Haskell

Took a while to figure out what part 2 was all about. Didn't have the energy to golf this one further today, so looking forward to seeing the other solutions!

import Data.Char
import Data.List
import Data.List.Split
import qualified Data.Vector as V

hash :: String -> Int
hash = foldl' (\a c -> ((a + ord c) * 17) `rem` 256) 0

hashmap :: [String] -> Int
hashmap = focus . V.toList . foldl' step (V.replicate 256 [])
  where
    focus = sum . zipWith focusBox [1 ..]
    focusBox i = sum . zipWith (\j (_, z) -> i * j * z) [1 ..] . reverse
    step boxes s =
      let (label, op) = span isLetter s
          i = hash label
       in case op of
            ['-'] -> V.accum (flip filter) boxes [(i, (/= label) . fst)]
            ('=' : z) -> V.accum replace boxes [(i, (label, read z))]
    replace ls (n, z) =
      case findIndex ((== n) . fst) ls of
        Just j ->
          let (a, _ : b) = splitAt j ls
           in a ++ (n, z) : b
        Nothing -> (n, z) : ls

main = do
  input <- splitOn "," . head . lines <$> readFile "input15"
  print $ sum . map hash $ input
  print $ hashmap input

Python

0.248 line-seconds (sixth simplest so far after days 6, 2, 1, 4 and 9).

Also on Github

import collections
import re

from .solver import Solver


def _hash(string: str) -> int:
  result = 0
  for c in string:
    result = (result + ord(c)) * 17 % 256
  return result

def _assert_full_match(pattern: str, string: str):
  m = re.fullmatch(pattern, string)
  if not m:
    raise RuntimeError(f'pattern {pattern} does not match {string}')
  return m

class Day15(Solver):
  input: list[str]

  def __init__(self):
    super().__init__(15)

  def presolve(self, input: str):
    self.input = input.rstrip().split(',')

  def solve_first_star(self) -> int:
    return sum(_hash(string) for string in self.input)

  def solve_second_star(self) -> int:
    boxes = [collections.OrderedDict() for _ in range(256)]
    for instruction in self.input:
      label, op, value = _assert_full_match(r'([a-z]+)([=-])(\d*)', instruction).groups()
      box = boxes[_hash(label)]
      match op:
        case '-':
          if label in box:
            del box[label]
        case '=':
          box[label] = value
    return sum((1 + box_idx) * (1 + lens_idx) * int(value)
                for box_idx, box in enumerate(boxes)
                for lens_idx, (_, value) in enumerate(box.items()))

Had to take a couple days off, but this was a nice one to come back to. Will have to find some time today to go back and do one or two of the 3 that I missed. I don't have much to say about this one - I had an idea almost immediately and it worked out without much struggle. There's probably some cleaner ways to write parts of this, but I'm not too disappointed with how it turned out.

https://github.com/capitalpb/advent_of_code_2023/blob/main/src/solvers/day15.rs

use crate::Solver;
use std::collections::HashMap;

#[derive(Debug)]
struct Lens {
    label: String,
    focal_length: u32,
}

fn hash_algorithm(input: &str) -> u32 {
    input
        .chars()
        .fold(0, |acc, ch| (acc + ch as u32) * 17 % 256)
}

pub struct Day15;

impl Solver for Day15 {
    fn star_one(&self, input: &str) -> String {
        input
            .trim_end()
            .split(',')
            .map(hash_algorithm)
            .sum::()
            .to_string()
    }

    fn star_two(&self, input: &str) -> String {
        let mut boxes: HashMap> = HashMap::new();

        for instruction in input.trim_end().split(',') {
            let (label, focal_length) = instruction
                .split_once(|ch| char::is_ascii_punctuation(&ch))
                .unwrap();

            let box_number = hash_algorithm(label);
            let lenses = boxes.entry(box_number).or_insert(vec![]);

            if focal_length == "" {
                lenses.retain(|lens| lens.label != label);
                continue;
            }

            let new_lens = Lens {
                label: label.to_string(),
                focal_length: focal_length.parse().unwrap(),
            };

            if let Some(lens_index) = lenses.iter().position(|lens| lens.label == new_lens.label) {
                lenses[lens_index].focal_length = new_lens.focal_length;
            } else {
                lenses.push(new_lens);
            }
        }

        boxes
            .iter()
            .map(|(box_number, lenses)| {
                lenses
                    .iter()
                    .enumerate()
                    .map(|(lens_index, lens)| {
                        (box_number + 1) * (lens_index as u32 + 1) * lens.focal_length
                    })
                    .sum::()
            })
            .sum::()
            .to_string()
    }
}

Scala3

def hash(s: String): Long = s.foldLeft(0)((h, c) => (h + c)*17 % 256)

extension [A] (a: List[A])
    def mapAtIndex(idx: Long, f: A => A): List[A] =
        a.zipWithIndex.map((e, i) => if i == idx then f(e) else e)

def runProcedure(steps: List[String]): Long =
    @tailrec def go(boxes: List[List[(String, Int)]], steps: List[String]): List[List[(String, Int)]] =
        steps match
            case s"$label-" :: t =>
                go(boxes.mapAtIndex(hash(label), _.filter(_._1 != label)), t)
            case s"$label=$f" :: t =>
                go(boxes.mapAtIndex(hash(label), b => 
                    val slot = b.map(_._1).indexOf(label)
                    if slot != -1 then b.mapAtIndex(slot, (l, _) => (l, f.toInt)) else (label, f.toInt) :: b
                ), t)
            case _ => boxes
    
    go(List.fill(256)(List()), steps).zipWithIndex.map((b, i) =>
        b.zipWithIndex.map((lens, ilens) => (1 + i) * (b.size - ilens) * lens._2).sum
    ).sum

def task1(a: List[String]): Long = a.head.split(",").map(hash).sum
def task2(a: List[String]): Long = runProcedure(a.head.split(",").toList)

C

Yes, it's a hash table. Did I pick a language with built in hash tables? Of course I didn't. Could I have used one of the many libraries implementing one? Sure. But the real question is, can we make do with stuffing things into a few static arrays at nearly zero memory and runtime cost? Yes!

In the spirit of Fred Brooks, it’ll suffice here to show my data structures:

struct slot { char label[8]; int lens; };
struct box { struct slot slots[8]; int nslots; };

static struct box boxes[256];

https://github.com/sjmulder/aoc/blob/master/2023/c/day15.c

This felt ... too simple. I think the hardest part of part two for me was reading comprehension. My errors were typically me not reading exactly was there.

import re
import math
import argparse
import itertools

def int_hash(string:str) -> int:
    hash = 0
    for c in [*string]:
        hash += ord(c)
        hash *= 17
        hash = hash % 256
    return hash

class Instruction:
    def __init__(self,string:str) -> None:
        label,action,strength = re.split('([-=])',string)
        self.label = label
        self.action = action
        if not strength:
            strength = 0
        self.strength = int(strength)
    
    def __repr__(self) -> str:
        return f"Instruction(l={self.label}, a={self.action}, s={self.strength})"
    
    def __str__(self) -> str:
        stren = str(self.strength if self.strength > 0 else '')
        return f"{self.label}{self.action}{stren}"


class Lens:
    def __init__(self,label:str,focal_length:int) -> None:
        self.label:str = label
        self.focal_length:int = focal_length

    def __repr__(self) -> str:
        return f"Lens(label:{self.label},focal_length:{self.focal_length})"
    
    def __str__(self) -> str:
        return f"[{self.label} {self.focal_length}]"

def main(line_list:str,part:int):
    init_sequence = line_list.splitlines(keepends=False)[0].split(',')
    sum = 0
    focal_array = dict[int,list[Lens]]()
    for i in range(0,256):
        focal_array[i] = list[Lens]()
    for s in init_sequence:
        hash_value = int_hash(s)
        sum += hash_value

        # part 2 stuff
        action = Instruction(s)
        position = int_hash(action.label)
        current_list = focal_array[position]
        existing_lens = list(filter(lambda x:x.label == action.label,current_list))
        if len(existing_lens) > 1:
            raise Exception("multiple of same lens in box, what do?")
        match action.action:
            case '-':
                if len(existing_lens) == 1:
                    current_list.remove(existing_lens[0])
            case '=':
                if len(existing_lens) == 0:
                    current_list.append(Lens(action.label,action.strength))
                if len(existing_lens) == 1:
                    existing_lens[0].focal_length = action.strength
            case _:
                raise Exception("unknown action")

    print(f"Part1: {sum}")
    #print(focal_array)

    sum2 = 0
    for i,focal_box in focal_array.items():
        for l,lens in enumerate(focal_box):
            sum2 += ( (i+1) * (l+1) * lens.focal_length )

    print(f"Part2: {sum2}")


if __name__ == "__main__":
    parser = argparse.ArgumentParser(description="template for aoc solver")
    parser.add_argument("-input",type=str)
    parser.add_argument("-part",type=int)
    args = parser.parse_args()
    filename = args.input
    if filename == None:
        parser.print_help()
        exit(1)
    part = args.part
    file = open(filename,'r')
    main(file.read(),part)
    file.close()