7.rb

class AoC7

  attr_reader :weights, :child_map, :root_name

  PATTERN = /^([a-z]+) \(([0-9]+)\)( -> ([a-z ,]+))?$/
  def initialize
    lines = IO.read("./input7.txt").strip.split("\n")
    @weights = {}
    @child_map = {}
    lines.each do |line|
      PATTERN =~ line
      @weights[$1] = $2.to_i
      @child_map[$1] = ($4 || "").split(", ")
    end
    all_children = @child_map.values.flatten

    @root_name = @weights.keys.find{|k| @child_map[k].size > 0 && !all_children.member?(k)}
  end

  def find_imbalance(name=nil)
    name ||= @root_name

    children = @child_map[name]

    if children.empty?
      return [name, @weights[name]]
    end

    child_results = children.collect{|n| find_imbalance(n)}

    child_weights = child_results.collect(&:last)
    if child_weights.uniq.size == 1
      return [name, @weights[name] + child_weights.inject(&:+)]
    else
      loner_weight, good_weight = child_weights.partition{|w| w == child_weights.first}.sort_by(&:size).collect(&:first)
      loner_name = child_results.find{|name, weight| weight == loner_weight}.first
      diff = loner_weight - good_weight
      raise (weights[loner_name] - diff).to_s
    end
  end
end

AoC7.new.tap{|x| puts x.root_name}.find_imbalance