1 vote

Day 12: Christmas Tree Farm

Automatically posted
Tags: advent of code.2025

Today's problem description: https://adventofcode.com/2025/day/12

Please post your solutions in your own top-level comment. Here's a template you can copy-paste into your comment to format it nicely, with the code collapsed by default inside an expandable section with syntax highlighting (you can replace python with any of the "short names" listed in this page of supported languages):

<details>
<summary>Part 1</summary>

```python
Your code here.
```

</details>

2 comments

  1. lily
    Link
    This was a strange one to end things off with. The problem, as described, seems incredibly hard - I'm not sure at all how I'd go about solving it. I spent about half an hour at first hopelessly...

    This was a strange one to end things off with. The problem, as described, seems incredibly hard - I'm not sure at all how I'd go about solving it. I spent about half an hour at first hopelessly writing a naive bruteforce search solution that had no chance of ever finishing, just to try and make some kind of progress. It turns out the input is really, really convenient, though, and so just checking for the absolute upper bound on required space is enough (not for the test input, only the real input). This seems to be intentional, given the puzzle's hover text. When taking advantage of that, the puzzle becomes trivial; the shape definitions can actually be completely ignored, even. Almost all of my code is just for parsing the input.

    Solution (Lily) 
    print("Part 1: {}".format(
    File
        .read_to_string("inputs/day_12.txt")
        .split("\n\n")[-1]
        .split("\n")
        .count(|line|
            var parts = line.split(": ")
            var size = parts[0].split("x")

            parts[1].split(" ").map(|count| count.parse_i().unwrap() * 9).fold(
                0, (|sum, count| sum + count)
            ) <= size[0].parse_i().unwrap() * size[1].parse_i().unwrap()
        )
))