Today I want to propose a new function for Elixir kernel: String.to_aoc_grid/1. But it’s not yet implemented so here we go (again)
Ps. Yes, there is a boolean arg. Don’t try that at home.
Pps. Counter + count. Naming is hard.
defmodule Aoc2025.Solutions.Y25.Day04 do
alias AoC.Input
@max_surrounding_rolls 3
@roll ?@
def parse(input, _part) do
Input.read!(input)
|> String.trim()
|> String.split("\n", split: true)
|> Enum.map(&String.to_charlist/1)
|> build_initial_grid()
end
def part_one(problem), do: count_accessible_rolls(problem)
def part_two(problem), do: count_accessible_rolls(problem, true)
def build_initial_grid(bare_grid_rows) do
for {row_data, row_idx} <- Enum.with_index(bare_grid_rows),
{col_data, col_idx} <- Enum.with_index(row_data),
into: %{} do
{{row_idx, col_idx}, col_data == @roll}
end
end
def count_surrounding_rolls(grid, {row, col}) do
for y <- [row - 1, row, row + 1],
x <- [col - 1, col, col + 1],
!(row == y && col == x),
reduce: 0 do
counter -> if grid[{y, x}], do: counter + 1, else: counter
end
end
def count_accessible_rolls(problem, reduce \\ false, counter \\ 0) do
{grid, count} =
Enum.reduce(problem, {problem, 0}, fn
{_cell, false}, acc ->
acc
{cell, true}, {grid, count} = acc ->
if count_surrounding_rolls(problem, cell) > @max_surrounding_rolls,
do: acc,
else: {%{grid | cell => false}, count + 1}
end)
if reduce && count > 0 do
count_accessible_rolls(grid, reduce, counter + count)
else
counter + count
end
end
end
