Advent of Code Day 7, in Elixir!

concurrent brute force again, with fancy pattern matching!

I did Day 7 in Elixir! (Previously: Day 6, in Elixir, and Day 1, Day 2, Day 3, Day 4, Day 5, all in Raku).

Today’s solution is pretty straightforward. Again, part 2 is similar to part 1 so I’ll just cover part 2.

defmodule BridgeCalibrations do
  defp test(target, target, []), do: true
  defp test(_target, _acc, []), do: false
  defp test(target, acc, [i | rest]) do
    test(target, acc + i, rest) or test(target, acc * i, rest)
    or test(target, concat(acc, i), rest)
  end
  def test(target, [i | rest]), do: test(target, i, rest)

  defp concat(a, b), do: a * 10 ** (b |> Integer.digits |> Enum.count) + b
end

test pattern matches on its inputs. If we’ve reached the target and there are no values remaining, the line succeeds. Otherwise, if they’re different, the line fails the test.¹ If the array isn’t empty, we test adding, multiplying, and concatenating.

concat uses the Integer.digits function, which returns a list of digits in the number.²

Now to use the module:

File.stream!("data.txt")
|> Task.async_stream(fn line ->
  [target, operands] = String.split(line, ": ")
  operands = operands |> String.trim |> String.split(" ") |> Enum.map(&String.to_integer/1)
  target = String.to_integer(target)

  if BridgeCalibrations.test(target, operands), do: target, else: 0
end, ordered: false)
|> Stream.map(fn {:ok, i} -> i end)
|> Enum.sum
|> IO.inspect

We spawn a task for each line that parses the line, then tests. Then we sum up the results. Spawning a task for each line with Task.async_stream cuts our runtime by 200ms on my machine compared to a Stream.map, which is pretty significant as our overall runtime (using tasks) is 400ms; that’s a 33% reduction! BEAM processes are pretty lightweight.