22.rb

players = data.split("\n\n").map do |s|
  s.lines[1..].join.ints
end

# part 1
loop do
  top = players.map(&:shift)
  (top[0] > top[1]) ? players[0] += top : players[1] += top.reverse
  if loser = players.find_index(&:empty?)
    p players[1 - loser].reverse.map.with_index { |c, ii| c * (ii + 1) }.sum
    exit
  end
end

# part 2
def combat(players)
  seen = Set.new
  loop do
    return 0 if seen.include?(players)
    seen << players.map(&:dup)

    top = players.map(&:shift)
    round = if players.zip(top).all? { |p, t| p.length >= t }
      combat(players.map.with_index { |p, ii| p[0, top[ii]] })
    else
      top[0] > top[1] ? 0 : 1
    end

    (round == 0) ? players[0] += top : players[1] += top.reverse
    if loser = players.find_index(&:empty?)
      return 1 - loser
    end
  end
end

winner = combat(players)
p players[winner].reverse.map.with_index { |c, ii| c * (ii + 1) }.sum