#include <iostream>
#include <vector>
#include <numeric>
#include <algorithm>
#include <set>
#include <iterator>

int main() {
  std::multiset<int> l, r;
  int a, b;
  while (std::cin >> a >> b) {
    l.insert(a); r.insert(b);
  }
  std::vector<int> delta;
  std::transform(l.begin(), l.end(), r.begin(), std::back_inserter(delta),
    [](int x, int y) { return std::abs(x-y); }
  );
  std::cout << std::accumulate(delta.begin(), delta.end(), 0) << std::endl;
}

#include <iostream>
#include <numeric>
#include <set>

int main() {
  std::multiset<int> l, r;
  int a, b;
  while (std::cin >> a >> b) {
    l.insert(a); r.insert(b);
  }
  std::cout << std::accumulate(l.begin(), l.end(), 0, [&r](int acc, int x) {
    return acc + x * r.count(x);
  }) << std::endl;
}

"./input.actual"
|> System.IO.File.ReadAllText
|> fun source -> 
    System.Text.RegularExpressions.Regex.Matches(source, @"don't\(\)|do\(\)|mul\((\d+),(\d+)\)")
    |> Seq.fold
        (fun (acc, enabled) m ->
            match m.Value with
            | "don't()" -> acc, false
            | "do()" -> acc, true
            | mul when enabled && mul.StartsWith("mul") ->
                let (x, y) = int m.Groups.[1].Value, int m.Groups.[2].Value
                acc + x * y, enabled
            | _ -> acc, enabled ) 
        (0, true)
    |> fst
|> printfn "The sum of all valid multiplications with respect to do() and don't() is %A"

There's probably a smarter way to do this...

For part 1, I dumped everything into a matrix. Then I scanned it element by element. If I found an X, I searched in 8 directions from there and counted up if I found M A S in sequence.

For part 2 I searched for an A, checked each diagonal corner, and counted up if the opposites were M S or S M

https://github.com/gustafe/aoc2024/blob/main/d04-Ceres-Search.pl

urgh this took me much longer than it should have... part 1 was easy enough but then I got tied up in knots with part 2. Finally I just sorto-bogo-sorted it all into shape

Perl: https://github.com/gustafe/aoc2024/blob/main/d05-Print-Queue.pl

Recheck array size: 98
All rechecks passed after 5938 passes
Duration: 00h00m00s (634.007 ms)

I expect much wailing and gnashing of teeth regarding the parsing, which of course is utterly trivial if you know a bit if regex.

I got bit by the input being more than one line. Embarrasing.

I wonder if any input starts with a "don't()" or if it's too early for Erik to pull such trickery.

[ inputs / "" ] | [.,.[0]|length] as [$H,$W] |

#----- In bound selectors -----#
def x: select(. >= 0 and . < $W);
def y: select(. >= 0 and . < $H);

reduce (
  [
    to_entries[] | .key as $y | .value |
    to_entries[] | .key as $x | .value |
    [ [$x,$y],. ]  | select(last!=".")
  ] | group_by(last)[] # Every antenna pair #
    | combinations(2)  | select(first < last)
) as [[[$ax,$ay]],[[$bx,$by]]] ({};
  # Assign linear anti-nodes #
  .[ range(-$H;$H) as $i | "\(
    [($ax+$i*($ax-$bx)|x), ($ay+$i*($ay-$by)|y)] | select(length==2)
  )"] = true
) | length

use strict;
use List::Util qw( min max );

open(FH, '<', $ARGV[0]) or die $!;

my @left;
my @right;

while (<FH>) {
	my @nums = split /\s+/, $_;
	push(@left, $nums[0]);
	push(@right, $nums[1]);
}

@left = sort { $b <=> $a } @left;
@right = sort { $b <=> $a } @right;

my $dist = 0;
my $sim = 0;
my $i = 0;

foreach my $lnum (@left) {
	$sim += $lnum * grep { $_ == $lnum } @right;

	my $rnum = $right[$i++];
	$dist += max($lnum, $rnum) - min($lnum, $rnum);
}

print 'Part 1: ', $dist, "\n";
print 'Part 2: ', $sim, "\n";

close(FH);

RegExp(r"(?:don\'t\(\)(?:.(?<!do\(\)))*do\(\))|(?:mul\((\d*),(\d*)\))") .allMatches(input) .fold<int>( 0, (p, e) => p + (e.group(0)![0] != 'd' ? e.groups([1, 2]).fold<int>(1, (p, f) => p * int.parse(f!)) : 0))

*ecma balls

reduce (
  inputs |   scan("do\\(\\)|don't\\(\\)|mul\\(\\d+,\\d+\\)")
         | [[scan("(do(n't)?)")[0]], [ scan("\\d+") | tonumber]]
) as [[$do], [$a,$b]] (
  { do: true, s: 0 };
    if $do == "do" then .do = true
  elif $do         then .do = false
  elif .do         then .s = .s + $a * $b end
) | .s

#include <iostream>
#include <vector>
#include <algorithm>
#include <sstream>
#include <string>
#include <unordered_map>

std::unordered_multimap<int, int> ordering;

bool sorted_before(int a, int b) {
  auto range = ordering.equal_range(a);
  for (auto it = range.first; it != range.second; ++it) {
    if (it->second == b) return true;
  }
  return false;
}

int main() {
  int sum = 0;
  std::string line;
  while (std::getline(std::cin, line) && !line.empty()) {
    int l, r;
    char c;
    std::istringstream iss(line);
    iss >> l >> c >> r;
    ordering.insert(std::make_pair(l,r));
  }
  while (std::getline(std::cin, line)) {
    std::istringstream iss(line);
    std::vector<int> pages;
    int page;
    while (iss >> page) {
      pages.push_back(page);
      iss.get();
    }   
    std::vector<int> sorted_pages = pages;
    std::stable_sort(sorted_pages.begin(), sorted_pages.end(), sorted_before);
    if (pages == sorted_pages) {  // Change to != for 5-2
      sum += sorted_pages[sorted_pages.size()/2];
    }
  }
  std::cout << "Sum: " << sum << std::endl;
}

Except that gives you all the ways to reach 9s from a 0, which is part 2. For part 1, I changed the scores to be sets of reachable 9s, and the score of a square was the size of the set at that position.

Part 1. A quick look at the data showed that the input length was short enough to perform an O(2^n^) search with some early exits. I coded it as a dfs.

Part 2. Adding concatenation just changes the base from 2 to 3, which, while strictly slower, wasn't much slower for this input.

void d7(bool sub) => print(getLines()
    .map((l) => l.split(RegExp(r':? ')).map(int.parse).toList())
    .fold<int>(
        0, (p, ops) => test(ops, ops[0], ops[1], 2, sub) ? ops[0] + p : p));

bool test(List<int> l, int cal, int cur, int i, bool sub) =>
    cur == cal && i == l.length ||
    (i < l.length && cur <= cal) &&
        (test(l, cal, cur + l[i], i + 1, sub) ||
            test(l, cal, cur * l[i], i + 1, sub) ||
            (sub && test(l, cal, cur.concat(l[i]), i + 1, sub)));

I am pretty sure I did an OK job at avoiding edge cases, though I suspect this problem has many of them.

The complex part was the "look for a hole" part. My input size was 19999, meaning an O(n^2^) solution was probably fast enough, but I decided to optimise and use a min heap for each space size prematurely. I foresaw that you need to split up a space if it is oversized for a particular piece of data, i.e. pop the slot from the heap, reduce the size of the slot, and put it in the heap corresponding to the new size.

I treated the input as a list of strings and searched for "XMAS" and "SAMX", rotating and transposing the list to scan for vertical and diagonal words.

In my arrogance, I thought I could do the diagonal rotation easily, but I was very wrong. I got it out in the end, though.

As it turns out, that instinct was also correct. By drawing the sample data (or counting or printing it out), I noticed that every page number had a defined relation with every other page number. This meant that a total ordering (rather than a lattice) existed, meaning it was possible to construct a comparison function.

So, the algorithm for part 1 was to check if a list was sorted, and part 2 was to sort the list. There's probably a 1-3 line solution for both parts a and b, but that's for Mr. The Reader.

Context: I participated (and completed!) in AoC last year and pragmatically wrote my code as a set of utility modules for solving these pathological problems. So, I had about 80% of the boilerplate for this problem written, waiting for me to read and relearn.

Anyway, the analysis: P1. was pretty straightforward. Just walk along the map, turn right if you hit an obstacle, and stop when you leave the map. I guessed that there may be a case where one needs to turn in place more than once to escape an obstacle, but I never checked if that was true. Either way, I got the answer out. This is a worst-case O(n^2^) solution, which was fast for n = 130.

P2. I chose to brute force this, and it was fine. I iterated through the grid, placing a wall if possible, and checked if this produced a loop using an explored set. This is worst case O(n^4^), which, for n=130, takes a few seconds to spit out the answer. It's parallelisable, though, so there's that. If a faster solution existed, I'd love to know.