AdventOfCode/2020/elixir/day23/day23part2.exs

82 lines
2.1 KiB
Elixir

defmodule Day23Part2 do
@input "137826495"
def run do
{a, b} =
@input
|> String.graphemes()
|> Enum.map(&String.to_integer/1)
|> init_cups()
|> play(10_000_000)
|> next_two_after_1()
IO.puts("Next two cups: #{a}, #{b}")
IO.puts("Answer: #{a * b}")
end
def init_cups([first, second | _] = input) do
cups = :ets.new(:circle, [])
(input ++ for(i <- 10..1_000_000, do: i))
|> Stream.chunk_every(3, 1)
|> Stream.each(fn
[a, b, c] ->
:ets.insert(cups, {b, {a, c}})
[penultimate, last] ->
:ets.insert(cups, {first, {last, second}})
:ets.insert(cups, {last, {penultimate, first}})
end)
|> Stream.run()
{first, cups}
end
def play({_current, cups}, 0), do: cups
def play(state, times), do: move(state) |> play(times - 1)
def move({current, cups}) do
holding = take_three(cups, current)
dest = find_dest(holding, current)
cups = insert(holding, dest, cups)
[{^current, {_prev, next}}] = :ets.lookup(cups, current)
{next, cups}
end
def find_dest(holding, 1), do: find_dest(holding, 1_000_001)
def find_dest(holding, current) do
dest = current - 1
if dest in holding, do: find_dest(holding, dest), else: dest
end
def take_three(cups, current) do
[{^current, {prev, a}}] = :ets.lookup(cups, current)
[{^a, {^current, b}}] = :ets.lookup(cups, a)
[{^b, {^a, c}}] = :ets.lookup(cups, b)
[{^c, {^b, next}}] = :ets.lookup(cups, c)
[{^next, {^c, last}}] = :ets.lookup(cups, next)
:ets.insert(cups, {current, {prev, next}})
:ets.insert(cups, {next, {current, last}})
[a, b, c]
end
def insert([a, b, c], dest, cups) do
[{^dest, {prev, next}}] = :ets.lookup(cups, dest)
[{^next, {^dest, last}}] = :ets.lookup(cups, next)
:ets.insert(cups, {dest, {prev, a}})
:ets.insert(cups, {a, {dest, b}})
:ets.insert(cups, {c, {b, next}})
:ets.insert(cups, {next, {c, last}})
cups
end
def next_two_after_1(cups) do
[{1, {_prev, next}}] = :ets.lookup(cups, 1)
[{^next, {1, nextnext}}] = :ets.lookup(cups, next)
{next, nextnext}
end
end
Day23Part2.run()