Day 17

Input

input = "target area: x=241..275, y=-75..-49"

test_input = "target area: x=20..30, y=-10..-5"

parse_input = fn input ->
  Regex.run(~r"target area: x=(-?\d+)\.\.(-?\d+), y=(-?\d+)..(-?\d+)", input)
  |> Enum.drop(1)
  |> Enum.map(&String.to_integer/1)
end

step = fn %{x: x, y: y, vx: vx, vy: vy, max_y: max_y, t: t} = probe ->
  # probe |> IO.inspect(label: "probe")
  x = x + vx
  y = y + vy

  vx =
    case vx do
      vx when vx > 0 -> vx - 1
      vx when vx < 0 -> vx + 1
      0 -> 0
    end

  vy = vy - 1
  max_y = max(y, max_y)

  %{probe | x: x, y: y, vx: vx, vy: vy, max_y: max_y, t: t + 1}
end

step_until_target = fn probe ->
  Stream.iterate(0, &(&1 + 1))
  |> Enum.reduce_while(probe, fn _, %{x: x, y: y, x1: x1, x2: x2, y1: y1, y2: y2} = acc ->
    cond do
      x > x2 || y < y1 ->
        {:halt, {:miss, acc}}

      x in x1..x2 &amp;&amp; y in y1..y2 ->
        {:halt, {:hit, acc}}

      true ->
        {:cont, step.(acc)}
    end
  end)
end

launch = fn vx, vy, x1, x2, y1, y2 ->
  probe = %{
    x: 0,
    y: 0,
    vx: vx,
    vy: vy,
    max_y: 0,
    t: 0,
    x1: x1,
    x2: x2,
    y1: y1,
    y2: y2,
    sx: vx,
    sy: vy
  }

  step_until_target.(probe)
end

Part 1

[x1, x2, y1, y2] = parse_input.(input)

for(x <- 0..100, y <- 0..100, do: {x, y})
|> Enum.map(fn {x, y} -> launch.(x, y, x1, x2, y1, y2) end)
|> Enum.filter(fn
  {:miss, _} -> false
  {:hit, _} -> true
end)
|> Enum.map(fn {:hit, p} -> p.max_y end)
|> Enum.max()

Part 2

[x1, x2, y1, y2] = parse_input.(input)

max_x = x2

for(x <- 0..max_x, y <- -y1..y1, do: {x, y})
|> Enum.map(fn {x, y} -> launch.(x, y, x1, x2, y1, y2) end)
|> Enum.filter(fn
  {:miss, _} -> false
  {:hit, _} -> true
end)
|> Enum.count()