Zig

const std = @import("std");
const List = std.ArrayList;
const Map = std.AutoHashMap;

const splitSeq = std.mem.splitSequence;
const splitScalar = std.mem.splitScalar;
const parseInt = std.fmt.parseInt;
const print = std.debug.print;
const sort = std.sort.block;

var gpa = std.heap.GeneralPurposeAllocator(.{}){};
const alloc = gpa.allocator();

const Answer = struct {
    distance: u32,
    similarity: u32,
};

fn lessThan(_: void, lhs: []const u8, rhs: []const u8) bool {
    return std.mem.lessThan(u8, lhs, rhs);
}

pub fn solve(input: []const u8) !Answer {
    var rows = splitScalar(u8, input, '\n');
    var left_list = List([]const u8).init(alloc);
    defer left_list.deinit();
    var right_list = List([]const u8).init(alloc);
    defer right_list.deinit();

    // PART 1

    // split the rows into two lists
    while (rows.next()) |row| {
        var sides = splitSeq(u8, row, "   ");
        try left_list.append(sides.next() orelse break);
        try right_list.append(sides.next() orelse break);
    }
    _ = left_list.pop(); // last null

    // sort both lists
    sort([]const u8, left_list.items, {}, lessThan);
    sort([]const u8, right_list.items, {}, lessThan);

    var distance: u32 = 0;
    for (left_list.items, right_list.items) |left, right| {
        distance += @abs(try parseInt(i32, left, 10) - try parseInt(i32, right, 10));
    }

    // PART 2
    var right_scores = Map(i32, u32).init(alloc);
    defer right_scores.deinit();

    // count number of item appearances in the right list
    for (right_list.items) |item| {
        const value = try parseInt(i32, item, 10);
        const result = try right_scores.getOrPut(value);
        if (!result.found_existing) {
            result.value_ptr.* = 1;
        } else {
            result.value_ptr.* += 1;
        }
    }

    // sum up similarity between items in left list and right list scores
    var similarity: u32 = 0;
    for (left_list.items) |item| {
        const value = try parseInt(i32, item, 10);
        const result = right_scores.get(value) orelse 0;
        similarity += @as(u32, @intCast(value)) * result;
    }
    return Answer{ .distance = distance, .similarity = similarity };
}

pub fn main() !void {
    const answer = try solve(@embedFile("input.txt"));
    print("Part 1: {d}\n", .{answer.distance});
    print("Part 2: {d}\n", .{answer.similarity});
}

test "test input" {
    const answer = try solve(@embedFile("test.txt"));
    try std.testing.expectEqual(answer.distance, 11);
    try std.testing.expectEqual(answer.similarity, 31);
}

Elixir

Total noob, but it’s fun to learn.

{left, right} =
  File.read!("./input.txt")
  |> String.split("\n", trim: true)
  |> Enum.map(fn line ->
    String.split(line)
    |> Enum.map(&String.to_integer/1)
    |> List.to_tuple()
  end)
  |> Enum.unzip()
  |> then(fn {left, right} ->
    {Enum.sort(left), Enum.sort(right)}
  end)

diffs =
  Enum.zip(left, right)
  |> Enum.map(fn {l, r} -> abs(l - r) end)
  |> Enum.sum()

freqs =
  Enum.filter(right, fn r -> r in left end)
  |> Enum.frequencies()

freqsum =
  Enum.map(left, fn n ->
    freq = Map.get(freqs, n, 0)
    n * freq
  end)
  |> Enum.sum()

IO.puts("part 1: #{diffs}")
IO.puts("part 2: #{freqsum}")

Factor

: get-input ( -- left-list right-list )
  "vocab:aoc-2024/01/input.txt" utf8 file-lines
  [ split-words harvest ] map unzip
  [ [ string>number ] map ] bi@ ;

: part1 ( -- n )
  get-input
  [ sort ] bi@
  [ - abs ] 2map-sum ;

: part2 ( -- n )
  get-input
  histogram
  '[ dup _ at 0 or * ] map-sum ;

https://github.com/AndydeCleyre/aoc-2024

Go

package main

import (
	"bufio"
	"fmt"
	"os"
	"sort"
	"strconv"
	"strings"
)

func main() {
	input, _ := os.Open("input.txt")
	defer input.Close()

	left, right := []int{}, []int{}

	scanner := bufio.NewScanner(input)
	for scanner.Scan() {
		line := scanner.Text()
		splitline := strings.Split(line, "   ")
		l, _ := strconv.Atoi(splitline[0])
		r, _ := strconv.Atoi(splitline[1])
		left, right = append(left, l), append(right, r)
	}

	fmt.Printf("part 1 - total diff: %d\n", part1(left, right))
	fmt.Printf("part 2 - new total: %d\n", part2(left, right))
}

func part1(left, right []int) int {
	diff := 0
	sort.Ints(left)
	sort.Ints(right)

	for i, l := range left {
		if l > right[i] {
			diff += (l - right[i])
		} else {
			diff += (right[i] - l)
		}
	}
	return diff
}

func part2(left, right []int) int {
	newTotal := 0

	for _, l := range left {
		matches := 0
		for _, r := range right {
			if l == r {
				matches++
			}
		}
		newTotal += l * matches
	}
	return newTotal
}

Kotlin

No 💜 for Kotlin here?

import kotlin.math.abs

fun part1(input: String): Int {
    val diffs: MutableList<Int> = mutableListOf()
    val pair = parse(input)
    pair.first.sort()
    pair.second.sort()
    pair.first.forEachIndexed { idx, num ->
        diffs.add(abs(num - pair.second[idx]))
    }
    return diffs.sum()
}

fun part2(input: String): Int {
    val pair = parse(input)
    val frequencies = pair.second.groupingBy { it }.eachCount()
    var score = 0
    pair.first.forEach { num ->
        score += num * frequencies.getOrDefault(num, 0)
    }
    return score
}

private fun parse(input: String): Pair<MutableList<Int>, MutableList<Int>> {
    val left: MutableList<Int> = mutableListOf()
    val right: MutableList<Int> = mutableListOf()
    input.lines().forEach { line ->
        if (line.isNotBlank()) {
            val parts = line.split("\\s+".toRegex())
            left.add(parts[0].toInt())
            right.add(parts[1].toInt())
        }
    }
    return left to right
}

Lua

Combined Solution

C#

using System;
using System.Linq;

public record Point(int X, int Y); 

static class Program
{
    static async Task Main(string[] args)
    {
        var data = (await ReadInputFromFile("data.txt")).ToArray();

        var part1Answer = CalculateTotalDifference(data);
        Console.WriteLine($"Part 1 = {part1Answer}");

        var part2Answer = CountFrequencies(data);
        Console.WriteLine($"Part 2 = {part2Answer}");
    }

    public static int CountFrequencies(ICollection<Point> points)
    {
        var freq = points
            .GroupBy(p => p.Y)
            .ToDictionary(g => g.Key, g => g.Count());
        return points
            .Sum(p => freq.GetValueOrDefault(p.X, 0) * p.X);
    }

    public static int CalculateTotalDifference(ICollection<Point> points)
        => points.OrderBy(p => p.X)
            .Zip(
                points.OrderBy(p => p.Y),
                (px, py) => Math.Abs(px.X - py.Y))
            .Sum();

    public static readonly char[] Delimiter = new char[] { ' ' };
    public static async Task<IEnumerable<Point>> ReadInputFromFile(string path)
        => (await File.ReadAllLinesAsync(path))
            .Select(l =>
            {
                var parts = l.Split(
                    Delimiter,
                    StringSplitOptions.TrimEntries | StringSplitOptions.RemoveEmptyEntries);
                return new Point(int.Parse(parts[0]), int.Parse(parts[1]));
            });
}

JavaScript

After writing a procedural to-the-point version in C, tried a JavaScript solution too because it’s just perfect for list comprehension. The part 2 search is inefficient but the data size is small.

const fs = require("fs");
const U = require("./util");

const pairs = fs
    .readFileSync(process.argv[2] || process.stdin.fd, "utf8")
    .split("\n")
    .filter(x => x != "")
    .map(x => x.split(/ +/).map(Number));

const ls = pairs.map(x => x[0]); ls.sort();
const rs = pairs.map(x => x[1]); rs.sort();

const p1 = U.sum(ls.map((l, i) => Math.abs(l - rs[i])));
const p2 = U.sum(ls.map(l => l * U.count(rs, l)));

console.log("01:", p1, p2);

https://github.com/sjmulder/aoc/blob/master/2024/js/day01.js

Smalltalk

day1p12: input    
	| list1 list2 nums dist sim |
	
	list1 := OrderedCollection new.
	list2 := OrderedCollection new.
	
	input linesDo: [ :l |
		nums := l substrings collect: [ :n | n asInteger ].
		list1 add: (nums at: 1).
		list2 add: (nums at: 2).
	].

	list1 sort.
	list2 sort.
	
	dist := 0.
	list1 with: list2 do: [ :a :b | dist := dist + (a - b) abs ].
	
	sim := list1 sumNumbers: [ :x | x * (list2 occurrencesOf: x) ].
	
	^ Array with: dist with: sim.

Haskell

import Control.Arrow
import Control.Monad
import Data.List
import Data.Map

part1 [a, b] = sum $ abs <$> zipWith (-) (sort a) (sort b)
part2 [a, b] = sum $ ap (zipWith (*)) (fmap (flip (findWithDefault 0) (freq b))) a
  where
    freq = fromListWith (+) . fmap (,1)

main = getContents >>= (print . (part1 &&& part2)) . transpose . fmap (fmap read . words) . lines

TypeScript

This is for part #2 only.

import { readFileSync } from 'fs'

const f = readFileSync('./input.txt', 'utf-8')
const lines = f.split("\n")

let rights = {}
for (const i in lines) {
	if (lines[i] == '') { continue }

	const [, right] = lines[i].split(/\s+/)
	if (rights[right] === undefined) {
		rights[right] = 0
	}

	rights[right]++
}

let ans = 0
for (const i in lines) {
	const [left] = lines[i].split(/\s+/)
	const similarity = rights[left]

	if (similarity) {
		ans += (Number(left) * rights[left])
	}
}

console.dir(ans)

Is it possible to get this more efficient? I would love a way that only required iterating over the list once, but I don’t really have the focus to puzzle it out any less than O(2n) (probably more than that, even, if you count reading in the data…).

Uiua

For entertainment purposes only, I’ll be trying a solution in Uiua each day until it all gets too much for me…

$ 3   4
$ 4   3
$ 2   5
$ 1   3
$ 3   9
$ 3   3
⊜∘⊸≠@\n     # Partition at \n.
⊜(⍆∵⋕)⊸≠@\s # Partition at space, parse ints, sort.

&p/+/(⌵-). # Part1 : Get abs differences, sum, print.

&p/+×⟜(/+⍉≡⌕)°⊂ # Part 2 : Count instances, mul out, sum, print.

Haskell

Plenty of scope for making part 2 faster, but I think simple is best here. Forgot to sort the lists in the first part, which pushed me waaay off the leaderboard.

import Data.List

main = do
  [as, bs] <- transpose . map (map read . words) . lines <$> readFile "input01"
  print . sum $ map abs $ zipWith (-) (sort as) (sort bs)
  print . sum $ map (\a -> a * length (filter (== a) bs)) as

Viml

I think viml is a very fun language, i like weird languages lol, so this year im doing it in viml while trying to use as many of the original ed/ex commands as i can (:d, :p, :a, :g, …)

!cp ./puzzle1 ./puzzle1.editing
e ./puzzle1.editing

1,$sort
let row1 = []

g/^\d/let row1 = add(row1, str2nr(expand("<cword>"))) | norm 0dw
1d
1,$sort
g/^\d/execute 'norm cc' .. string(abs(expand("<cword>") - row1[line('.') - 1]))

$a|---ANSWER---
0
.
1,$-1g/^\d/call setline("$", str2nr(getline("$")) + str2nr(expand("<cword>")))

read ./puzzle1

let cnt = 0
g/^\d/let cnt += expand("<cword>") *
            \ searchcount(#{pattern: '\s\+' .. expand("<cword>")}).total

echo cnt .. "\n"

w! ./puzzle1.editing

Rust

Right IDs are directly read into a hash map counter.

use std::str::FromStr;
use std::collections::HashMap;

fn part1(input: String) {
    let mut left = Vec::new();
    let mut right = Vec::new();
    for line in input.lines() {
        let mut parts = line.split_whitespace()
            .map(|p| u32::from_str(p).unwrap());
        left.push(parts.next().unwrap());
        right.push(parts.next().unwrap());
    }
    left.sort_unstable();
    right.sort_unstable();
    let diff: u32 = left.iter().zip(right)
        .map(|(l, r)| l.abs_diff(r))
        .sum();
    println!("{diff}");
}

fn part2(input: String) {
    let mut left = Vec::new();
    let mut right: HashMap<u32, u32> = HashMap::new();
    for line in input.lines() {
        let mut parts = line.split_whitespace()
            .map(|p| u32::from_str(p).unwrap());
        left.push(parts.next().unwrap());
        *right.entry(parts.next().unwrap()).or_default() += 1;
    }
    let similar: u32 = left.iter()
        .map(|n| n * right.get(n).copied().unwrap_or_default())
        .sum();
    println!("{similar}");
}

util::aoc_main!();