This is not a win today. I wrote the code, and it had an off-by-one error with the sample input in the final cycle. I couldn’t figure it out at all.
Someone else on reddit ran into the same problem, and I ended up peeking at their code. It’s a one-line Enum.reject in my code to fix it, but I don’t fully understand why I needed it.
The neighbor selection/search could be better.
defmodule Day17 do
def readinput() do
File.read!("17.input.txt")
|> String.split("\n", trim: true)
|> Enum.map(&String.split(&1, "", trim: true))
end
def neighbors3({x, y, z}) do
for i <- -1..1 do
for j <- -1..1 do
for k <- -1..1 do
{x + i, y + j, z + k}
end
end
end
|> List.flatten()
# we have to return the neighbors of a point, so we explicitly
# remove the point from the output
|> Enum.reject(fn {a, b, c} -> a == x and b == y and c == z end)
end
def neighbors4({x, y, z, w}) do
for i <- -1..1 do
for j <- -1..1 do
for k <- -1..1 do
for l <- -1..1 do
{x + i, y + j, z + k, w + l}
end
end
end
end
|> List.flatten()
|> Enum.reject(fn {a, b, c, d} -> a == x and b == y and c == z and d == w end)
end
def activeneighbors(state, neighborfn, point) do
Enum.count(neighborfn.(point), &Map.has_key?(state, &1))
end
def cycle(state, _, 0) do
state
|> Map.values()
|> Enum.count(fn cell -> cell == "#" end)
end
def cycle(state, neighborfn, count) do
# find the neighbors of active cells
tocheck =
state
|> Map.keys()
|> Enum.flat_map(neighborfn)
|> Enum.frequencies()
Enum.map(tocheck, fn {point, _} ->
cell = Map.get(state, point)
countactive = activeneighbors(state, neighborfn, point)
cond do
cell == "#" and countactive not in [2, 3] -> {point, :inactive}
countactive == 3 -> {point, :active}
true -> nil
end
end)
|> Enum.filter(& &1)
|> Enum.reduce(state, fn {p, newstate}, acc ->
case newstate do
:inactive -> Map.delete(acc, p)
:active -> Map.put(acc, p, "#")
end
end)
# why does this happen?
|> Enum.reject(fn {point, _} -> !Map.get(tocheck, point) end)
|> Map.new()
|> cycle(neighborfn, count - 1)
end
def part1(input \\ readinput()) do
rows = length(input)
cols = length(Enum.at(input, 0))
for c <- 0..(cols - 1) do
for r <- 0..(rows - 1) do
{{c, r, 0}, Enum.at(input, r) |> Enum.at(c)}
end
|> Enum.filter(fn {_, v} -> v == "#" end)
|> Enum.into(%{})
end
# i don't like this
|> Enum.reduce(%{}, fn m, acc -> Map.merge(acc, m) end)
|> cycle(&neighbors3/1, 6)
end
def part2(input \\ readinput()) do
rows = length(input)
cols = length(Enum.at(input, 0))
for c <- 0..(cols - 1) do
for r <- 0..(rows - 1) do
{{c, r, 0, 0}, Enum.at(input, r) |> Enum.at(c)}
end
|> Enum.filter(fn {_, v} -> v == "#" end)
|> Enum.into(%{})
end
# i don't like this
|> Enum.reduce(%{}, fn m, acc -> Map.merge(acc, m) end)
|> cycle(&neighbors4/1, 6)
end
end
