<Advent of Code 2025 day 12> (spoilers) :thread:
# advent-of-codea
Advent of Code 2023 day 12
12/12/2025, 5:00 AMAdvent of Code 2025 day 12 (spoilers) 🧵
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Kroppeb
12/12/2025, 5:48 AMI don't like today at all.
💯 3
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Dan Fingal-Surma
12/12/2025, 6:04 AMNP hard, what's the problem
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vanshg
12/12/2025, 6:10 AMThis ended up being waaaaay simpler than it looked
🧠 1
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Dan Fingal-Surma
12/12/2025, 6:11 AMWell like I see some number of my lines can fit with 0 need to pack because there are enough 3x3 grids for them
Dan Fingal-Surma
12/12/2025, 6:11 AMAnd then one shape can make a 4x3 grid with 2
Dan Fingal-Surma
12/12/2025, 6:14 AMNeed to look at the shapes more closely
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vanshg
12/12/2025, 6:24 AMHint: there's a shortcut
🧠 1
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Dan Fingal-Surma
12/12/2025, 6:35 AMAlright I will exit spoiler chat for now 🙃
Dan Fingal-Surma
12/12/2025, 6:43 AMOMG LOL
same 2
Dan Fingal-Surma
12/12/2025, 6:43 AMSolve
😀 1
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Michael de Kaste
12/12/2025, 6:46 AMwell... I kinda did the 'trick' to at least know beforehand to not check... I didn't know the predicate also worked the other way. 😂
Michael de Kaste
12/12/2025, 6:46 AMI solved it the brute force way but the 'trick' way works as well. Wonder if there's a poor soul that it doesn't work for
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Dan Fingal-Surma
12/12/2025, 6:48 AMRight my next step was going to be filter out based on that criteria, then “may as well check”
Dan Fingal-Surma
12/12/2025, 6:49 AMDan Fingal-Surma
12/12/2025, 6:55 AMah so actually. i had an error in the “3x3 fits” (> instead of >=). you can just see that working forward and backward give the same number, therefore there are no areas that need special packing
Dan Fingal-Surma
12/12/2025, 6:56 AMScreenshot 2025-12-11 at 10.56.19 PM.png
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Jakub Gwóźdź
12/12/2025, 7:03 AMOh soooo disappointing puzzle today 😞
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phldavies
12/12/2025, 7:04 AMI'm kinda disappointed so much of that wasn't needed 😞 I shouldn't have checked initial predicate 😞
Copy code
@AoKSolution
object Day12 {
context(input: PuzzleInput)
fun trees() =
input.lines.mapNotNull {
if (it.indexOf('x') != 2) return@mapNotNull null
val dims = it.substringBefore(':').split('x').let { (a, b) -> a.toInt() to b.toInt() }
val presents = it.substringAfter(' ').split(' ').map(String::toInt)
dims to presents
}
context(_: PuzzleInput)
fun part1() = trees().count { (dims, presents) -> presents.sum() * 9 <= dims.first * dims.second }
}j
Jakub Gwóźdź
12/12/2025, 7:05 AM Copy code
data class Region(val w: Int, val h: Int, val counts: List<Int>) {
fun pack(shapes: List<Shape>): Boolean {
if (counts.sum() <= (w / 3) * (h / 3)) return true
.debug { "Trivially fits by count" }
if (shapes.zip(counts).sumOf { (s, n) -> n * s.size } > h * w) return false
.debug { "Too much area needed" }
val toGo = shapes.flatMapIndexed { index, shape -> List(counts[index]) { shape } }
.debug { "To go: ${it.size} shapes" }
TODO("Implement packing algorithm")
}
}TODO()😂 3
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Dan Fingal-Surma
12/12/2025, 7:07 AMModifying my code to add a TODO
advent of code intensifies 1
Dan Fingal-Surma
12/12/2025, 7:10 AMSaved 1 LOC in the process
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Jakub Gwóźdź
12/12/2025, 7:22 AMThings I already implemented and won't be needed anyway:
• parsing shapes (instead I could just say
data class(val size: Int =7)d
Dan Fingal-Surma
12/12/2025, 7:24 AMMy shapes had different # counts
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Jakub Gwóźdź
12/12/2025, 7:25 AMmy shapes were exactly the same as in puzzle's example
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Dan Fingal-Surma
12/12/2025, 7:26 AM7 7 7 7 6 5 for me
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Jakub Gwóźdź
12/12/2025, 7:27 AMoh damn. NO you're right, for me it was also different, I didn't check correctly open eye crying laughing . so even worse 🙂
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Marcin Wisniowski
12/12/2025, 7:28 AMWHAT
😂 1
Marcin Wisniowski
12/12/2025, 7:29 AMI finished implementing all the rotations, the packing, recursive search with pruning, got it to work on the examples but it was too slow on the real data, came here for hints...
😭 1
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phldavies
12/12/2025, 7:32 AMyou mean you did more than
Copy code
@AoKSolution
object Day12Min {
context(_: PuzzleInput)
fun part1() = lines.count {
if('x' !in it) return@count false
val ints = it.split("x", ": ", " ").map(String::toInt)
ints.let { (x, y) -> x * y } >= ints.subList(2, ints.size).sum() * 9
}
}m
Marcin Wisniowski
12/12/2025, 7:33 AMinternal screaming
same 2
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Jakub Gwóźdź
12/12/2025, 7:34 AMto be fair, this is safer:
Copy code
counts.sum() <= (w / 3) * (h / 3)m
Michael de Kaste
12/12/2025, 7:39 AMj
Jakub Gwóźdź
12/12/2025, 7:39 AMGOLFING TIME!
Jakub Gwóźdź
12/12/2025, 7:40 AMbut frankly,
sumOf(9::times)m
Michael de Kaste
12/12/2025, 7:40 AMI love me some function references
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phldavies
12/12/2025, 9:55 AM🏌️ 67 tokens
Copy code
fun main() = println(generateSequence(::readLine).count {
Regex("\\D").split(it).mapNotNull(String::toIntOrNull)
.run { size == 8 && let { (a, b) -> a * b / 9 >= sum() - a - b } }
})nice spin 1
6️⃣ 1
7️⃣ 1
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Kai Yuan
12/12/2025, 10:41 AM@phldavies can be shorter
Copy code
...
'x' in it && it.split(Regex("\\D+")).map { it.toInt() }
...🏌️ 1
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Endre Deak
12/12/2025, 3:20 PMthe proof it got me first:
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bj0
12/12/2025, 6:19 PMbut.. but.. i originally tried the easy answer that people are showing, but i tried it against the sample data first which it doesn't work against
bj0
12/12/2025, 6:19 PMso i didn't try it against my own data
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Jakub Gwóźdź
12/12/2025, 6:20 PMAnd you’re not the only one
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bj0
12/12/2025, 6:24 PMhow rude of the example input
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Endre Deak
12/12/2025, 6:34 PMyeah took me so long to realize that the right approach doesn't work for the sample input...
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Jaap Beetstra
12/15/2025, 9:51 AMThe puzzle input was lame, but it was a good day for visualizations.
❤️ 1
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