this post was submitted on 27 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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Quest 16: Harmonics of Stone

  • 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/

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[โ€“] Pyro@programming.dev 2 points 4 weeks ago

Python

def part1(data: str):
    elements = map(int, data.split(','))
    total = 0
    for ele in elements:
        # a spell element will add (90 // number) bricks for 90 columns
        total += 90 // ele
    return total

assert part1("1,2,3,5,9") == 193

# Gets all spell elements needed to build a wall
# Useful for part 2 and part 3
def get_elements(data: str):
    wall = map(int, data.split(','))
    elements = []   # spell elements

    for col_idx, fragments in enumerate(wall):
        # fix for 1-based indexing
        col = col_idx + 1

        # account bricks for recorded elements
        for ele in elements:
            if col % ele == 0:
                fragments -= 1
        
        # if we still have fragments, we need to add a new element with that column number
        if fragments > 0:
            elements.append(col)
    
    return elements

def part2(data: str):
    # Get all spell elements needed to build the wall
    elements = get_elements(data)
    # return product of all elements
    ans = 1
    for ele in elements:
        ans *= ele
    return ans

assert part2("1,2,2,2,2,3,1,2,3,3,1,3,1,2,3,2,1,4,1,3,2,2,1,3,2,2") == 270

# Part 3: Binary Search for maximum full columns
def part3(data: str):
    BRICKS = 202520252025000
    elements = get_elements(data)

    # Check if we can build 'cols' full columns within BRICKS bricks
    def can_build(cols: int) -> bool:
        bricks_used = 0
        for ele in elements:
            bricks_used += cols // ele
            if bricks_used > BRICKS:
                return False
        return True

    # binary search: break on first column size we cannot build
    lo = 1
    hi = BRICKS
    while lo < hi:
        mid = lo + (hi - lo) // 2
        if can_build(mid):
            lo = mid + 1
        else:
            hi = mid
    
    # if lo is the first we cannot build, lo - 1 is the maximum we can build
    return lo - 1

assert part3("1,2,2,2,2,3,1,2,3,3,1,3,1,2,3,2,1,4,1,3,2,2,1,3,2,2") == 94439495762954
[โ€“] hades@programming.dev 1 points 1 month ago (1 children)

Rust

fn build_wall(numbers: &[i64], length: usize) -> Vec<i64> {
    let mut divisors = vec![0; length];
    for n in numbers {
        for (i, d) in divisors.iter_mut().enumerate() {
            if (i + 1) as i64 % n == 0 {
                *d += 1;
            }
        }
    }
    divisors
}

pub fn solve_part_1(input: &str) -> String {
    let numbers = input
        .split(",")
        .map(|n| n.parse::<i64>().unwrap())
        .collect::<Vec<_>>();
    build_wall(&numbers, 90).iter().sum::<i64>().to_string()
}

fn solve(
    divisor_masks: &Vec<Vec<i32>>,
    selected_divisors: Vec<i64>,
    values: &Vec<i64>,
    next_divisor_to_try: i64,
) -> Option<Vec<i64>> {
    if values.iter().all(|v| *v == 0) {
        return Some(selected_divisors);
    }
    if values.iter().any(|v| *v < 0) {
        return None;
    }
    if next_divisor_to_try as usize > values.len() {
        return None;
    }
    let next_values = values
        .iter()
        .zip(divisor_masks[(next_divisor_to_try - 1) as usize].iter())
        .map(|(v, d)| *v - *d as i64)
        .collect();
    let mut next_selected_divisors = selected_divisors.clone();
    next_selected_divisors.push(next_divisor_to_try);
    if let Some(result) = solve(
        &divisor_masks,
        next_selected_divisors,
        &next_values,
        next_divisor_to_try + 1,
    ) {
        return Some(result);
    }
    return solve(
        &divisor_masks,
        selected_divisors,
        &values,
        next_divisor_to_try + 1,
    );
}

pub fn solve_part_2_raw(input: &str) -> Vec<i64> {
    let values = input
        .split(",")
        .map(|n| n.parse::<i64>().unwrap())
        .collect::<Vec<_>>();
    let mut divisor_masks = vec![vec![0; values.len()]; values.len()];
    for (i, mask) in divisor_masks.iter_mut().enumerate() {
        let divisor = i + 1;
        for (j, d) in mask.iter_mut().enumerate() {
            let number = j + 1;
            if number % divisor == 0 {
                *d += 1;
            }
        }
    }
    let result = solve(&divisor_masks, vec![], &values, 1).unwrap();
    result
}

pub fn solve_part_2(input: &str) -> String {
    solve_part_2_raw(input).iter().product::<i64>().to_string()
}

pub fn solve_part_3(input: &str) -> String {
    let divisors = solve_part_2_raw(input);
    let blocks = 202520252025000i64;
    let mut l = 1i64;
    let mut r = 108420091881608i64;
    while r - l > 1 {
        let mid = (r + l) / 2;
        let b = divisors.iter().map(|d| mid / d).sum::<i64>();
        if b > blocks {
            r = mid;
        } else {
            l = mid;
        }
    }
    l.to_string()
}
[โ€“] LiseAndreasen@mastodon.world 1 points 1 month ago (1 children)

@hades
I don't get these posts. I can't read them. Newlines have disappeared?

[โ€“] hades@programming.dev 1 points 1 month ago (1 children)

Sadly, every client renders code blocks differently :/ Yours apparently doesn't keep line breaks for some reason. I assume this happens to other posts on programming.dev with code blocks, not just mine, right?

Either way, you can open it on the web to read the code.

[โ€“] LiseAndreasen@mastodon.world 1 points 4 weeks ago* (last edited 4 weeks ago) (1 children)

@hades
My client is called mastodon. I assume it is official in some way.
I don't read a lot of code here.
I don't think there's a link to a site. Oh wait, found it.

[โ€“] hades@programming.dev 1 points 4 weeks ago

Ah, makes sense. Here's a link to this comment so you can view it on the web: https://programming.dev/post/41427600/20744400