Day 4: Ceres Search
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Elixir
defmodule AdventOfCode.Solution.Year2024.Day04 do
use AdventOfCode.Solution.SharedParse
defmodule Map do
defstruct [:chars, :width, :height]
end
@impl true
def parse(input) do
chars = String.split(input, "\n", trim: true) |> Enum.map(&String.codepoints/1)
%Map{chars: chars, width: length(Enum.at(chars, 0)), height: length(chars)}
end
def at(%Map{} = map, x, y) do
cond do
x < 0 or x >= map.width or y < 0 or y >= map.height -> ""
true -> map.chars |> Enum.at(y, []) |> Enum.at(x, "")
end
end
def part1(map) do
dirs = for dx <- -1..1, dy <- -1..1, {dx, dy} != {0, 0}, do: {dx, dy}
xmas = String.codepoints("XMAS") |> Enum.with_index() |> Enum.drop(1)
for x <- 0..(map.width - 1),
y <- 0..(map.height - 1),
"X" == at(map, x, y),
{dx, dy} <- dirs,
xmas
|> Enum.all?(fn {c, n} -> at(map, x + dx * n, y + dy * n) == c end),
reduce: 0 do
t -> t + 1
end
end
def part2(map) do
for x <- 0..(map.width - 1),
y <- 0..(map.height - 1),
"A" == at(map, x, y),
(at(map, x - 1, y - 1) <> at(map, x + 1, y + 1)) in ["MS", "SM"],
(at(map, x - 1, y + 1) <> at(map, x + 1, y - 1)) in ["MS", "SM"],
reduce: 0 do
t -> t + 1
end
end
end
