/
[Lil]: https://beyondloom.com/decker/lil.html
[liltutorial]: https://beyondloom.com/decker/learnlil.html
[lilquickref]: https://beyondloom.com/decker/lilquickref.html
[lilt]: https://beyondloom.com/decker/lilt.html
Want to jump ahead? [Day 1](#day1) | [Day 2](#day2) | [Day 3](#day3) | [Thoughts](#thoughts)
## <a name="day1"></a> Day 1
On Day 1 you get two lists side-by-side, need to order each one individually, then take the absolute differences and add them up.
Part 2 requires you to find the number of times an element shows up in the other list.
That can be done in Lil like this:
```
example:"%i %i" parse "\n" split "
3 4
4 3
2 5
1 3
3 9
3 3
"
on sort l do
extract value orderby value asc from l
end
on reverse l do
extract value orderby index desc from l
end
on part1 input do
sorted:sort @ flip input
result:sum each x in (sorted[0] - sorted[1]) mag x end
print[result]
end
on dump name x do
print[name]
print["," fuse (list "%J") format x]
end
on part2 input do
lists:flip input
left:extract value where value > 0 from lists[0]
right:extract value where value > 0 from lists[1]
counts:each elem in left
sum right=elem
end
result:sum left*counts
print[result]
end
part1[example]
part2[example]
```
Example:
```
; lilt aoc1.lil
11
31
```
## <a name="day2"></a> Day 2
On Day 2 you have to determine which reports are safe.
A report is a line of numbers and if they match some rules it is considered safe.
Count those safe ones.
And part 2 allows you to make some reports safe by ignoring some results.
```
example:"7 6 4 2 1
1 2 7 8 9
9 7 6 2 1
1 3 2 4 5
8 6 4 4 1
1 3 6 7 9"
on spreadcheck elems do
prod each elem in -2 window elems
diff:mag elem[0]-elem[1]
(diff-1) > 0 and (diff-1) < 3
end
end
on allincr elems do
prod each elem in -2 window elems elem[0] < elem[1] end
end
on alldecr elems do
prod each elem in -2 window elems elem[0] > elem[1] end
end
on reportcheck report do
report:on _ elem do "%i" parse elem end @ report
(max allincr[report],alldecr[report]) * spreadcheck[report]
end
on part1 input do
reports:on _ line do " " split line end @ "\n" split input
result:sum reportcheck @ reports
show[result]
end
on part2 input do
reports:on _ line do " " split line end @ "\n" split input
result:each report in reports
report:on _ elem do "%i" parse elem end @ report
valid:(max allincr[report],alldecr[report]) & spreadcheck[report]
if valid
1
else
max each i in range count report
newreport:extract value where !(index=i) from report
(max allincr[newreport],alldecr[newreport]) & spreadcheck[newreport]
end
end
end
show[sum result]
end
part1[example]
part2[example]
```
Example:
```
; lilt aoc2.lil
2
4
```
## <a name="day3"></a> Day 3
I couldn't stop, so I also solved day 3.
Day 3 has a garbled string containing some instructions to multiply some numbers.
Part 2 has additional instructions that turns on or off those instructions.
I opted to iterate the string input and match for the expected patterns.
```
example:"xmul(2,4)%&mul[3,7]!@^do_not_mul(5,5)+mul(32,64]then(mul(11,8)mul(8,5))"
example2:"xmul(2,4)&mul[3,7]!^don't()_mul(5,5)+mul(32,64](mul(11,8)undo()?mul(8,5))"
on part1 input do
f:"mul(%i,%i)"
result:sum each i in range count input
curinp:i drop input
ab:f parse i drop input
offset:(count "mul(")+(count "," fuse ab)
if curinp[offset]=")"
prod ab
end
end
show[result]
end
on part2 input do
f:"mul(%i,%i)"
do_:"do()"
dont:"don't()"
enabled:1
result:sum each i in range count input
curinp:i drop input
if ((count do_) limit curinp)=do_
enabled:1
end
if ((count dont) limit curinp)=dont
enabled:0
end
if (4 limit curinp)="mul("
ab:f parse curinp
offset:(count "mul(")+(count "," fuse ab)
if enabled & curinp[offset]=")"
prod ab
end
end
end
show[result]
end
part1[example]
part2[example2]
```
Run it:
```
; lilt aoc3.lil
159892596
92626942
```
## <a name="thoughts"></a> Thoughts
I like Lil.
It's small, it's simple and it's surprisingly fast.
It also gets the job done.
These `name:data` variable assignments threw me off a bit, but I got over that hump.
[jq](https://jqlang.github.io/jq/) is a powerful tool to work with JSON, but for the sake of it I can't remember any more of its syntax than the basic `.[]`.
Reading and transforming a JSON file using Lil instead felt very intuitive.
I opted to write a script I needed to transform some JSON into some other output it was intuitive.
This is how you read and parse a JSON file[^3]:
```
json:"%j" parse read["droids.json"]
```
And then you can access the data:
```
extract n:name d:description where name like "Astro.*" from table json.data
{"n":("Astromech Droid"),"d":("Astromech droids are a series of versatile utility robots generally used for the maintenance and repair of starships and related technology. These small droids are often equipped with a variety of tool-tipped appendages that are stowed in recessed compartments. The R2 unit is a popular example of an astromech droid.")}
```
Not needing to deal with missing fields or inconsistencies in the format
makes it easy to ignore the error cases,
something that comes in handy for one-off scripts.
Now I need to take a closer look at Decker and how this powerful language can be used with it.
---
_Footnotes:_
[^1]: Decker is a "multimedia platform for creating and sharing interactive documents". I have yet to actually do something cool with Decker itself.
[^2]: I didn't _know_ I would be doing 3. But I finished Day 1 quickly, Day 2 not long after and Day 3 was done an hour later. I looked at Day 4 and how to solve it was not immediately clear, so I stopped.
[^3]: Data from the [`droids` endpoint of the Star Wars Databank](https://starwars-databank-server.vercel.app/api/v1/droids/)