From: Benoit Daloze <eregontp@...>
Date: 2010-05-17T08:30:24+09:00
Subject: Re: Array conversion

Hi,
On 17 May 2010 00:51, Paul A. <cyril.staff@gmail.com> wrote:
> Just a last question: how can we do the oposite operation?
>

I suppose this is a way:

array_32bit = [1717922406, 543519338, 543585641, 1701343776,
1718969704, 544368245, 1751457906, 1713399157]
array_128bit = [136107835579490162288229994478942319136,
136190811075650685875822190301307495797]

class Array
  def convert_digest(from_bits, to_bits)
    if from_bits < to_bits
      self.each_slice(to_bits/from_bits).map do |slice|
        slice.inject(0) do |sum,hash|
          (sum<<from_bits) + hash
        end
      end
    else
      self.inject([]) { |result, hash|
        result + (from_bits/to_bits).times.inject([]) { |ary|
          hash, rest = hash.divmod(2**to_bits)
          ary << rest
        }.reverse
      }
    end
  end
end

p array_32bit.convert_digest(32,128) == array_128bit
p array_128bit.convert_digest(128,32) == array_32bit
p array_32bit.convert_digest(32,128).convert_digest(128,32) == array_32bit
p array_128bit.convert_digest(128,32).convert_digest(32,128) == array_128bit

(always be careful with Integer#<<(n), as it can easily 'steal' all
your memory if you are a bit heavy on the n ;) )

Regards,
B.D.