Ruby/Hash/Hash Extensions

class Hash
  def safe_invert
    new_hash = {}
    self.each do |k,v|
      if v.is_a? Array
        v.each { |x| new_hash.add_or_append(x, k) }
      else
        new_hash.add_or_append(v, k)
      end
    end
    return new_hash
  end
  def add_or_append(key, value)
    if has_key?(key)
      self[key] = [value, self[key]].flatten
    else
      self[key] = value
    end
  end
end
phone_directory = {  "A" => "1",
                     "B" => "2",
                     "M" => "3" }
p phone_directory.safe_invert
p phone_directory.safe_invert.safe_invert

class Hash
  def remove_hash(other_hash)
    delete_if { |k,v| other_hash[k] == v }
  end
end
squares = { 1 => 1, 2 => 4, 3 => 9 }
doubles = { 1 => 2, 2 => 4, 3 => 6 }
squares.remove_hash(doubles)
squares                                   # => {1=>1, 3=>9}

class Hash
  def find_all
    new_hash = Hash.new
    each { |k,v| new_hash[k] = v if yield(k, v) }
    new_hash
  end
end
squares = {0=>0, 1=>1, 2=>4, 3=>9}
squares.find_all { |key, value| key > 1 }        # => {2=>4, 3=>9}

class Hash
  def grep(pattern)
    inject([]) do |res, kv|
      res << kv if kv[0] =~ pattern or kv[1] =~ pattern
      res
    end
  end
end
h = { "apple tree" => "plant", "ficus" => "plant",
      "shrew" => "animal", "plesiosaur" => "animal" }
 
p h.grep(/pl/)
p h.grep(/plant/)        # => [["ficus", "plant"], ["apple tree", "plant"]]
p h.grep(/i.*u/)         # => [["ficus", "plant"], ["plesiosaur", "animal"]]