Advent of code 2025 - Day 2

Description

Day 2: Gift Shop

defmodule Load do
  def input do
    File.read!("#{__DIR__}/inputs/day02.txt")
  end
end

defmodule Day2 do
  defp parse(input) do
    input
    |> String.replace("\n", "")
    |> String.split(",")
    |> Enum.map(fn range -> String.split(range, "-") end)
    |> Enum.map(fn [f, s] -> {String.to_integer(f), String.to_integer(s)} end)
  end

  def invalid_part1?(number) do
    string = Integer.to_string(number)
    {f, s} = String.split_at(string, div(String.length(string), 2))
    f == s
  end

  def invalid_part2?(number) do
    Regex.match?(~r/^(.+)\1+$/, Integer.to_string(number))
  end

  def invalid_value(value, invalidator) do
    if invalidator.(value) do
      value
    else
      0
    end
  end

  def range_invalid({s, e}, invalidator) do
    Enum.reduce(s..e, [], fn value, acc -> if invalidator.(value) do [value | acc] else acc end end)
  end 

  def solve(input, invalidator) do
    parse(input) 
    |> Enum.map(fn range -> range_invalid(range, invalidator) end)
    |> List.flatten()
    |> Enum.reduce(0, fn value, acc -> invalid_value(value, invalidator) + acc end)
  end
  
  def part1(input), do: solve(input, &invalid_part1?/1)

  def part2(input), do: solve(input, &invalid_part2?/1)
end

ExUnit.start(autorun: false)

defmodule Test do
  use ExUnit.Case, async: true

  @input """
  11-22,95-115,998-1012,1188511880-1188511890,222220-222224,
  1698522-1698528,446443-446449,38593856-38593862,565653-565659,
  824824821-824824827,2121212118-2121212124
  """

  test "part 1 - invalid ids add up" do
    assert Day2.part1(@input) == 1_227_775_554
  end

  test "part 2 - invalid ids add up" do
    assert Day2.part2(@input) == 4_174_379_265
  end
end

ExUnit.run()

Day2.part1(Load.input())

Day2.part2(Load.input())