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);

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)")

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()

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

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

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))))))

Rust

use std::collections::{HashMap, HashSet};

use crate::solver::DaySolver;
use crate::grid::{Coordinate, Grid};

fn add_distance(coordinate: Coordinate, distance: (i64, i64)) -> Option<Coordinate> {
    coordinate.try_add(distance)
}

fn sub_distance(coordinate: Coordinate, distance: (i64, i64)) -> Option<Coordinate> {
    coordinate.try_sub(distance)
}

fn part2_possible_antinodes<F>(
    grid: &Grid<Option<char>>,
    coordinate: Coordinate,
    distance: (i64, i64),
    op: F,
    mut accumulator: Vec<Coordinate>
) -> Vec<Coordinate>
where F: Fn(Coordinate, (i64, i64)) -> Option<Coordinate> {
    match op(coordinate, distance).filter(|c| grid.get(*c).is_some()) {
        None => accumulator,
        Some(next_coord) => {
            accumulator.push(next_coord);
            part2_possible_antinodes(grid, next_coord, distance, op, accumulator)
        }
    }
}

trait Pairable<T> {
    fn pairs(&self) -> Vec<(&T, &T)>;
}

impl<T> Pairable<T> for HashSet<T> {
    fn pairs(&self) -> Vec<(&T, &T)> {
        let v: Vec<&T> = self.iter().collect();

        let mut p = vec![];

        for i in 0..v.len() {
            let thing1 = v[i];

            for thing2 in &v[i+1..] {
                p.push((thing1, *thing2));
            }
        }

        p
    }
}

fn parse_input(input: String) -> (Grid<Option<char>>, HashMap<char, HashSet<Coordinate>>) {
    let g: Grid<Option<char>> =
        input.lines()
        .map(|line| line.chars()
             .map(|c| if c == '.' {
                 None
             } else {
                 Some(c)
             }).collect::<Vec<Option<char>>>()
        )
        .collect::<Vec<Vec<Option<char>>>>()
        .into();

    let mut freq_to_coords: HashMap<char, HashSet<Coordinate>> = HashMap::new();

    for (coord, freq_opt) in g.iter() {
        match freq_opt {
            None => (),
            Some(freq) => {
                freq_to_coords.entry(*freq)
                    .and_modify(|coords| {
                        coords.insert(coord);
                    })
                    .or_insert(HashSet::from([coord]));
            }
        }
    }

    (g, freq_to_coords)
}

pub struct Day08Solver;

impl DaySolver for Day08Solver {
    fn part1(&self, input: String) -> usize {
        let (g, freq_to_coords) = parse_input(input);

        let mut antinodes: HashSet<Coordinate> = HashSet::new();

        for (_, coords) in freq_to_coords {
            // println!("Freq = {}", freq);
            for (c1, c2) in coords.pairs() {
                let distance = c1.xy_distance_to(c2);
                let possible_antinodes: Vec<Coordinate> = [c1.try_sub(distance), c2.try_add(distance)].into_iter()
                    .flat_map(|co| co.filter(|c| g.get(*c).is_some()))
                    .collect();

                // println!("Pair = ({},{}), antinodes = {:?}", c1, c2, possible_antinodes);

                for antinode in possible_antinodes {
                    antinodes.insert(antinode);
                }
            }
        }

        antinodes.len()
    }

    fn part2(&self, input: String) -> usize {
        let (g, freq_to_coords) = parse_input(input);

        let mut antinodes: HashSet<Coordinate> = HashSet::new();

        for (freq, coords) in freq_to_coords {
            println!("Freq = {}", freq);
            for (c1, c2) in coords.pairs() {
                let distance = c1.xy_distance_to(c2);

                let possible_antinodes: Vec<Coordinate> = [
                    part2_possible_antinodes(&g, *c1, distance, add_distance, vec![*c1]),
                    part2_possible_antinodes(&g, *c1, distance, sub_distance, vec![*c1]),
                    part2_possible_antinodes(&g, *c2, distance, add_distance, vec![*c2]),
                    part2_possible_antinodes(&g, *c2, distance, sub_distance, vec![*c2]),
                ].into_iter().flatten().collect();

                println!("Pair = ({},{}), antinodes = {:?}", c1, c2, possible_antinodes);

                for antinode in possible_antinodes {
                    antinodes.insert(antinode);
                }
            }
        }

        antinodes.len()
    }
}

https://gitlab.com/bricka/advent-of-code-2024-rust/-/blob/main/src/days/day08.rs?ref_type=heads

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;
  }
}

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

Nim

Overall really simple puzzle, but description is so confusing, that I mostly solved it based on example diagrams.
Edit: much shorter and faster one-pass solution. Runtime: 132 us

type Vec2 = tuple[x,y: int]
func delta(a, b: Vec2): Vec2 = (a.x-b.x, a.y-b.y)
func outOfBounds[T: openarray | string](pos: Vec2, grid: seq[T]): bool =
  pos.x < 0 or pos.y < 0 or pos.x > grid[0].high or pos.y > grid.high

proc solve(input: string): AOCSolution[int, int] =
  var grid = input.splitLines()
  var antennas: Table[char, seq[Vec2]]

  for y, line in grid:
    for x, c in line:
      if c != '.':
        discard antennas.hasKeyOrPut(c, newSeq[Vec2]())
        antennas[c].add (x, y)

  var antinodesP1: HashSet[Vec2]
  var antinodesP2: HashSet[Vec2]

  for _, list in antennas:
    for ind, ant1 in list:
      antinodesP2.incl ant1 # each antenna is antinode
      for ant2 in list.toOpenArray(ind+1, list.high):
        let d = delta(ant1, ant2)
        for dir in [-1, 1]:
          var i = dir
          while true:
            let antinode = (x: ant1.x+d.x*i, y: ant1.y+d.y*i)
            if antinode.outOfBounds(grid): break
            if i in [1, -2]: antinodesP1.incl antinode
            antinodesP2.incl antinode
            i += dir
  result.part1 = antinodesP1.len
  result.part2 = antinodesP2.len

Codeberg repo