At this rate, I might catch up. Here’s my first pass at part 1:
#!/usr/bin/env elixir
defmodule Day10.Part1 do
@ascii_adjustment 48
@trail_head 0
@destination 9
@search_depth @destination
defp parse(str) do
[row | _rows] =
rows =
str
|> String.split("\n", trim: true)
|> Stream.map(&to_charlist/1)
|> Enum.map(fn row ->
Enum.map(row, &(&1 - @ascii_adjustment))
end)
height = Enum.count(rows)
length = Enum.count(row)
trail_heads =
for y <- 0..(height - 1), x <- 0..(length - 1) do
{x, y}
end
|> Enum.reduce(
[],
fn {x, y} = pos, trail_heads = acc ->
c = rows |> Enum.at(y) |> Enum.at(x)
case c do
@trail_head -> [pos | trail_heads]
_ -> acc
end
end
)
%{
map: %{
height: height,
length: length,
rows: rows
},
trail_heads: trail_heads
}
end
defp on_board?({x, y}, height, length) when x < 0 or x >= length or y < 0 or y >= height,
do: false
defp on_board?(_pair, _height, _length), do: true
defp reachable_neighbors({x, y}, %{height: height, length: length, rows: rows}) do
current_value = rows |> Enum.at(y) |> Enum.at(x)
[{x + 1, y}, {x - 1, y}, {x, y + 1}, {x, y - 1}]
|> Enum.filter(fn {neighbor_x, neighbor_y} = pair ->
on_board?(pair, height, length) and
(rows |> Enum.at(neighbor_y) |> Enum.at(neighbor_x)) - current_value == 1
end)
end
defp reachable_destinations({x, y} = pos, %{rows: rows} = _map, 0 = _search_depth) do
if rows |> Enum.at(y) |> Enum.at(x) == @destination, do: MapSet.new([pos]), else: MapSet.new()
end
defp reachable_destinations(pos, map, search_depth) do
reachable_neighbors(pos, map)
|> Enum.reduce(
MapSet.new(),
fn neighbor, reachable_destinations ->
reachable_destinations(neighbor, map, search_depth - 1)
|> Enum.into(reachable_destinations)
end
)
end
defp unique_destination_count(trail_head, map, search_depth) do
reachable_neighbors(trail_head, map)
|> Enum.reduce(
MapSet.new(),
fn neighbor, destinations_already_seen ->
reachable_destinations(neighbor, map, search_depth - 1)
|> Enum.into(destinations_already_seen)
end
)
|> Enum.count()
end
defp trail_head_scores(%{
map: map,
trail_heads: trail_heads
}) do
trail_heads
|> Enum.map(&unique_destination_count(&1, map, @search_depth))
|> Enum.sum()
end
def solve() do
File.read!("10/input.txt")
|> parse()
|> trail_head_scores()
|> IO.puts()
end
end
Day10.Part1.solve()
I misinterpreted part one and as I fixed it and got to part 2 I saw 81 and was like ‘wait, I saw that’.
