From: han.holl@... (Han Holl)
Date: 2004-07-08T21:42:34+09:00
Subject: Proposal for Hash#invert

Hello,

The current Hash#invert implementation will silently throw away keys that
have the same value associated with them.
Because I often have many to one hashes, but have preferred keys, I'd like
to propose having an optional arbitrating block, that gets called with
the alternative valuse in an array. Something like this:

class Hash
  def invert(&blk)
    rsl = {}
    arbitrate = {}
    each_pair do |k, v|
      if rsl.has_key?(v) && block_given?
        (arbitrate[v] ||= [rsl[v]]) << k
      else
        rsl[v] = k
      end
    end
    arbitrate.each_pair do |k,v|
      rsl[k] = yield v
    end
    rsl
  end
end

You can call this with something like:
hash.invert {|x| x.find {|el| fields.include?(el)} || 
       x.sort_by{|el| el.size}[-1]  }

or even the simple:
hash.invert { raise 'This hash is not invertible' }

Does this sound generally useful, or is it just me ?

Cheers,

Han Holl