class Hash

  def fast_sort_keys
    keys = self.keys.sort
    Hash[keys.zip(self.values_at(*keys))]
  end

  # TODO write fast sort values

  def populate_with_keys(min: nil, max: nil, explicit_default_value: 0)
    start_key = min || self.keys.min
    end_key = max || self.keys.max

    result = self.dup
    (start_key..end_key).each do |k|
      result[k] = self[k] || explicit_default_value
    end

    return result
  end

  # def collapse_values_on_modulo(integer, min: nil, max: nil, shift: nil)
  #   raise ArgumentError.new("Modulo integer had better be larger than shift") if integer <= shift.to_i
  #
  #   result = Hash.new(0)
  #   hash = self.dup.populate_with_keys(min: min, max: max)
  #   hash = hash.reject{|k, v| k < min.to_i}
  #   hash = hash.reject{|k, v| k > max if max.present?}
  #
  #   hash.each do |k, v|
  #     if k%integer == shift.to_i
  #       result[k] = (k..(k + integer - 1)).map{|i| hash[i].to_i }.sum
  #     end
  #   end
  #
  #   return result.fast_sort_keys
  # end

  def self.sum(h1, h2)
    # 1. take keys from both hashes
    keys = (h1.keys + h2.keys).uniq

    sum_hash = {}

    # 2. loop over the keys, summing numerics and recursing into hash-like structures
    keys.each do |key|
      v1 = h1[key]
      v2 = h2[key]

      sum_value = if v1.respond_to?(:keys) && v2.respond_to?(:keys)
        # O-1 recuring needed only if both are hash-like
        sum(v1, v2)
      elsif v1.present? && v2.blank?
        # O-3a sum possibe because either is nil (one of the hashes does not have the key at all)
        v1
      elsif v1.blank? && v2.present?
        # O-3b sum possibe because either is nil (one of the hashes does not have the key at all)
        v2
      elsif v1.respond_to?(:to_f) && v2.respond_to?(:to_f)
        # O-2 sum possible because both numerics
        v1 + v2
      elsif v1.present? && v2.present? && (v1.respond_to?(:to_f) != v2.respond_to?(:to_f))
        # 0-4 error because types mismatch
        raise ArgumentError.new("Sorry, can not sum these hashes, there's value collision in '#{key}' keys")
      end

      sum_hash[key] = sum_value
    end

    return sum_hash
  end

end