re-implementation of ActiveSupport's `Hash#deep_merge`

deep_merge.rb

# frozen_string_literal: true

# Implemented in terms of Ruby's built-in ability to pass a block to handle duplicate keys in +merge+
class Hash
  def deep_merge(other_hash, &block)
    dup.deep_merge!(other_hash, &block)
  end

  def deep_merge!(other, &block)
    merge!(other) do |_key, old_val, new_val|
      if old_val.is_a?(Hash) && new_val.is_a?(Hash)
        old_val.dup.deep_merge!(new_val, &block)
      elsif block_given?
        block.call(_key, old_val, new_val)
      else
        new_val
      end
    end
  end
end