The recursion rules tripped me up several times. My part 2 returned the right answer when I ran it with both the infinite example and the worked-out example, and it was only when I traced through the latter that I realized I had to copy a slice of the deck of cards, not the entire rest of the deck.
It does too much work.
defmodule Day22 do
def readinput() do
File.read!("22.input.txt")
|> String.split("\n\n", trim: true)
|> Enum.map(fn s ->
String.split(s, "\n", trim: true) |> tl() |> Enum.map(&String.to_integer/1)
end)
end
def part1([player1, player2] \\ readinput()) do
play1(player1, player2)
end
def play1([], cards), do: score(Enum.reverse(cards))
def play1(cards, []), do: score(Enum.reverse(cards))
def play1([c1 | cards1], [c2 | cards2]) when c1 > c2, do: play1(cards1 ++ [c1, c2], cards2)
def play1([c1 | cards1], [c2 | cards2]) when c2 > c1, do: play1(cards1, cards2 ++ [c2, c1])
def part2([player1, player2] \\ readinput()) do
play2(player1, player2)
end
def play2(cards1, cards2, p1set \\ MapSet.new(), p2set \\ MapSet.new())
def play2([], cards, _, _), do: {:player2, score(Enum.reverse(cards))}
def play2(cards, [], _, _), do: {:player1, score(Enum.reverse(cards))}
def play2([c1 | cards1] = p1cards, [c2 | cards2] = p2cards, p1set, p2set) do
cond do
p1cards in p1set or p2cards in p2set ->
{:player1, true}
c1 <= length(cards1) and c2 <= length(cards2) ->
case play2(Enum.take(cards1, c1), Enum.take(cards2, c2)) do
{:player1, _} ->
play2(
cards1 ++ [c1, c2],
cards2,
MapSet.put(p1set, p1cards),
MapSet.put(p2set, p2cards)
)
{:player2, _} ->
play2(
cards1,
cards2 ++ [c2, c1],
MapSet.put(p1set, p1cards),
MapSet.put(p2set, p2cards)
)
end
c1 > c2 ->
play2(cards1 ++ [c1, c2], cards2, MapSet.put(p1set, p1cards), MapSet.put(p2set, p2cards))
true ->
play2(cards1, cards2 ++ [c2, c1], MapSet.put(p1set, p1cards), MapSet.put(p2set, p2cards))
end
end
def score(cards) do
Stream.zip(cards, Stream.iterate(1, &(&1 + 1)))
|> Stream.map(fn {a, b} -> a * b end)
|> Enum.sum()
end
end
