Day 17: No Such Thing as Too Much

Mix.install([
  {:kino, "~> 0.12.3"}
])

Modules

defmodule Parser do
  def parse(input) do
    input
    |> String.split("\n")
    |> Enum.map(&String.to_integer/1)
  end
end

defmodule Combinations do
  @moduledoc """
  Functions to facilitate finding combinations of lists of elements.
  """

  @doc """
  Finds all combinations of a given list of positive integers that sum
  to the total.
  """
  @spec sum_to(list(pos_integer()), pos_integer()) :: list(list(pos_integer()))
  def sum_to(elts, total)

  # base case when find a succesful combination
  def sum_to(_elts, 0), do: [[]]

  # base case when there are no elts remaining and non zero total
  def sum_to([], _total), do: []

  # general case
  def sum_to(elts, total) do
    [first | rest] = elts

    # we find all sequences that include the first element
    left =
      sum_to(rest, total - first)
      |> Enum.map(fn elts -> [first | elts] end)

    # and all sequences that do not include the first element
    right = sum_to(rest, total)

    # and then join them
    left ++ right
  end
end

Input

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

Part 1

input
|> Kino.Input.read()
|> Parser.parse()
|> Combinations.sum_to(150)
|> Enum.count()

Part 2

combinations =
  input
  |> Kino.Input.read()
  |> Parser.parse()
  |> Combinations.sum_to(150)

minimum =
  combinations
  |> Enum.map(&length/1)
  |> Enum.min()

combinations
|> Enum.count(&(length(&1) == minimum))