Advent 2024 - Day 7

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

Setup

input = Kino.Input.textarea("Please paste your input file")

input = input
  |> Kino.Input.read()
  |> String.split("\n")
  |> Enum.map(fn line ->
    [target, nums] = String.split(line, ": ")

    {
      String.to_integer(target),
      nums |> String.split(" ") |> Enum.map(&(String.to_integer(&1)))
    }
  end)

defmodule ElephantMath do
  def run(goal, nums, symbols) do
    for combo <- possible_combos(nums, symbols) do
      calculate(combo) == goal
    end
  end

  def calculate([a | [symbol | [b | rest]]]) do
    result = case symbol do
      :+ -> a + b
      :* -> a * b
      :|| -> String.to_integer("#{Integer.to_string(a)}#{Integer.to_string(b)}")
    end

    calculate([result | rest])
  end

  def calculate([value]) do
    value
  end

  def possible_combos([a | [b | rest]], symbols) do
    for section <- possible_combos([b | rest], symbols) do
      symbols |> Enum.map(&amp;([a, &amp;1] ++ section))
    end
    |> Enum.concat()
  end

  def possible_combos([x], _symbols) do
    [[x]]
  end
end

Part 1

for {goal, nums} <- input do
  if ElephantMath.run(goal, nums, [:+, :*]) |> Enum.any?() do
    goal
  end
end
|> Enum.filter(&amp;(&amp;1))
|> Enum.sum()

Part 2

for {goal, nums} <- input do
  if ElephantMath.run(goal, nums, [:+, :*, :||]) |> Enum.any?() do
    goal
  end
end
|> Enum.filter(&amp;(&amp;1))
|> Enum.sum()