From: Martin DeMello <martindemello@...>
Date: 2002-04-15T07:21:15+09:00
Subject: Re: Reverse hash lookup?

Alex Liebowitz <stopm@attbi.com> wrote:
> Is there any way to specify a value and hash, and get the key corresponding
> to that value (in O(1) time, preferably)?

How would this work in the case of more than one key having the same value? 
Anyway, the following (untested) hack should work if you don't mind having
reverse just returning the last key to be assigned a particular value:

class InvertibleHash < Hash
  def initialize(*args)
    super(*args)
    @reverse = self.invert
  end

  def []=(key, val)
    super(key, val)
    @reverse[val] = key
  end

  def reverse_lookup(val)
    @reverse[val]
  end
end

or, enforcing a 1-1 mapping:

class Bijection < Hash
  def initialize(*args)
    super(*args)
    @reverse = self.invert
  end

  def []=(key, val)
    super(key, val)
    if @reverse.has_key?(val)
      self.delete(@reverse[val])
    end
    @reverse[val] = key
  end

  def reverse_lookup(val)
    @reverse[val]
  end
end

-- 
Martin DeMello