class Tree
  attr_reader :data

  def initialize(data)
    @data = data
  end

  def +(other)
    Tree.new(deep_merge(@data, other.is_a?(Tree) ? other.data : other))
  end

  delegate :empty?, to: :data

  def replace(other)
    @data.replace(other.data)
  end

  def root
    level(0)
  end

  def levels
    output = []
    i = -1
    while i += 1
      current_level = level(i)
      break if current_level.all?(&:nil?)
      output << current_level
    end
    output
  end

  def level(level_number)
    all_elements_at_level(@data, level_number)
  end

  private

  def deep_merge(a, b)
    case a
    when Hash
      return merge_hashes(a, b) if b.is_a?(Hash)
      return merge_array_hash(b, a) if b.is_a?(Array)
      [b, a]
    when Array
      return merge_arrays(a, b) if b.is_a?(Array)
      return merge_array_hash(a, b) if b.is_a?(Hash)
      [b] + a
    else
      return [a, b] if b.is_a?(Hash)
      return [a] + b if b.is_a?(Array)
      a == b ? a : [a, b]
    end
  end

  def merge_array_hash(a, b)
    if a.last.is_a? Hash
      a[0...-1] + [merge_hashes(a.last, b)]
    else
      a + [b]
    end
  end

  def merge_hashes(a, b)
    a.deep_merge(b) do |_, this_val, other_val|
      deep_merge(this_val, other_val)
    end
  end

  def merge_arrays(a, b)
    keys = merge_array_keys(a, b)
    hashes = merge_hashes(a.last.is_a?(Hash) ? a.last : {}, b.last.is_a?(Hash) ? b.last : {})
    if hashes.empty?
      keys
    else
      (keys - hashes.keys) + [hashes]
    end
  end

  def merge_array_keys(a, b)
    (a.reject { |e| e.is_a?(Hash) } + b.reject { |e| e.is_a?(Hash) }).uniq
  end

  def all_elements_at_level(data, level_number)
    return ground_level(data) if level_number == 0
    case data
    when Hash
      data.map { |_, v| all_elements_at_level(v, level_number - 1) }
    when Array
      data.map { |e| all_elements_at_level(e, level_number) }.flatten
    end
  end

  def ground_level(data)
    case data
    when Hash
      data.keys
    when Array
      data.map { |e| all_elements_at_level(e, 0) }.flatten
    else
      data
    end
  end
end