Recursive Circus — Ruby — #adventofcode Day 7

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Today's challenge introduces a set of processes balancing precariously on top of each other. We find them stuck and unable to get down because one of the processes is the wrong size, unbalancing the whole circus. Our job is to figure out the root from the input and then find the correct weight for the single incorrect process.

→ Full code on GitHub


So I didn't really intend to take a full polyglot approach to Advent of Code, but it turns out to have been quite fun, so I made a shortlist of languages to try. Building a tree is a classic application for object-orientation using a class to represent tree nodes, and I've always liked the feel of Ruby's class syntax, so I gave it a go.

First make sure we have access to Set, which we'll use later.

require 'set'

Now to define the CircusNode class, which represents nodes in the tree. attr :s automatically creates a function s that returns the value of the instance attribute @s

class CircusNode
  attr :name, :weight

  def initialize(name, weight, children=nil)
    @name = name
    @weight = weight
    @children = children || []

Add a << operator (the same syntax for adding items to a list) that adds a child to this node.

  def <<(c)
    @children << c
    @total_weight = nil

total_weight recursively calculates the weight of this node and everything above it. The @total_weight ||= blah idiom caches the value so we only calculate it once.

  def total_weight
    @total_weight ||= @weight + {|c| c.total_weight}.sum

balance_weight does the hard work of figuring out the proper weight for the incorrect node by recursively searching through the tree.

  def balance_weight(target=nil)
    by_weight ={|h, k| h[k] = []}
    @children.each{|c| by_weight[c.total_weight] << c}

    if by_weight.size == 1 then
      if target
        return @weight - (total_weight - target)
        raise ArgumentError, 'This tree seems balanced!'
      odd_one_out = {|k, v| v.length == 1}.first[1][0]
      child_target = {|k, v| v.length > 1}.first[0]
      return odd_one_out.balance_weight child_target

A couple of utility functions for displaying trees finish off the class.

  def to_s
    "#{@name} (#{@weight})"

  def print_tree(n=0)
    puts "#{'    '*n}#{self} -> #{self.total_weight}"
    @children.each do |child|
      child.print_tree n+1


build_circus takes input as a list of lists [name, weight, children]. We make two passes over this list, first creating all the nodes, then building the tree by adding children to parents.

def build_circus(data)
  all_nodes = {}
  all_children =

  data.each do |name, weight, children|
    all_nodes[name] = name, weight

  data.each do |name, weight, children|
    children.each {|child| all_nodes[name] << all_nodes[child]}
    all_children.merge children

  root_name = (all_nodes.keys.to_set - all_children).first
  return all_nodes[root_name]

Finally, build the tree and solve the problem! Note that we use String.to_sym to convert the node names to symbols (written in Ruby as :symbol), because they're faster to work with in Hashes and Sets as we do above.

data = do |line|
  match = /(?<parent>\w+) \((?<weight>\d+)\)(?: -> (?<children>.*))?/.match line
   match['children'] ? match['children'].split(', ').map {|x| x.to_sym} : []]

root = build_circus data

puts "Root node: #{root}"

puts root.balance_weight


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