instructions, network = ARGF.read.split("\n\n")

network = network
  .scan(/(\w+) = \((\w+), (\w+)\)/)
  .to_h { [_1, { L: _2, R: _3 }] }

steps = ->(start, finish) {
  node = start
  instructions.chars.map(&:to_sym)
    .cycle.with_index.lazy
    .filter_map {|dir, i|
      node = network.fetch(node).fetch(dir)
      (node =~ finish) ? i+1 : nil
    }
    .first
}

# part one
p steps.("AAA", /ZZZ/)

# part two
nodes = network.keys.select { _1.end_with?(?A) }
node_steps = nodes.map { steps.(_1, /Z$/) }
p node_steps.inject(1, :lcm)