Rust

For the first time, I can post my solution, because I actually solved it on the day :D Probably not the cleanest or optimal solution, but it does solve the problem.

Very long, looking forward to someone solving it in 5 lines of unicode :D

#[cfg(test)]
mod tests {

    fn get_frequences(input: &str) -> Vec<char> {
        let mut freq = vec![];
        for char in input.chars() {
            if char == '.' {
                continue;
            }
            if !freq.contains(&char) {
                freq.push(char);
            }
        }
        freq
    }

    fn find_antennas(board: &Vec<Vec<char>>, freq: char) -> Vec<(isize, isize)> {
        let mut antennas = vec![];
        for (i, line) in board.iter().enumerate() {
            for (j, char) in line.iter().enumerate() {
                if *char == freq {
                    antennas.push((i as isize, j as isize));
                }
            }
        }
        antennas
    }

    fn calc_antinodes(first: &(isize, isize), second: &(isize, isize)) -> Vec<(isize, isize)> {
        let deltax = second.0 - first.0;
        let deltay = second.1 - first.1;

        if deltax == 0 && deltay == 0 {
            return vec![];
        }

        vec![
            (first.0 - deltax, first.1 - deltay),
            (second.0 + deltax, second.1 + deltay),
        ]
    }

    #[test]
    fn test_calc_antinodes() {
        let expected = vec![(0, -1), (0, 2)];
        let actual = calc_antinodes(&(0, 0), &(0, 1));
        for i in &expected {
            assert!(actual.contains(i));
        }
        let actual = calc_antinodes(&(0, 1), &(0, 0));
        for i in &expected {
            assert!(actual.contains(i));
        }
    }

    fn calc_all_antinodes(board: &Vec<Vec<char>>, freq: char) -> Vec<(isize, isize)> {
        let antennas = find_antennas(&board, freq);

        let mut antinodes = vec![];

        for (i, first) in antennas.iter().enumerate() {
            for second in antennas[i..].iter() {
                antinodes.extend(calc_antinodes(first, second));
            }
        }

        antinodes
    }

    fn prune_nodes(
        nodes: &Vec<(isize, isize)>,
        height: isize,
        width: isize,
    ) -> Vec<(isize, isize)> {
        let mut pruned = vec![];
        for node in nodes {
            if pruned.contains(node) {
                continue;
            }
            if node.0 < 0 || node.0 >= height {
                continue;
            }
            if node.1 < 0 || node.1 >= width {
                continue;
            }
            pruned.push(node.clone());
        }
        pruned
    }

    fn print_board(board: &Vec<Vec<char>>, pruned: &Vec<(isize, isize)>) {
        for (i, line) in board.iter().enumerate() {
            for (j, char) in line.iter().enumerate() {
                if pruned.contains(&(i as isize, j as isize)) {
                    print!("#");
                } else {
                    print!("{char}");
                }
            }
            println!();
        }
    }

    #[test]
    fn day8_part1_test() {
        let input: String = std::fs::read_to_string("src/input/day_8.txt").unwrap();

        let frequencies = get_frequences(&input);

        let board = input
            .trim()
            .split('\n')
            .map(|line| line.chars().collect::<Vec<char>>())
            .collect::<Vec<Vec<char>>>();

        let mut all_nodes = vec![];
        for freq in frequencies {
            let nodes = calc_all_antinodes(&board, freq);
            all_nodes.extend(nodes);
        }

        let height = board.len() as isize;
        let width = board[0].len() as isize;

        let pruned = prune_nodes(&all_nodes, height, width);

        println!("{:?}", pruned);

        print_board(&board, &pruned);

        println!("{}", pruned.len());

        // 14 in test
    }

    fn calc_antinodes2(first: &(isize, isize), second: &(isize, isize)) -> Vec<(isize, isize)> {
        let deltax = second.0 - first.0;
        let deltay = second.1 - first.1;

        if deltax == 0 && deltay == 0 {
            return vec![];
        }
        let mut nodes = vec![];
        for n in 0..50 {
            nodes.push((first.0 - deltax * n, first.1 - deltay * n));
            nodes.push((second.0 + deltax * n, second.1 + deltay * n));
        }

        nodes
    }

    fn calc_all_antinodes2(board: &Vec<Vec<char>>, freq: char) -> Vec<(isize, isize)> {
        let antennas = find_antennas(&board, freq);

        let mut antinodes = vec![];

        for (i, first) in antennas.iter().enumerate() {
            for second in antennas[i..].iter() {
                antinodes.extend(calc_antinodes2(first, second));
            }
        }

        antinodes
    }

    #[test]
    fn day8_part2_test() {
        let input: String = std::fs::read_to_string("src/input/day_8.txt").unwrap();

        let frequencies = get_frequences(&input);

        let board = input
            .trim()
            .split('\n')
            .map(|line| line.chars().collect::<Vec<char>>())
            .collect::<Vec<Vec<char>>>();

        let mut all_nodes = vec![];
        for freq in frequencies {
            let nodes = calc_all_antinodes2(&board, freq);
            all_nodes.extend(nodes);
        }

        let height = board.len() as isize;
        let width = board[0].len() as isize;

        let pruned = prune_nodes(&all_nodes, height, width);

        println!("{:?}", pruned);

        print_board(&board, &pruned);

        println!("{}", pruned.len());
    }
}

Haskell

I overslept 26 minutes (AoC starts at 06:00 here) which upsets me more than it should.
I thought this one was going to be hard on performance or memory but it was surprisingly easy.

import Control.Arrow hiding (first, second)
import Data.Bifunctor

import Data.Array.Unboxed (UArray)

import qualified Data.List as List
import qualified Data.Set as Set
import qualified Data.Array.Unboxed as Array

parse :: String -> UArray (Int, Int) Char
parse s = Array.listArray ((1, 1), (n, m)) . filter (/= '\n') $ s :: UArray (Int, Int) Char

        where
                n = takeWhile   (/= '\n') >>> length $ s
                m = List.filter (== '\n') >>> length >>> pred $ s

groupSnd:: Eq b => (a, b) -> (a', b) -> Bool
groupSnd = curry (uncurry (==) <<< snd *** snd)

cartesianProduct xs ys = [(x, y) | x <- xs, y <- ys]

calculateAntitone ((y1, x1), (y2, x2)) = (y1 + dy, x1 + dx)
        where
                dy = y1 - y2
                dx = x1 - x2

antennaCombinations = Array.assocs
        >>> List.filter (snd >>> (/= '.'))
        >>> List.sortOn snd
        >>> List.groupBy groupSnd
        >>> map (map fst)
        >>> map (\ xs -> cartesianProduct xs xs)
        >>> map (filter (uncurry (/=)))

part1 a = antennaCombinations
        >>> List.concatMap (map calculateAntitone)
        >>> List.filter (Array.inRange (Array.bounds a))
        >>> Set.fromList
        >>> Set.size
        $ a

calculateAntitones ((y1, x1), (y2, x2)) = iterate (bimap (+dy) (+dx)) (y1, x1)
        where
                dy = y1 - y2
                dx = x1 - x2

part2 a = antennaCombinations
        >>> List.map (map calculateAntitones)
        >>> List.concatMap (List.concatMap (takeWhile (Array.inRange (Array.bounds a))))
        >>> Set.fromList
        >>> Set.size
        $ a

main = getContents
        >>= print
        . (part1 &&& part2)
        . parse

Haskell

Not a very pretty solution today, I’m afraid.

import Control.Arrow
import Control.Monad
import Data.Biapplicative
import Data.Ix
import Data.Map (Map)
import Data.Map qualified as Map
import Data.Set qualified as Set

type Coords = (Int, Int)

readInput :: String -> Map Coords Char
readInput s =
  Map.fromAscList
    [ ((i, j), c)
      | (i, l) <- zip [0 ..] (lines s),
        (j, c) <- zip [0 ..] l
    ]

(.+.), (.-.) :: Coords -> Coords -> Coords
(.+.) = join biliftA2 (+)
(.-.) = join biliftA2 (-)

part1, part2 :: (Coords -> Bool) -> (Coords, Coords) -> [Coords]
part1 valid (p1, p2) =
  let s = p2 .-. p1
   in filter valid [p1 .-. s, p2 .+. s]
part2 valid (p1, p2) =
  let (si, sj) = p2 .-. p1
      d = gcd si sj
      s = (si `div` d, sj `div` d)
   in takeWhile valid (iterate (.+. s) p1)
        ++ takeWhile valid (drop 1 $ iterate (.-. s) p2)

pairs (x : xs) = map (x,) xs ++ pairs xs
pairs _ = []

main = do
  input <- readInput <$> readFile "input08"
  let antennas = Map.filter (/= '.') input
      antennaGroups =
        Map.foldrWithKey
          (\p c m -> Map.insertWith (++) c [p] m)
          Map.empty
          antennas
      valid =
        inRange
          . (Set.findMin &&& Set.findMax)
          $ Map.keysSet input
      antiNodes model =
        Set.fromList
          . concatMap (concatMap (model valid) . pairs)
          $ antennaGroups
  print . Set.size $ antiNodes part1
  print . Set.size $ antiNodes part2

Uiua

Adapting the part one solution for part two took me longer than part one did today, but I didn’t want to change much anymore.

I even got scolded by the interpreter to split the evaluating line onto multiple ones because it got too long.
Can’t say it’s pretty but it does it’s job ^^’

Run with example input here

PartOne ← (
  &rs ∞ &fo "input-8.txt"
  ⟜(▽¬∈".\n".◴)
  ⊜∘≠@\n.
  :¤⟜(:¤-1△)
  ≡(□⊚⌕)
  ◴/◇⊂⍚(≡(-:⟜-°⊟)⧅≠2)
  ⧻▽¬:⊙(/+⍉+)⟜⊓><,0
)

PartTwo ← (
  &rs ∞ &fo "input-8.txt"
  ⟜(▽¬∈".\n".◴⟜¤
    ▽:⟜≡(>1⧻⊚⌕)
  )
  ⊜∘≠@\n.
  :¤⟜(:¤-1△)
  ≡(□⊚⌕)
  ⊸⍚(
    ⧅≠2⊙¤
    ≡(:¤⟜-°⊟
      ⍢(⊙⊂⟜-⊙⊸⊢
      | ⋅(=0/++⊓><,0⊢))
      □⊙◌◌
    )
  )
  ◴/◇⊂/◇⊂
  ⧻▽¬:⊙(/+⍉+)⟜⊓><,0
)

&p "Day 8:"
&pf "Part 1: "
&p PartOne
&pf "Part 2: "
&p PartTwo

C#

public class Day08 : Solver
{
  private ImmutableArray<string> data;
  private int width, height;

  public void Presolve(string input) {
    data = input.Trim().Split("\n").ToImmutableArray();
    width = data[0].Length;
    height = data.Length;
  }

  public string SolveFirst() {
    Dictionary<char, List<(int, int)>> antennae = [];
    HashSet<(int, int)> antinodes = [];
    for (int i = 0; i < width; i++) {
      for (int j = 0; j < height; j++) {
        if ('.' == data[j][i]) continue;
        antennae.TryAdd(data[j][i], []);
        foreach (var (oi, oj) in antennae[data[j][i]]) {
          int di = i - oi;
          int dj = j - oj;
          int ai = i + di;
          int aj = j + dj;
          if (ai >= 0 && aj >= 0 && ai < width && aj < height) {
            antinodes.Add((ai, aj));
          }
          ai = oi - di;
          aj = oj - dj;
          if (ai >= 0 && aj >= 0 && ai < width && aj < height) {
            antinodes.Add((ai, aj));
          }
        }
        antennae[data[j][i]].Add((i, j));
      }
    }
    return antinodes.Count.ToString();
  }

  public string SolveSecond() {
    Dictionary<char, List<(int, int)>> antennae = [];
    HashSet<(int, int)> antinodes = [];
    for (int i = 0; i < width; i++) {
      for (int j = 0; j < height; j++) {
        if ('.' == data[j][i]) continue;
        antennae.TryAdd(data[j][i], []);
        foreach (var (oi, oj) in antennae[data[j][i]]) {
          int di = i - oi;
          int dj = j - oj;
          for (int ai = i, aj = j;
               ai >= 0 && aj >= 0 && ai < width && aj < height; 
               ai += di, aj +=dj) {
            antinodes.Add((ai, aj));
          }
          for (int ai = oi, aj = oj;
               ai >= 0 && aj >= 0 && ai < width && aj < height; 
               ai -= di, aj -=dj) {
            antinodes.Add((ai, aj));
          }
        }
        antennae[data[j][i]].Add((i, j));
      }
    }
    return antinodes.Count.ToString();
  }
}

Dart

This really does feel like a weekend break this year, maybe Eric and co have begun to realise that family time is more precious than work time :-)

import 'dart:math';
import 'package:more/more.dart';

solve(List<String> lines, int min, int max) {
  var map = ListMultimap<String, Point<int>>();
  for (var r in lines.indices()) {
    for (var ci in lines[r].split('').indexed()) {
      if (ci.value != '.') map[ci.value].add(Point(ci.index, r));
    }
  }
  var anti = <Point<int>>{};
  for (var k in map.keys) {
    for (var p in map[k].combinations(2, repetitions: false)) {
      var diff = p.last - p.first;
      for (var m in min.to(max)) {
        anti.addAll([p.first - diff * m, p.last + diff * m]);
      }
    }
  }

  return anti.count((e) =>
      e.x.between(0, lines.first.length - 1) &&
      e.y.between(0, lines.length - 1));
}

part1(List<String> lines) => solve(lines, 1, 2);

part2(List<String> lines) => solve(lines, 0, 50);

python

import aoc

def setup():
    lines = aoc.get_lines(8, stripped=True)
    ll = len(lines)
    fm = {f: [(x, y) for y, r in enumerate(lines)
              for x, z in enumerate(r) if z == f]
          for f in {z for r in lines for z in r if z != '.'}}
    return ll, fm

def fa(fm, ll, rh=False):
    ans = set()
    for cd in fm.values():
        l = len(cd)
        for i in range(l):
            x1, y1 = cd[i]
            for j in range(i + 1, l):
                x2, y2 = cd[j]
                dx, dy = x2 - x1, y2 - y1
                if rh:
                    for k in range(-ll, ll):
                        x, y = x1 + k * dx, y1 + k * dy
                        if 0 <= x < ll and 0 <= y < ll:
                            ans.add((x, y))
                else:
                    x3, y3, x4, y4 = x1 - dx, y1 - dy, x2 + dx, y2 + dy
                    if 0 <= x3 < ll and 0 <= y3 < ll:
                        ans.add((x3, y3))
                    if 0 <= x4 < ll and 0 <= y4 < ll:
                        ans.add((x4, y4))
    return len(ans)

def one():
    ll, fm = setup()
    print(fa(fm, ll))

def two():
    ll, fm = setup()
    print(fa(fm, ll, rh=True))

one()
two()

C

Not hard but a little fiddly.

#include "common.h"

#define GZ 52

static char g[GZ][GZ];
#define ANTI_P1 1
#define ANTI_P2 2
static uint8_t anti[GZ][GZ];
static int w,h;

int
main(int argc, char **argv)
{
	int p1=0,p2=0, x,y, x1,y1, ax,ay, i;
	char *lf;

	if (argc > 1)
		DISCARD(freopen(argv[1], "r", stdin));
	for (h=0; h<GZ && fgets(g[h], GZ, stdin); h++)
		;

	assert(feof(stdin));
	lf = strchr(g[0], '\n');
	assert(lf);
	w = lf - g[0];

	/*
	 * Find antenna pairs, then project backwards from the first,
	 * forwards from the second. Don't like the repetition but it
	 * makes for easy code.
	 */
	for (y=0; y<h; y++)
	for (x=0; x<w; x++) {
		if (!isalnum(g[y][x]))
			continue;

		for (y1=y; y1<h; y1++)
		for (x1=(y==y1?x+1:0); x1<w; x1++) {
			if (g[y][x] != g[y1][x1])
				continue;

			for (i=0; ; i++) {
				if ((ax = x-(x1-x)*i) <0 || ax>w ||
				    (ay = y-(y1-y)*i) <0 || ay>h)
					break;
				anti[ay][ax] |= ANTI_P1 * i==1;
				anti[ay][ax] |= ANTI_P2;
			}

			for (i=0; ; i++) {
				if ((ax = x1+(x1-x)*i) <0 || ax>w ||
				    (ay = y1+(y1-y)*i) <0 || ay>h)
					break;
				anti[ay][ax] |= ANTI_P1 * i==1;
				anti[ay][ax] |= ANTI_P2;
			}
		}
	}

	for (y=0; y<h; y++)
	for (x=0; x<w; x++) {
		p1 += !!(anti[y][x] & ANTI_P1);
		p2 += !!(anti[y][x] & ANTI_P2);
	}

	printf("08: %d %d\n", p1, p2);
	return 0;
}

https://github.com/sjmulder/aoc/blob/master/2024/c/day08.c

Julia

I was surprised when my solution worked for part 2 since I thought you also had to include fractions of antenna distances, but apparently not.

function readInput(inputFile::String)::Matrix{Char}
	f = open(inputFile,"r")
	lines::Vector{String} = readlines(f)
	close(f)
	cityMap = Matrix{Char}(undef,length(lines),length(lines[1]))
	for (i,l) in enumerate(lines)
		cityMap[i,:] = collect(l)
	end
	return cityMap
end

function getAntennaLocations(cityMap::Matrix{Char})::Dict
	antennaLocations = Dict{Char,Vector{Vector{Int}}}()
	for l=1 : size(cityMap)[1]
		for c=1 : size(cityMap)[2]
			cityMap[l,c]=='.' ? continue : nothing
			if !haskey(antennaLocations,cityMap[l,c])
				antennaLocations[cityMap[l,c]] = []
			end
			push!(antennaLocations[cityMap[l,c]],[l,c])
		end
	end
	return antennaLocations
end

function countAntinodes(cityMap::Matrix{Char},antLoc::Dict{Char,Vector{Vector{Int}}},withHarmonics::Bool)::Int #antLoc: antenna locations
	lBounds = 1:size(cityMap)[1]; cBounds = 1:size(cityMap)[2]
	anodeLocs::Matrix{Bool} = zeros(size(cityMap))
	for key in keys(antLoc)
		for i=1 : length(antLoc[key])
			withHarmonics&&length(antLoc[key])>1 ? anodeLocs[antLoc[key][i][1],antLoc[key][i][2]]=1 : nothing #add antenna locations as antinodes
			#should also add fractions of antenna distances, but works without
			for j=i+1 : length(antLoc[key])
				harmonic::Int = 1
				while true
					n1l = antLoc[key][i][1]+harmonic*(antLoc[key][i][1]-antLoc[key][j][1])
					n1c = antLoc[key][i][2]+harmonic*(antLoc[key][i][2]-antLoc[key][j][2])
					n2l = antLoc[key][j][1]+harmonic*(antLoc[key][j][1]-antLoc[key][i][1])
					n2c = antLoc[key][j][2]+harmonic*(antLoc[key][j][2]-antLoc[key][i][2])
					if n1l in lBounds && n1c in cBounds
						anodeLocs[n1l,n1c] = 1
					end
					if n2l in lBounds && n2c in cBounds
						anodeLocs[n2l,n2c] = 1
					end
					withHarmonics ? nothing : break
					!(n1l in lBounds) && !(n1c in cBounds) && !(n2l in lBounds) && !(n2c in cBounds) ? break : harmonic+=1
				end
			end
		end
	end
	return sum(anodeLocs)
end

@info "Part 1"
println("antinode count $(countAntinodes(getAntennaLocations(readInput("day08Input"))),false)")
@info "Part 2"
println("antinode count $(countAntinodes(getAntennaLocations(readInput("day08Input"))),faltrue)")

Uiua

Getting closer to an elegant solution, but still a way to go…

How to read this

Try it live!

Grid      ← ⊜∘⊸≠@\n "............\n........0...\n.....0......\n.......0....\n....0.......\n......A.....\n............\n............\n........A...\n.........A..\n............\n............"
Pairs     ← ∧(⊂⧅≠2⊚⌕)⊙¤⊸(◴▽⊸≠@.♭)Grid[] # get pairs of nodes in grid (both ways round).
InGrid    ← ▽≡/××<⧻Grid:≥0..            # (posns) Keeps those in range.
AntiNodes ← ↯∞_2≡(+¤⊙(פ)⊃⊣/-):¤        # (range, pairs) Find antinodes by taking offset, mul by range, add to last node, check it's in grid.
&p ⧻InGrid◴ AntiNodes ↘1⇡2 Pairs
&p ⧻InGrid◴ AntiNodes ↘0⇡50 Pairs

Lisp

Could probably just write points right to the results instead of to an intermediate list, but it runs instantly, so my motivation to do so was low.

(defun p1-process-line (line)
   (to-symbols line 'advt2024-d8))
  
(defun count-results (results)
  (loop for i from 0 below (array-total-size results)
        count (row-major-aref results i)))

(defun place-annode (pos results)
  (let ((x (first pos)) (y (second pos)))
    (when (in-map results x y) 
      (setf (aref results y x) t))))

(defun create-annodes-p1 (x1 y1 x2 y2)
  (let ((delta-x (- x2 x1)) (delta-y (- y2 y1)))
    (list (list (- x1 delta-x) (- y1 delta-y)) (list (+ x2 delta-x) (+ y2 delta-y)))))

(defun place-annodes (positions results create-annodes)
  (when positions
     (loop with a = (car positions)
           with x1 = (first a)
           with y1 = (second a)
           for b in (cdr positions)
           for ans = (funcall create-annodes x1 y1 (first b) (second b))
           do (dolist (a ans) (place-annode a results)))
     (place-annodes (cdr positions) results create-annodes)))

(defun place-all-annodes (xmits map &optional (create-annodes #'create-annodes-p1))
  (let ((results (make-array (array-dimensions map) :element-type 'boolean :initial-element nil)))
    (loop for k being the hash-key of xmits
          do (place-annodes (gethash k xmits) results create-annodes))
    results))

(defun find-transmitters (map)
  "look throught the map and record where the transmitters are in a hash map"
  (let ((h (make-hash-table)))
    (destructuring-bind (rows cols) (array-dimensions map)
      (loop for j from 0 below rows
            do (loop for i from 0 below cols
                     for v = (aref map j i)
                     unless (eql v '|.|)
                       do (push (list i j) (gethash v h))
                     )))
    h))

(defun run-p1 (file) 
  (let* ((map (list-to-2d-array (read-file file #'p1-process-line))))
    (count-results (place-all-annodes (find-transmitters map) map))
    ))

(defun create-annodes-2 (x1 y1 x2 y2 map)
  (destructuring-bind (rows cols) (array-dimensions map)
    (let* ((m (/ (- y2 y1) (- x2 x1) ))
           (b (- y2 (* m x2))))
      (loop for x from 0 below cols
            for y = (+ b (* x m))
            for r = (nth-value 1 (floor y))
            when (and (= r 0) (>= y 0) (< y rows))
              collect (list x y)))))

(defun run-p2 (file) 
  (let* ((map (list-to-2d-array (read-file file #'p1-process-line))))
    (count-results (place-all-annodes (find-transmitters map) map
                                      (lambda (x1 y1 x2 y2)
                                        (create-annodes-2 x1 y1 x2 y2 map))))))

And I of course misread and wasted a bunch of time debugging the second part, entirely missed the fact that antinodes occurred on top of the emanating antennae as well…

public static class LINQExt
{
  public static IEnumerable<(T,T)> PermutatePairs<T>(this IEnumerable<T> source) {
    return source.SelectMany(k => source.Where(v => !v?.Equals(k) ?? false).Select(v => (k, v)));
  }
}

struct Antenna
{
  public int X, Y;
  public char Frequency;
}

List<Antenna> antennae = new List<Antenna>();
int width, height;

public void Input(IEnumerable<string> lines)
{
  char[] map = string.Join("", lines).ToCharArray();
  width = lines.First().Length;
  height = lines.Count();

  for (int y = 0; y < height; ++y)
    for (int x = 0; x < width; ++x)
    {
      char at = map[y * width + x];
      if (at == '.')
        continue;

      antennae.Add(new Antenna{ X = x, Y = y, Frequency = at });
    }
}

public void Part1()
{
  HashSet<(int, int)> antinodes = new HashSet<(int, int)>();

  foreach (var antinode in antennae.GroupBy(k => k.Frequency).SelectMany(g => g.PermutatePairs()).SelectMany(v => GetOpposing(v.Item1, v.Item2)).Where(InRange))
    antinodes.Add(antinode);

  Console.WriteLine($"Unique antinodes: {antinodes.Count}");
}
public void Part2()
{
  HashSet<(int, int)> antinodes = new HashSet<(int, int)>();

  foreach (var antennaePair in antennae.GroupBy(k => k.Frequency).SelectMany(g => g.PermutatePairs()))
  {
    // Iterate separately, to make the handling of bound exit easier
    foreach (var antinode in GetAllOpposing(antennaePair.Item1, antennaePair.Item2).TakeWhile(InRange))
      antinodes.Add(antinode);
    foreach (var antinode in GetAllOpposing(antennaePair.Item2, antennaePair.Item1).TakeWhile(InRange))
      antinodes.Add(antinode);
  }
  Console.WriteLine($"Unique antinodes: {antinodes.Count}");
}

bool InRange((int, int) point) {
  return point.Item1 >= 0 && point.Item1 < width && point.Item2 >= 0 && point.Item2 < height;
}
(int, int)[] GetOpposing(Antenna a, Antenna b) {
  return new[] { (a.X + (a.X - b.X), a.Y + (a.Y - b.Y)), (b.X + (b.X - a.X), b.Y + (b.Y - a.Y)) };
}
IEnumerable<(int, int)> GetAllOpposing(Antenna a, Antenna b) {
  (int, int) diff = (a.X - b.X, a.Y - b.Y);
  (int, int) at = (a.X, a.Y);
  yield return at;

  while (true)
  {
    at.Item1 += diff.Item1;
    at.Item2 += diff.Item2;

    yield return at;
  }
}

J

J really doesn’t have hashes! Or anything like hashes! And it’s really annoying after a while!

What it does have is automatic internal optimization via hashing of the “index of” operation m i. n where m is a fixed list (the object being searched) and n is the query, which can vary. But as soon as you update m the hash table is thrown away. And you still have to choose some kind of numeric key, or store a list of boxed pairs where the first coordinate is the key – effectively this is an old-style Lisp association list, but with extra steps because you have to use boxing to defeat J’s automatic array concatenation and reshaping. If you want non-cubical shapes (J calls these “ragged arrays”), or heterogeneous lists, you end up writing u &.> a lot – this means “unbox, apply u then rebox”. J arrays are required to be rectangular and homogeneous, but a boxed anything is a single atom just like a number is.

It’s just a really bad choice of language if you want data structures other than essentially-cubical arrays. On the other hand, once you beat the list manipulation primitives into producing your 1970s Lisp data structure of choice, the rest of the program is as nice as it usually is.

data_file_name =: '8.data'
grid =: ,. > cutopen fread data_file_name
'rsize csize' =: $ grid
inbounds =: monad : '(*/ y >: 0 0) * (*/ y < rsize, csize)'
antenna_types =: (#~ (~: & '.')) ~. , grid
NB. list_antennas gives a list of boxed matrices of shape 2 n_k in cell k, where
NB. n_k is the number of antennas of type k and the rows are coordinates of that type
list_antennas =: monad define
   antenna_locs =. (# antenna_types) $ a:
   for_r. i. rsize do.
      for_c. i. csize do.
         cell =. y {~ <(r, c)
         if. '.' ~: cell do.
            at =. antenna_types i. cell
            antenna_locs =. ((<(r, c)) ,&.> at { antenna_locs) at} antenna_locs
         end.
      end.
   end.
   NB. _2 ]\ l reshapes l into length 2 rows without finding its length ahead of time
   (_2 & (]\))&.> antenna_locs
)
NB. a1 pair_antinodes a2 gives the two antinodes from that pair
pair_antinodes =: dyad : '(#~ inbounds"1) ((2 * x) - y) ,: (2 * y) - x'
NB. if u is a symmetric dyad expecting rank 1 arguments, u on_pairs is a monad
NB. expecting a list of rank 1 arguments, and yields the concatenation of x u y
NB. where (x, y) is drawn from the (unordered) pairs of elements of the argument
NB. see page_pairs in 5.ijs for a non-point-free version of pair enumeration
on_pairs =: adverb define
   ; @: (< @: u/"2) @: ({~ (; @: (< @: (,~"0 i.)"0) @: i. @: #))
)
NB. antinodes antennas gives a list (may contain duplicates) of all the antinodes from
NB. that set of antennas
antinodes =: pair_antinodes on_pairs
NB. on_antennas concatenates and uniquifies result lists from all antennas
on_antennas =: adverb define
   ~. @: ; @: (u &.>) @: list_antennas
)
result1 =: # antinodes on_antennas grid

NB. a1 res_antinodes a2 gives the list of antinodes from that pair with resonance
res_antinodes =: dyad define
   step =. (% +./) x - y
   NB. lazy: max_steps doesn't take location of x into account
   max_steps =. <. (rsize % 1 >. | 0 { step) <. (csize % 1 >. 1 { step)
   (#~ inbounds"1) x +"1 step *"1 0 i: max_steps
)
result2 =: # res_antinodes on_pairs on_antennas grid

Haskell

import Control.Arrow
import Control.Monad
import Data.List
import Data.Map qualified as M

type Pos = [Int]

parse :: String -> (Pos, [(Char, Pos)])
parse s = ([n, m], [(c, [i, j]) | i <- [0 .. n], j <- [0 .. m], c <- [l !! i !! j], c /= '.'])
  where
    l = lines s
    n = pred $ length $ head l
    m = pred $ length l

buildMap :: [(Char, Pos)] -> M.Map Char [Pos]
buildMap = M.fromListWith (++) . fmap (second pure)

allPairs :: [Pos] -> [(Pos, Pos)]
allPairs l = [(x, y) | (x : xs) <- tails l, y <- xs]

add = zipWith (+)
sub = zipWith (-)

antinodes :: Pos -> Pos -> [Pos]
antinodes a b = [a `sub` ab, b `add` ab]
  where
    ab = b `sub` a

inBounds [x', y'] [x, y] = x >= 0 && y >= 0 && x <= x' && y <= y'

antinodes' :: Pos -> Pos -> Pos -> [Pos]
antinodes' l a b = al ++ bl
  where
    ab = b `sub` a
    al = takeWhile (inBounds l) $ iterate (`sub` ab) a
    bl = takeWhile (inBounds l) $ iterate (`add` ab) b

part1 l = length . nub . filter (inBounds l) . concat . M.elems . fmap (allPairs >=> uncurry antinodes)
part2 l = length . nub . concat . M.elems . fmap (allPairs >=> uncurry (antinodes' l))

main = getContents >>= print . (uncurry part1 &&& uncurry part2) . second buildMap . parse

Rust

Pretty happy with my solution today. I took my time today as it was a bit of a slow day and did it in Rust instead of python. Having proper Vec2 types is very nice.

use std::{collections::HashMap, error::Error, io::Read};

use glam::{IVec2, Vec2};

fn permutations_of_size_two(antennas: &[Vec2]) -> Vec<[&Vec2; 2]> {
    let mut permutations = vec![];
    for (i, antenna) in antennas.iter().enumerate() {
        for j in 0..antennas.len() {
            if i == j {
                continue;
            }
            permutations.push([antenna, &antennas[j]])
        }
    }
    permutations
}

fn main() -> Result<(), Box<dyn Error>> {
    let mut input = String::new();
    std::io::stdin().read_to_string(&mut input)?;

    let height = input.lines().count() as i32;
    let width = input.lines().next().unwrap().len() as i32;

    let antenna_positions = input
        .lines()
        .enumerate()
        .flat_map(|(y, l)| 
            l.chars().enumerate().map(move |(x, c)| (Vec2::new(x as f32, y as f32), c))
        )
        .filter(|(_v, c)| *c != '.')
        .fold(HashMap::new(), |mut acc: HashMap<char, Vec<_>> , current| {
            acc.entry(current.1).or_default().push(current.0);
            acc
        });

    let mut antinodes = vec![];
    for (_c, antennas) in antenna_positions {
        let perms = permutations_of_size_two(&antennas);
        for [first, second] in perms {
            let mut i = 1.;
            loop {
                let antinode = (first + (second-first) * i).round();
                if (0..height).contains(&(antinode.x as i32)) &&
                    (0..width).contains(&(antinode.y as i32)) {
                        antinodes.push(antinode);
                } else {
                    break;
                }
                i += 1.;
            }
        }
    }

    let mut antinode_count = 0;
    let map = input
        .lines()
        .enumerate()
        .map(|(y, l)| 
            l.chars().enumerate().map(|(x, c)| {
                if antinodes.contains(&Vec2::new(x as f32, y as f32)) {
                    println!("({x},{y})");
                    antinode_count += 1;
                    return '#';
                }
                c
            }).collect::<String>()
        )
        .collect::<Vec<_>>()
        .join("\n");

    println!("{map}");
    println!("{antinode_count}");

    Ok(())
}