This one was a real bear for me. I think I’m not yet thinking in proper functional terms, when I see some of the short elegant solutions. Here’s mine:
defmodule Day10 do
@input [
# original array of numbers given, removed here for brevity
] |> Enum.sort
@full_input @input ++ [List.last(@input) + 3]
def part1() do
jumps = count_jumps(@full_input, 0, %{})
jumps[1] * jumps[3]
end
def part2() do
find_ways_to_each([0|@full_input], %{0 => 1})[List.last(@full_input)]
end
defp count_jumps([next|rest], last, acc) do
diff = next - last
old = Map.get(acc, diff, 0)
count_jumps(rest, next, Map.put(acc, diff, old + 1))
end
defp count_jumps([], _, acc), do: acc
defp find_ways_to_each([here|rest], counts) do
new_counts =
rest
|> Enum.take(3)
|> Enum.filter(&(&1 <= here + 3))
|> add_to_map(counts[here], counts)
find_ways_to_each(rest, new_counts)
end
defp find_ways_to_each([], counts), do: counts
defp add_to_map([target|rest], ways_here, counts) do
add_to_map(rest, ways_here,
Map.update(counts, target, ways_here, &(&1 + ways_here)))
end
defp add_to_map([], _, counts), do: counts
end
