aoc_1.ex

defmodule AdventOfCode.Day01 do
  def solve(part) when part == :one do
    nums = parse_input()
    init = delta(Enum.at(nums, 0), Enum.at(nums, -1))
    
    Enum.chunk_every(nums, 2, 1, :discard)
      |> Enum.reduce(init, fn([a, b], acc) ->
        acc + delta(a, b)
      end)
  end

  def solve(part) when part == :two do
    nums = parse_input()

    count = Enum.count(nums)
    middle = Float.floor(count/2) |> Kernel.trunc
    
    { _, total } = Enum.reduce(nums, { 0, 0 }, fn(n, { index, total }) ->
      b_index = rem((index + middle), count)
      b = Enum.at(nums, b_index)
      { index + 1, total + delta(n, b) }
    end)

    total  
  end

  defp parse_input do
    File.read!("./data/day1.txt")
      |> String.split("", trim: true)
      |> Enum.map(&String.to_integer/1)
  end

  defp delta(a, b) when a == b do; a end
  defp delta(_, _) do; 0 end
end