h = {:abc => 123, :xyz => 789, :qaz => 789, :wsx => [888, 999]}
h.fetch(:xyz) # => 789
h.rfetch(123) # => abc
h.rfetch(789) # => [:xyz, :qaz]
h.rfetch(888) # => :wsx

class BiHash
    def initialize
        @forward = Hash.new { |h, k| h[k] = [ ] }
        @reverse = Hash.new { |h, k| h[k] = [ ] }
    end

    def insert(k, v)
        @forward[k].push(v)
        @reverse[v].push(k)
        v
    end

    def fetch(k)
        fetch_from(@forward, k)
    end

    def rfetch(v)
        fetch_from(@reverse, v)
    end

    protected

    def fetch_from(h, k)
        return nil if(!h.has_key?(k))
        v = h[k]
        v.length == 1 ? v.first : v.dup
    end
end

b = BiHash.new
b.insert(:a, 'a')
b.insert(:a, 'b')
b.insert(:a, 'c')
b.insert(:b, 'a')
b.insert(:c, 'x')

puts b.fetch(:a).inspect            # ["a", "b", "c"]
puts b.fetch(:b).inspect            # "a"
puts b.rfetch('a').inspect          # [:a, :b]
puts b.rfetch('x').inspect          # :c
puts b.fetch(:not_there).inspect    # nil
puts b.rfetch('not there').inspect  # nil

h = {:abc => 123, :xyz => 789, :qaz => 789, :wsx => [888, 999]}
h.rassoc(123) # => [:abc, 123]

h = MyBidirectionalArray.new(:foo => 42, :bar => [:hello, :world])
h.rfetch(:world) # => :bar
h[:bar].shift
h[:bar] # => [:world]
h.rfetch(:world) # => should be nil, but how to detect this??

class Hash
  def rfetch val
    select { |k,v| v.is_a?(Array) ? v.include?(val) : v == val }.map { |x| x[0] }
  end
end