hauleth
Advent of Code 2025 - Day 11
Graph traversals
Parse
graph =
puzzle_input
|> String.split("\n", trim: true)
|> Map.new(fn <<from::binary-3>> <> ": " <> rest ->
{from, String.split(rest)}
end)
Implementation
defmodule Servers do
def paths(graph, from, to), do: paths(graph, from, to, [from])
defp paths(_graph, to, to, acc), do: [Enum.reverse(acc)]
defp paths(graph, from, to, acc) do
if next = graph[from] do
Stream.flat_map(next, &paths(graph, &1, to, [&1 | acc]))
else
[]
end
end
def paths_through(graph, from, to, required),
do: path_through(graph, from, to, MapSet.new(required), %{})
defp path_through(_graph, to, to, required, memo),
do: {if(Enum.empty?(required), do: 1, else: 0), memo}
defp path_through(graph, from, to, required, memo) do
state = MapSet.delete(required, from)
with :error <- Map.fetch(memo, {from, state}),
{:ok, next} <- Map.fetch(graph, from) do
{sum, memo} =
Enum.reduce(next, {0, memo}, fn n, {sum, acc} ->
{c, next_acc} = path_through(graph, n, to, state, acc)
{c + sum, next_acc}
end)
{sum, Map.put(memo, {from, state}, sum)}
else
:error -> {0, memo}
{:ok, val} -> {val, memo}
end
end
end
Part 1
Servers.paths(graph, "you", "out") |> Enum.count()
Part 2
Servers.paths_through(graph, "svr", "out", ["dac", "fft"])
|> elem(0)
First Post!
lud
A similar solution but I divided the work in multiple steps:
count(svr -> fft) * count(fft -> dac) * count(dac -> out)
defmodule AdventOfCode.Solutions.Y25.Day11 do
alias AoC.Input
def parse(input, _part) do
input
|> Input.stream!(trim: true)
|> Map.new(&parse_line/1)
end
defp parse_line(line) do
<<inp::binary-size(3), ": ", rest::binary>> = line
{inp, String.split(rest)}
end
def part_one(full_io) do
count_paths(full_io, "you", "out")
end
defp count_paths(full_io, start, finish) do
counts = count_paths_loop(full_io, start, finish, Map.put(%{"out" => 0}, finish, 1))
Map.fetch!(counts, start)
end
defp count_paths_loop(full_io, current, finish, acc) do
exits = Map.fetch!(full_io, current)
acc =
Enum.reduce(exits, acc, fn exit_, acc ->
if Map.has_key?(acc, exit_) do
acc
else
count_paths_loop(full_io, exit_, finish, acc)
end
end)
value = Enum.sum_by(exits, &Map.fetch!(acc, &1))
Map.put(acc, current, value)
end
def part_two(full_io) do
{step1, step2} =
if count_paths(full_io, "fft", "dac") > 0 do
{"fft", "dac"}
else
true = count_paths(full_io, "dac", "fft") > 0
{"dac", "fft"}
end
count_paths(full_io, "svr", step1) *
count_paths(full_io, step1, step2) *
count_paths(full_io, step2, "out")
end
end
Edit: simplified the parse, taking inspiration from hauleth’s ![]()
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