23.rb

# append to data
data += (data.max + 1..1000000).to_a
max = data.max

#
# build cups/lookup from data
#

Node = Struct.new(:value, :next)

lookup = {}
cups = nn = Node.new
data.each do |x|
  lookup[x] = nn.next = Node.new(x)
  nn = nn.next
end
cups = cups.next
nn.next = cups

10000000.times do
  # pick up three cups
  nn = cups
  4.times { nn = nn.next }
  pickup = cups.next
  cups.next = nn

  # pick destination label
  label = cups.value - 1
  loop do
    label = max if label == 0
    if pickup.value == label || pickup.next.value == label || pickup.next.next.value == label
      label -= 1
    else
      break
    end
  end

  # insert after label
  nn = lookup[label]
  after = nn.next
  nn.next = pickup
  pickup.next.next.next = after

  # advance
  cups = cups.next
end

# done
nn = lookup[1]
puts nn.next.value * nn.next.next.value