Wow, some really interesting solutions here. Mine is a fairly basic recursive approach that relies on processing the input into a list of strings. It is brittle because if the strings are of unequal length it would fail. I also hardcoded the string length in the rem/2 call, which I could have avoided by introducing another module attribute like @line_length or something.
defmodule Day3 do
@input File.stream!("lib/input") |> Enum.into([])
@finish Kernel.length(@input)
def progress(_, y, _, _, trees) when y >= @finish do
trees
end
def progress(x, y, run, rise, trees) do
case @input
|> Enum.at(y)
|> String.at(rem(x, 31)) do
"#" -> progress(x + run, y + rise, run, rise, trees + 1)
_ -> progress(x + run, y + rise, run, rise, trees)
end
end
end
(Day3.progress(0, 0, 1, 1, 0) * Day3.progress(0, 0, 3, 1, 0) * Day3.progress(0, 0, 5, 1, 0) *
Day3.progress(0, 0, 7, 1, 0) * Day3.progress(0, 0, 1, 2, 0))
|> IO.inspect()
