# Day 6
## Part 1
```elixir
# input = File.stream!("6/example.txt")
input = File.stream!("6/input.txt")
```
```elixir
defmodule U do
def parse(input) do
{_, obstacles, init_pos} = Enum.reduce(input, {0, MapSet.new(), nil}, fn line, {i, obstacles, init_pos} ->
{_, obstacles, init_pos} = Enum.reduce(String.codepoints(line), {0, obstacles, init_pos},
fn char, {j, obstacles, init_pos} ->
case char do
"." -> {j + 1, obstacles, init_pos}
"#" -> {j + 1, MapSet.put(obstacles, {i, j}), init_pos}
"^" -> {j + 1, obstacles, {i, j, :up}}
_ -> {j + 1, obstacles, init_pos}
end
end)
{i + 1, obstacles, init_pos}
end)
size = {(Enum.at(input, 0) |> String.codepoints() |> Enum.count()) - 1, input |> Enum.to_list |> Enum.count()}
{init_pos, obstacles, size}
end
end
```
```elixir
{pos, obstacles, size} = input |> U.parse
```
```elixir
defmodule P1 do
def step(pos, obstacles, size) do
step(pos, MapSet.new(), obstacles, size)
end
defp step({i, j, dir}, path, obstacles, {w, h} = size) do
path = MapSet.put(path, {i, j})
{i_next, j_next} =
case dir do
:up -> {i - 1, j}
:right -> {i, j + 1}
:down -> {i + 1, j}
:left -> {i, j - 1}
end
cond do
i_next < 0 || i_next >= h || j_next < 0 || j_next >= w ->
path
MapSet.member?(obstacles, {i_next, j_next}) ->
dir_next = case dir do
:up -> :right
:right -> :down
:down -> :left
:left -> :up
end
step({i, j, dir_next}, path, obstacles, size)
true ->
step({i_next, j_next, dir}, path, obstacles, size)
end
end
end
```
```elixir
P1.step(pos, obstacles, size) |> Enum.count
```
## Part 2
```elixir
candidates = P1.step(pos, obstacles, size)
```
```elixir
defmodule P2 do
def step(pos, obstacles, size) do
step(pos, MapSet.new(), obstacles, size)
end
defp step({i, j, dir} = pos, path, obstacles, {w, h} = size) do
if MapSet.member?(path, pos) do
true
else
path = MapSet.put(path, pos)
{i_next, j_next} =
case dir do
:up -> {i - 1, j}
:right -> {i, j + 1}
:down -> {i + 1, j}
:left -> {i, j - 1}
end
cond do
i_next < 0 || i_next >= h || j_next < 0 || j_next >= w ->
false
MapSet.member?(obstacles, {i_next, j_next}) ->
dir_next =
case dir do
:up -> :right
:right -> :down
:down -> :left
:left -> :up
end
step({i, j, dir_next}, path, obstacles, size)
true ->
step({i_next, j_next, dir}, path, obstacles, size)
end
end
end
end
```
```elixir
Enum.count(candidates, fn new_obst -> P2.step(pos, MapSet.put(obstacles, new_obst), size) end)
```
