AdventOfCode/2020/day11/day11part2.exs

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Elixir
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defmodule Day11Part2 do
def run do
File.read!("input")
|> String.split("\n", trim: true)
|> Enum.map(&String.to_charlist/1)
|> parse_seat_plan()
|> run_until_stable()
|> count_occupied()
|> IO.inspect()
end
def run_until_stable(seats) do
new_seats = tick(seats)
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if new_seats == seats do
seats
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else
run_until_stable(new_seats)
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end
end
def tick(seats) do
Map.new(seats, fn {{x, y}, old_seat} -> toggle(old_seat, x, y, seats) end)
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end
def toggle(:empty, x, y, seats) do
new_value =
if occupied_visible(x, y, seats) == 0 do
:occupied
else
:empty
end
{{x, y}, new_value}
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end
def toggle(:occupied, x, y, seats) do
new_value =
if occupied_visible(x, y, seats) >= 5 do
:empty
else
:occupied
end
{{x, y}, new_value}
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end
def toggle(:floor, x, y, _), do: {{x, y}, :floor}
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def occupied_visible(x, y, seats) do
directions = [
{-1, -1},
{-1, 0},
{-1, +1},
{0, -1},
{0, +1},
{+1, -1},
{+1, 0},
{+1, +1}
]
Enum.count(directions, fn {vx, vy} -> check_visible_in_direction(x, y, vx, vy, 1, seats) end)
end
def count_occupied(seats) do
Enum.count(seats, &match?({_, :occupied}, &1))
end
def check_visible_in_direction(x, y, vx, vy, distance, seats) do
next = seats[{x + vx * distance, y + vy * distance}]
case next do
nil -> false
:empty -> false
:occupied -> true
:floor -> check_visible_in_direction(x, y, vx, vy, distance + 1, seats)
end
end
def parse_seat_plan(input) do
{_, seats} =
Enum.reduce(input, {0, %{}}, fn row, {y, seats} ->
{_, seats} =
Enum.reduce(row, {0, seats}, fn seat, {x, seats} ->
seat =
case seat do
?L -> :empty
?. -> :floor
?# -> :occupied
end
{x + 1, Map.put(seats, {x, y}, seat)}
end)
{y + 1, seats}
end)
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seats
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end
end
Day11Part2.run()