My first defeat.
I implemented part 2 by doing this:
It works on the sample input, but says it’s too low on the real one. Must be a bug somewhere, but time’s up.
Yes, the guard only turns right. So my thinking is if there is already a travelled path to the right of the current position, putting a boulder immediately ahead will create a cycle.
From the sample:
....#.....
....+---+#
....|...|.
..#.|...|.
....|..#|.
....|...|.
.#.O^---+.
........#.
#.........
......#...
When we “look right” after reaching the start position a 2nd time, we see the already travelled path in the same direction, put obstacle.
edit:
I think I forgot to account for cases where the guard could walk backwards along the same path: like this:
.#.
->#
#..
.O.
Anyway…
finally got time to complete Part1 <3
https://github.com/jarlah/advent_of_code/blob/master/lib/2024/day_6/Part1.ex
its pretty simple to do, IF you have the hunch early to create a tile formatter .. which I didn’t and made much later than I should have … this can easily display the grid back in the terminal … much easier to see offset errors
Hey there, I completed Part 1 & 2, but I’m not proud of my code 
so I keep it for myself 
Part2 runs in 3.5 sec, with Task.async_stream 
Haha. One thing ive learned in 16 years as developer. Never keep things for your self. Always something to learn, and some «bad code» can even look good to others 
but good job. I almost never think about speed. Unless i see its slow
impressive cod .. I mean cuP <3 
but seriously i see no issues with code 
how is this Mac mini going btw. and m1 model ? niiice clean desktop (for the picture I guess)
now ill shut up with the Segways
I’m weird. My desktop is always clean and tidy, otherwise I can’t work.
It’s a Mac Studio I bought almost 3 years ago, still so powerful I think I will keep it for years
{guard, grid} = :persistent_term.get(Day06)
grid = Map.put(grid, pos, ?\#)
@bjorng On my machine I have seen that if I modify the map it’s slower. I just carry the new obstacle pos around and return :obst arbitrarily when looking for that key.
I wonder if modifying the persistent term does a full copy of the term instead of having the rest of the map being pointed to the shared memory.
Edit: Hmmm I do not have this behaviour anymore. Should be caused by something else.
Anyway it runs under 1s at the expense of clarity, enough for today
defmodule AdventOfCode.Solutions.Y24.Day06 do
alias AdventOfCode.Grid
alias AoC.Input
def parse(input, _part) do
grid =
input
|> Input.stream!()
|> Grid.parse_stream(fn
_, "#" ->
{:ok, :obst}
{x, y}, "^" ->
Process.put(:start_pos, {x, y})
{:ok, :n}
_, _ ->
:ignore
end)
start = Process.get(:start_pos)
Process.delete(:start_pos)
{0, xo, 0, yo} = Grid.bounds(grid)
{grid, start, :n, {xo, yo}}
end
def part_one({grid, start, dir, {xo, yo}}) do
grid
|> loop_until_quits(start, dir, {xo, yo}, %{start => true})
|> map_size()
end
defp loop_until_quits(grid, pos, dir, {xo, yo}, acc) do
{x, y} = next_pos = Grid.translate(pos, dir, 1)
case Map.get(grid, next_pos) do
:obst ->
loop_until_quits(grid, pos, rotate(dir), {xo, yo}, acc)
nil when x in 0..xo//1 and y in 0..yo//1 ->
loop_until_quits(grid, next_pos, dir, {xo, yo}, Map.put(acc, next_pos, true))
nil ->
acc
_ ->
loop_until_quits(grid, next_pos, dir, {xo, yo}, Map.put(acc, next_pos, true))
end
end
defp rotate(:n), do: :e
defp rotate(:e), do: :s
defp rotate(:s), do: :w
defp rotate(:w), do: :n
def part_two({grid, start, dir, {xo, yo}}) do
candidates =
grid
|> loop_until_quits(start, dir, {xo, yo}, %{})
|> Map.keys()
:persistent_term.put(:grid, grid)
parent = self()
candidates
|> Enum.map(
&spawn_link(fn ->
grid = Map.put(:persistent_term.get(:grid), &1, :obst)
one_or_zero = loop_value(grid, start, dir, {xo, yo})
send(parent, one_or_zero)
end)
)
|> Enum.reduce(0, fn _, acc ->
receive do
n -> acc + n
end
end)
end
defp loop_value(grid, pos, dir, {xo, yo}) do
{x, y} = next_pos = Grid.translate(pos, dir, 1)
case Map.get(grid, next_pos) do
:obst ->
loop_value(grid, pos, rotate(dir), {xo, yo})
^dir ->
1
_ when x in 0..xo//1 and y in 0..yo//1 ->
loop_value(Map.put(grid, next_pos, dir), next_pos, dir, {xo, yo})
nil ->
0
end
end
end