day20.rb

if ARGV.empty?
  data = [1, 2, -3, 3, -2, 0, 4]
else
  data = File
    .readlines(ARGV[0], chomp: true)
    .map(&:to_i)
end

def mix(data)
  # iterate over all the elements and
  # shuffle them to the right positions
  data.length.times do |i|
    data.rotate! until data.first.last == i

    head, *tail = data
    data = [
      head,
      *tail.rotate(head.first)
    ].rotate(-head.first)
  end

  data.rotate! until data.first.last == 0
  data
end

data = data.each_with_index.to_a

# part 1
# numbers = mix(data).map(&:first)
# zero = numbers.find_index(0)
# numbers = numbers.cycle.take(zero + 3001)
# p numbers[zero + 1000] + numbers[zero + 2000] +
#   numbers[zero + 3000]

# part 2
data = data.map {|n, i| [n * 811589153, i] }
10.times do |i|
  puts "Round #{i}"
  data = mix(data)
end

numbers = data.map(&:first)
zero = numbers.find_index(0)
numbers = numbers.cycle.take(zero + 3001)
p numbers[zero + 1000] + numbers[zero + 2000] +
  numbers[zero + 3000]

https://youtu.be/oEDjAUS3sjc