Day 1

Mix.install([
  {:kino, "~> 0.14.2"}
])

Prep

input = Kino.Input.textarea("Puzzle input")

Part 1

defmodule Part1 do
  def solve(list) do
    left = Enum.take_every(list, 2) |> Enum.sort()
    right = Enum.take_every(tl(list), 2) |> Enum.sort()

    sum_arrays(left, right)
  end

  def sum_arrays([], []), do: 0

  def sum_arrays([h1 | left], [h2 | right]) do
    abs(h1 - h2) + sum_arrays(left, right)
  end
end

Kino.Input.read(input)
|> String.split(["\n", " "], trim: true)
|> Enum.map(&String.to_integer/1)
|> Part1.solve()

Alternative solutions

This solution is this one mixed with my own ideas

{left, right} =
  Kino.Input.read(input)
  |> String.split(["\n", " "], trim: true)
  |> Enum.map(&String.to_integer/1)
  |> Enum.chunk_every(2)
  |> Enum.map(&List.to_tuple/1)
  |> Enum.unzip()

Enum.zip(Enum.sort(left), Enum.sort(right))
|> Enum.map(fn {a, b} -> abs(a-b) end)
|> Enum.sum

Part 2

list =
  Kino.Input.read(input)
  |> String.split(["\n", " "], trim: true)
  |> Enum.map(&String.to_integer/1)

{left, right} = {Enum.take_every(list, 2), Enum.take_every(tl(list), 2)}

freqs = Enum.frequencies(right)
Enum.reduce(left, 0, fn v, acc -> acc + v * (freqs[v] || 0) end)

Alternative soltuion

So, let’s try some lessons learned in the alternative solution for the first part

# Same parsing pipeline
{left, right} =
  Kino.Input.read(input)
  |> String.split(["\n", " "], trim: true)
  |> Enum.map(&String.to_integer/1)
  |> Enum.chunk_every(2)
  |> Enum.map(&List.to_tuple/1)
  |> Enum.unzip()

freqs = Enum.frequencies(right)
# And then the trick with map |> sum instead of reduce (just looks cleaner)
Enum.map(left, fn a -> (freqs[a] || 0) * a end) |> Enum.sum

Today I Learned!

• Enum.unzip to convert list of tuples to multiple lists
• Sometimes Enum.reduce isn’t the cleanest option