Day 7

Deps & preparation

Mix.install([
  {:kino, "~> 0.4.0"}
])

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

Input

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

Part 1

# fuel cost is always 1
fuel_calc = fn positions, target_pos ->
  Enum.map(positions, fn position ->
    abs(position - target_pos)
  end)
  |> Enum.sum()
end

Enum.map(Enum.min(crabs)..Enum.max(crabs), fn target ->
  fuel_calc.(crabs, target)
end)
|> Enum.min()

Part 2

# fuel cost grows by step count (1 is 1, 2 is 2)
fuel_calc = fn positions, target_pos ->
  Enum.map(positions, fn position ->
    x = abs(position - target_pos)
    div(x * (x + 1), 2)
  end)
  |> Enum.sum()
end

Enum.map(Enum.min(crabs)..Enum.max(crabs), fn target ->
  fuel_calc.(crabs, target)
end)
|> Enum.min()