Advent of Code 2024

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

Day 2

input =
  "https://adventofcode.com/2024/day/2/input"
  |> Req.get!(headers: [cookie: "session=#{System.get_env("AOC_COOKIE")}"])
  |> Map.get(:body)

sample = """
7 6 4 2 1
1 2 7 8 9
9 7 6 2 1
1 3 2 4 5
8 6 4 4 1
1 3 6 7 9
"""

defmodule Day2 do
  def parse(input) do
    input
    |> String.split("\n", trim: true)
    |> Enum.map(&(String.split(&1, " ") |> Enum.map(fn s -> String.to_integer(s) end)))
  end

  def safe?(list) do
    list
    |> Enum.chunk_every(2, 1, :discard)
    |> Enum.map(fn [left, right] ->
      left - right
    end)
    |> then(fn diffs ->
      all_decreasing? = Enum.all?(diffs, &(&1 > 0))
      all_increasing? = Enum.all?(diffs, &amp;(&amp;1 < 0))
      safe_levels? = Enum.all?(diffs, &amp; abs(&amp;1) in 1..3)

      (all_decreasing? || all_increasing?) &amp;&amp; safe_levels?
    end)
  end
end

import Day2

Part 1

input
|> parse()
|> Enum.filter(&safe?/1)
|> Enum.count()

Part 2

input
|> parse()
|> Enum.filter(fn list ->
  if safe?(list) do
    true
  else
    0..(Enum.count(list) - 1)
    |> Enum.find(fn i ->
      list |> List.delete_at(i) |> safe?()
    end)
  end
end)
|> Enum.count()