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# Advent of Code 2023 - Day 23
```elixir
Mix.install([
{:req, "~> 0.4.0"},
{:libgraph, "~> 0.16.0"}
])
```
## Input
```elixir
opts = [headers: [{"cookie", "session=#{System.fetch_env!("LB_AOC_SESSION")}"}]]
puzzle_input = Req.get!("https://adventofcode.com/2023/day/23/input", opts).body
```
```elixir
input =
for {row, r} <- String.split(puzzle_input, "\n", trim: true) |> Enum.with_index(),
{col, c} <- String.graphemes(row) |> Enum.with_index(),
col != "#",
into: %{},
do: {{r, c}, col}
```
```elixir
size = Map.keys(input) |> Enum.max() |> Tuple.to_list() |> Enum.max()
```
```elixir
{start, "."} = Enum.find(input, nil, fn {{r, _c}, v} -> r == 0 and v == "." end)
```
```elixir
{goal, "."} = Enum.find(input, nil, fn {{r, _c}, v} -> r == size and v == "." end)
```
```elixir
defmodule ALongWalk do
def build_graph(g, grid, current, from, weight, seen, add_edge_fun) do
seen = [current | seen]
next(grid, current, seen)
|> do_build_graph(g, grid, current, from, weight, seen, add_edge_fun)
end
defp next(grid, {r, c}, seen) do
case grid[{r, c}] do
">" -> [{r, c + 1}]
"v" -> [{r + 1, c}]
"." -> [{r, c + 1}, {r + 1, c}, {r, c - 1}, {r - 1, c}]
end
|> Enum.reject(&is_nil(grid[&1]))
|> Enum.reject(&Enum.member?(seen, &1))
end
defp do_build_graph([], g, _grid, current, from, weight, _seen, add_edge_fun) do
add_edge_fun.(g, from, current, weight: weight * -1)
end
defp do_build_graph([next], g, grid, _current, from, weight, seen, add_edge_fun) do
build_graph(g, grid, next, from, weight + 1, seen, add_edge_fun)
end
defp do_build_graph(nexts, g, grid, current, from, weight, seen, add_edge_fun) do
g = add_edge_fun.(g, from, current, weight: weight * -1)
Enum.reduce(nexts, g, fn next, acc ->
build_graph(acc, grid, next, current, 1, seen, add_edge_fun)
end)
end
def longest_path(g, from, to) do
g
|> Graph.get_paths(from, to)
|> Stream.map(&path_weight(&1, g))
|> Enum.max()
end
def path_weight(path, g) do
path
|> Stream.chunk_every(2, 1, :discard)
|> Stream.map(fn [v1, v2] -> Graph.edge(g, v1, v2) end)
|> Stream.map(fn %Graph.Edge{weight: weight} -> abs(weight) end)
|> Enum.sum()
end
end
```
## Part 1
```elixir
Graph.new()
|> ALongWalk.build_graph(input, start, start, 0, [], &Graph.add_edge/4)
|> ALongWalk.longest_path(start, goal)
```
## Part 2
```elixir
Graph.new()
|> ALongWalk.build_graph(input, start, start, 0, [], fn g, v1, v2, opts ->
g
|> Graph.add_edge(v1, v2, opts)
|> Graph.add_edge(v2, v1, opts)
end)
|> ALongWalk.longest_path(start, goal)
```
[](https://livebook.dev/run?url=https%3A%2F%2Fgithub.com%2Fpehbehbeh%2Fadventofcode%2Fblob%2Fmain%2F2023%2F23.livemd)
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