Advent of Code 2024

Mix.install([
  {:req, "~> 0.3.2"}
])

Day 5

input =
  "https://adventofcode.com/2024/day/5/input"
  |> Req.get!(headers: [cookie: "session=#{System.get_env("AOC_COOKIE")}"])
  |> Map.get(:body)

sample = """
47|53
97|13
97|61
97|47
75|29
61|13
75|53
29|13
97|29
53|29
61|53
97|53
61|29
47|13
75|47
97|75
47|61
75|61
47|29
75|13
53|13

75,47,61,53,29
97,61,53,29,13
75,29,13
75,97,47,61,53
61,13,29
97,13,75,29,47
"""

defmodule Day5 do
  def parse(input) do
    [rules, updates] =
      input
      |> String.split("\n\n", trim: true)
      |> Enum.map(&String.split(&1, "\n", trim: true))

    rules =
      rules
      |> Enum.map(&String.split(&1, "|"))
      |> Enum.reduce(%{}, fn [left, right], rules ->
        Map.update(rules, left, MapSet.new([right]), &MapSet.put(&1, right))
      end)

    updates = updates |> Enum.map(&String.split(&1, ","))

    {rules, updates}
  end

  def safe?(rules, list) do
    safe?(rules, tl(list), hd(list))
  end

  def safe?(_rules, [], _), do: true

  def safe?(rules, [hd | rest] = list, cur) do
    m =
      Map.get(rules, cur, MapSet.new())
      |> MapSet.intersection(MapSet.new(list))

    if MapSet.size(m) == Enum.count(list) do
      safe?(rules, rest, hd)
    else
      false
    end
  end
end

import Day5

Part 1

{rules, updates} =
  input
  |> parse()

updates
|> Enum.filter(& safe?(rules, &1))
|> Enum.map(& &1 |> Enum.at(&1 |> Enum.count() |> div(2)) )
|> Enum.map(&String.to_integer/1)
|> Enum.sum()

Part 2

{rules, updates} =
  input
  |> parse()

updates
|> Enum.reject(&safe?(rules, &1))
|> Enum.map(fn update ->
  update
  |> Enum.sort(&(Map.get(rules, &1, MapSet.new()) |> MapSet.member?(&2)))
end)
|> Enum.map(& &1 |> Enum.at(&1 |> Enum.count() |> div(2)) )
|> Enum.map(&String.to_integer/1)
|> Enum.sum()