Day 2: Gift Shop

Day 2: Gift Shop

Day 2: Gift Shop

Part 1

defmodule AwesomeInvalidIDCheckerPart1 do
  def run(number) do
    charlist = number
    |> Integer.to_string() 
    |> String.to_charlist()

    maybe_invalid = charlist
    |> Enum.frequencies
    |> Enum.all?(fn {_, cnt} -> rem(cnt, 2) == 0 end)

    if maybe_invalid do
      len = Enum.count(charlist)
      head = charlist |> Enum.slice(0..(div(len, 2) - 1))
      tail = charlist |> Enum.slice(div(len, 2)..(len - 1))
      head == tail
    else
      false
    end
  end
end

defmodule AdventOfCode2025Day2Part1 do
  def run(input) do
    input
    |> parse_input()
    |> solve()
  end

  defp solve(list_of_ranges) do
    list_of_ranges
    |> Enum.reduce(0, fn range, acc ->
      range
      |> Enum.reduce(acc, fn number, acc ->
        fun(AwesomeInvalidIDCheckerPart1.run(number), number, acc)
      end)
    end)
  end

  defp fun(true, number, acc), do: number + acc
  defp fun(false, _number, acc), do: acc

  defp parse_input(input) do
    input
    |> String.split(",", trim: true)
    |> Enum.map(&parse/1)
  end

  defp parse(range) do
    %{"first" => first, "last" => last} = Regex.named_captures(~r/(?\d+)-(?\d+)/, range)
    Range.new(String.to_integer(first), String.to_integer(last))
  end
end

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

AdventOfCode2025Day2Part1.run(input)

Part 2

defmodule AwesomeInvalidIDCheckerPart2 do
  def run(number) do
    charlist = number
    |> Integer.to_string() 
    |> String.to_charlist()

    if Enum.count(charlist) > 1 do
      1..div(Enum.count(charlist), 2)
      |> Enum.any?(fn count ->
        [head | tail] = Enum.chunk_every(charlist, count)
        Enum.all?(tail, & &1 == head)
      end)
    else
      false
    end
  end
end

defmodule AdventOfCode2025Day2Part2 do
  def run(input) do
    input
    |> parse_input()
    |> solve()
  end

  defp solve(list_of_ranges) do
    list_of_ranges
    |> Enum.reduce(0, fn range, acc ->
      range
      |> Enum.reduce(acc, fn number, acc ->
        fun(AwesomeInvalidIDCheckerPart2.run(number), number, acc)
      end)
    end)
  end

  defp fun(true, number, acc), do: number + acc
  defp fun(false, _number, acc), do: acc

  defp parse_input(input) do
    input
    |> String.split(",", trim: true)
    |> Enum.map(&parse/1)
  end

  defp parse(range) do
    %{"first" => first, "last" => last} = Regex.named_captures(~r/(?\d+)-(?\d+)/, range)
    Range.new(String.to_integer(first), String.to_integer(last))
  end
end

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

AdventOfCode2025Day2Part2.run(input)