bjorng

Erlang Core Team

Advent of Code 2021 - Day 12

Here is my solution:

bjorng/advent-of-code-2021/blob/77bdef7a51b428b58ffb4ab76f540901e636f841/day12/lib/day12.ex

defmodule Day12 do
  def part1(input) do
    solve(input, 1)
  end

  def part2(input) do
    solve(input, 2)
  end

  defp solve(input, max_visits) do
    pairs = parse(input)

    map = Enum.reduce(pairs, pairs, fn {from, to}, acc ->
      [{to, from} | acc]
    end)
    |> Enum.group_by(fn {from, _} -> from end)
    |> Enum.map(fn {from, destinations} ->
      {from, Enum.map(destinations, fn {_, to} -> to end)}
    end)
    |> Map.new

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19 comments

#advent-of-code

12 1605 19

2021-12-13 15:27:30 UTC

Most Liked

dallagi

Here is my solution

At first it was pretty slow (~6s for part2), and I naively fixed it via memoizing the result of count_paths with an Agent.

Only then I realized that performance problems were due to the fact I was using a regex for the “small cave” check
On the bright side I learnt how to profile elixir code

github.com

dallagi/aoc2021/blob/main/lib/aoc2021/day12.ex

defmodule Aoc2021.Day12 do
  def part1(input) do
    input
    |> parse()
    |> paths_count(false)
  end

  def part2(input) do
    input
    |> parse()
    |> paths_count(true)
  end

  defp paths_count(
         connections,
         can_visit_small_cave_twice?,
         position \\ "start",
         visited \\ MapSet.new()
       )

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Post #5

milli

My solution.
It looked very difficult at first, but turned out quite nicely in the end

Post #8

egze

My recursions are getting better, but it’s still not easy

y2021/day-12.livemd

Post #7

princemaple

I’ve been able to run my code on fly.io free tier livebook… Today the streak ends.

github.com

princemaple/aoc/blob/main/2021/day12.exs

# Title: Day12

# ── Untitled ──

data =
  "input"
  |> IO.getn(1_000_000)
  |> String.trim()
  |> String.split(["\n", "\r\n"], trim: true)
  |> Enum.map(&String.split(&1, "-"))

data =
  for [from, to] <- data do
    [{from, to}, {to, from}]
  end
  |> List.flatten()
  |> Enum.reject(fn {from, to} -> from == "end" or to == "start" end)
  |> Enum.group_by(fn {f, _t} -> f end, fn {_f, t} -> t end)

defmodule P1 do

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Post #3

princemaple

After some optimization my solution can now finish (and is pretty fast) on fly.io free tier livebook!

github.com

princemaple/aoc/blob/main/2021/day12.exs

# Title: Day12

# ── Untitled ──

data =
  "input"
  |> IO.getn(1_000_000)
  |> String.trim()
  |> String.split(["\n", "\r\n"], trim: true)
  |> Enum.map(&String.split(&1, "-"))

data =
  for [from, to] <- data do
    [{from, to}, {to, from}]
  end
  |> List.flatten()
  |> Enum.reject(fn {from, to} -> from == "end" or to == "start" end)
  |> Enum.group_by(fn {f, _t} -> f end, fn {_f, t} -> t end)

defmodule P1 do

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Post #9

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Advent of Code 2021 - Day 12

bjorng

Advent of Code 2021 - Day 12

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dallagi

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