class Hash
  # Allows for reverse merging two hashes where the keys in the calling hash take precedence over those
  # in the <tt>other_hash</tt>. This is particularly useful for initializing an option hash with default values:
  #
  #   def setup(options = {})
  #     options.reverse_merge! :size => 25, :velocity => 10
  #   end
  #
  # Using <tt>merge</tt>, the above example would look as follows:
  #
  #   def setup(options = {})
  #     { :size => 25, :velocity => 10 }.merge(options)
  #   end
  #
  # The default <tt>:size</tt> and <tt>:velocity</tt> are only set if the +options+ hash passed in doesn't already
  # have the respective key.
  def reverse_merge(other_hash)
    other_hash.merge(self)
  end

  # Performs the opposite of <tt>merge</tt>, with the keys and values from the first hash taking precedence over the second.
  # Modifies the receiver in place.
  def reverse_merge!(other_hash)
    # right wins if there is no left
    merge!( other_hash ){|key,left,right| left }
  end

  alias_method :reverse_update, :reverse_merge!
end

class Hash
  # Allows for reverse merging two hashes where the keys in the calling hash take precedence over those
  # in the <tt>other_hash</tt>. This is particularly useful for initializing an option hash with default values:
  #
  #   def setup(options = {})
  #     options.reverse_merge! :size => 25, :velocity => 10
  #   end
  #
  # The default <tt>:size</tt> and <tt>:velocity</tt> are only set if the +options+ hash passed in doesn't already
  # have the respective key.
  #
  # As contrast, using Ruby's built in <tt>merge</tt> would require writing the following:
  #
  #   def setup(options = {})
  #     options = { :size => 25, :velocity => 10 }.merge(options)
  #   end
  def reverse_merge(other_hash)
    other_hash.merge(self)
  end

  # Performs the opposite of <tt>merge</tt>, with the keys and values from the first hash taking precedence over the second.
  # Modifies the receiver in place.
  def reverse_merge!(other_hash)
    # right wins if there is no left
    merge!( other_hash ){|key,left,right| left }
  end

  alias_method :reverse_update, :reverse_merge!
end

Correct example that did not do what it claimed. Rework explanation.
Correct example that did not do what it claimed. Rework explanation.
