this post was submitted on 30 Nov 2025
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Advent Of Code

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An unofficial home for the advent of code community on programming.dev! Other challenges are also welcome!

Advent of Code is an annual Advent calendar of small programming puzzles for a variety of skill sets and skill levels that can be solved in any programming language you like.

Everybody Codes is another collection of programming puzzles with seasonal events.

EC 2025

AoC 2025

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console.log('Hello World')

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๐Ÿฆ† Everybody.Codes 2025 Quest 20 Solutions ๐Ÿฆ† (programming.dev)
submitted 3 weeks ago by hades@programming.dev to c/advent_of_code@programming.dev
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Quest 20: Dream in Triangles

  • Keep top level comments as only solutions, if you want to say something other than a solution put it in a new post. (replies to comments can be whatever)
  • You can send code in code blocks by using three backticks, the code, and then three backticks or use something such as https://topaz.github.io/paste/ if you prefer sending it through a URL

Link to participate: https://everybody.codes/

And it's a wrap! Thanks to everyone who participated or lurked :)

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[โ€“] hades@programming.dev 1 points 3 weeks ago

Rust

use std::collections::{HashSet, VecDeque};

use itertools::Itertools;

fn neighbours_of(i: usize, j: usize, side: usize) -> impl Iterator<Item = (usize, usize)> {
    [
        (i, j.wrapping_sub(1)),
        (i, j + 1),
        (
            if (i + j) % 2 == 0 {
                i.wrapping_sub(1)
            } else {
                i + 1
            },
            j,
        ),
    ]
    .into_iter()
    .filter(move |&(i, j)| i < side && j >= i && j < (2 * side - i - 1))
}

pub fn solve_part_1(input: &str) -> String {
    let data = input
        .lines()
        .map(|l| l.chars().collect::<Vec<_>>())
        .collect::<Vec<_>>();
    let side = data.len();
    let mut queue = VecDeque::new();
    queue.push_back((0, 0));
    let mut pairs = 0;
    let mut visited = HashSet::new();
    while let Some((i, j)) = queue.pop_front() {
        if visited.contains(&(i, j)) {
            continue;
        }
        for (ni, nj) in neighbours_of(i, j, side) {
            if visited.contains(&(ni, nj)) {
                continue;
            }
            if data[i][j] == 'T' && data[ni][nj] == 'T' {
                pairs += 1;
            }
            queue.push_back((ni, nj));
        }
        visited.insert((i, j));
    }
    pairs.to_string()
}

pub fn solve_part_2(input: &str) -> String {
    let data = input
        .lines()
        .map(|l| l.chars().collect::<Vec<_>>())
        .collect::<Vec<_>>();
    let side = data.len();
    let mut front = (0..data.len())
        .cartesian_product(0..data[0].len())
        .filter(|&(i, j)| data[i][j] == 'S')
        .collect::<HashSet<_>>();
    let mut visited = HashSet::new();
    let mut steps = 0;
    while !front.is_empty() {
        let mut next_front = HashSet::new();
        for (i, j) in front.drain() {
            if data[i][j] == 'E' {
                return steps.to_string();
            }
            visited.insert((i, j));
            for (ni, nj) in neighbours_of(i, j, side) {
                if (data[ni][nj] == 'T' || data[ni][nj] == 'E') && !visited.contains(&(ni, nj)) {
                    next_front.insert((ni, nj));
                }
            }
        }
        steps += 1;
        front = next_front;
    }
    panic!("exit not found");
}

pub fn rotate(data: &[Vec<char>]) -> Vec<Vec<char>> {
    let mut result = vec![vec!['.'; data[0].len()]; data.len()];
    let side = data.len();
    for i in 0..data.len() {
        for j in i..(data[0].len() - i) {
            if (i + j) % 2 == 0 {
                result[i][j] = data[side - (i + j) / 2 - 1][i * 2 + side - (i + j) / 2 - 1];
            } else {
                result[i][j] = data[side - (i + j).div_ceil(2) - 1][side - (j - i).div_ceil(2) + i];
            }
        }
    }
    result
}

pub fn solve_part_3(input: &str) -> String {
    let data = input
        .lines()
        .map(|l| l.chars().collect::<Vec<_>>())
        .collect::<Vec<_>>();
    let side = data.len();
    let data = [data.clone(), rotate(&data), rotate(&rotate(&data))];
    let mut front = (0..data[0].len())
        .cartesian_product(0..data[0][0].len())
        .filter(|&(i, j)| data[0][i][j] == 'S')
        .map(|(i, j)| (i, j, 0))
        .collect::<HashSet<_>>();
    let mut visited = HashSet::new();
    let mut steps = 0;
    while !front.is_empty() {
        let mut next_front = HashSet::new();
        for (i, j, rotation) in front.drain() {
            if data[rotation][i][j] == 'E' {
                return steps.to_string();
            }
            visited.insert((i, j, rotation));
            let next_rotation = (rotation + 1) % 3;
            for (ni, nj) in neighbours_of(i, j, side) {
                if (data[next_rotation][ni][nj] == 'T' || data[next_rotation][ni][nj] == 'E')
                    && !visited.contains(&(ni, nj, next_rotation))
                {
                    next_front.insert((ni, nj, next_rotation));
                }
            }
            if (data[next_rotation][i][j] == 'T' || data[next_rotation][i][j] == 'E')
                && !visited.contains(&(i, j, next_rotation))
            {
                next_front.insert((i, j, next_rotation));
            }
        }
        steps += 1;
        front = next_front;
    }
    panic!("exit not found");
}