Day 17

Mix.install([
  {:kino, "~> 0.6.2"},
  {:vega_lite, "~> 0.1.5"}
])

Input

[_, ranges] = "target area: x=79..137, y=-176..-117" |> String.split(": ", trim: true)
["x", xmin, xmax, "y", ymin, ymax] = String.split(ranges, [", ", "=", ".."])

[xmin, xmax, ymin, ymax] = [xmin, xmax, ymin, ymax] |> Enum.map(&String.to_integer(&1))

range = {xmin, xmax, ymin, ymax}

print_target = fn {xmin, xmax, ymin, ymax}, locations ->
  {dxmin, dxmax, dymin, dymax} = {min(xmin, 0), max(xmax, 0), min(ymin, 0), max(ymax, 0)}

  {dxmin, dxmax, dymin, dymax} =
    {
      min(dxmin, locations |> Enum.map(&elem(&1, 0)) |> Enum.min()),
      max(dxmax, locations |> Enum.map(&elem(&1, 0)) |> Enum.max()),
      min(dymin, locations |> Enum.map(&elem(&1, 1)) |> Enum.min()),
      max(dymax, locations |> Enum.map(&elem(&1, 1)) |> Enum.max())
    }
    |> IO.inspect()

  for y <- dymax..dymin do
    for x <- dxmin..dxmax do
      cond do
        {x, y} == {0, 0} ->
          'S'

        {x, y} in locations ->
          '*'

        x >= xmin and x <= xmax and y >= ymin and y <= ymax ->
          'T'

        true ->
          '.'
      end
    end
    |> Enum.concat()
    |> IO.puts()
  end
end

Part 1

path = fn {vx, vy}, {x, y} = pos, f ->
  if x <= xmax and y >= ymin do
    x = x + vx
    y = y + vy

    vx = if(vx == 0, do: 0, else: (abs(vx) - 1) * Integer.floor_div(vx, abs(vx)))
    vy = vy - 1
    [pos | f.({vx, vy}, {x, y}, f)]
  else
    []
  end
end

vxopt = round(:math.sqrt(xmin + 1) + 1)
vyopt = -ymin - 1

# print_target.(range, path.({vxopt, vyopt}, {0, 0}, path) |> IO.inspect())

hits = fn {xmin, xmax, ymin, ymax}, path ->
  for {x, y} <- path do
    x >= xmin and x <= xmax and y >= ymin and y <= ymax
  end
  |> Enum.any?()
end

ymax = round(vyopt * (vyopt + 1) / 2)

Part 2

vxmin = round(:math.sqrt(xmin + 1) + 1)
vxmax = xmax

vymin = ymin
vymax = -ymin - 1

for vx <- vxmin..vxmax,
    vy <- vymin..vymax do
  hits.(range, path.({vx, vy}, {0, 0}, path))
end
|> Enum.filter(&amp; &amp;1)
|> Enum.count()