Day18a.rb

require 'json'

class Atom
  attr_accessor :val, :prv, :nxt, :become

  def initialize(val)
    @val = val
  end

  def leftmost
    self
  end

  def rightmost
    self
  end

  def split
    return false unless val >= 10
    val_l = val / 2
    val_r = val - val_l
    become.(Pair.new(Atom.new(val_l), Atom.new(val_r)))
    true
  end

  def explode(depth=0)
    false
  end

  def to_nomber
    val
  end

  def magnitude
    val
  end

  def inspect
    "#<Atom: #{to_nomber.inspect}>"
  end
end

class Pair
  attr_reader :left, :right
  attr_accessor :become

  def initialize(l, r)
    l.rightmost.nxt = r.leftmost
    r.leftmost.prv = l.rightmost
    l.become = method(:left=)
    r.become = method(:right=)
    @left = l
    @right = r
  end

  def leftmost
    left.leftmost
  end

  def rightmost
    right.rightmost
  end

  def left=(l)
    l.leftmost.prv = leftmost.prv
    leftmost.prv.nxt = l.leftmost if leftmost.prv
    l.rightmost.nxt = right.leftmost
    right.leftmost.prv = l.rightmost
    l.become = method(:left=)
    @left = l
  end

  def right=(r)
    r.rightmost.nxt = rightmost.nxt
    rightmost.nxt.prv = r.rightmost if rightmost.nxt
    r.leftmost.prv = left.rightmost
    left.rightmost.nxt = r.leftmost
    r.become = method(:right=)
    @right = r
  end

  def explode(depth=0)
    if depth >= 4
      left.prv.val += left.val if left.prv
      right.nxt.val += right.val if right.nxt
      become.(Atom.new(0))
      true
    else
      left.explode(depth + 1) or right.explode(depth + 1)
    end
  end

  def split
    left.split or right.split
  end

  def normalize
    loop do
      progress = false
      while explode
        progress = true
      end
      if split
        progress = true
      end
      break if !progress
    end
    self
  end

  def to_nomber
    [left.to_nomber, right.to_nomber]
  end

  def magnitude
    3 * left.magnitude + 2 * right.magnitude
  end

  def inspect
    "#<Pair: #{to_nomber.inspect}>"
  end
end

def construct(nomber)
  case nomber
  when Integer
    Atom.new(nomber)
  when Array
    nl, nr = nomber
    Pair.new(construct(nl), construct(nr))
  else
    raise "malformed nomber"
  end
end

ns = $<.map {|l| construct(JSON.parse(l))}
final = ns.reduce {|n1, n2| Pair.new(n1, n2).normalize}
puts final.magnitude