From: ptkwt@...1.aracnet.com (Phil Tomson)
Date: 2003-07-19T04:05:39+09:00
Subject: [SNIPPET] Binary counter

I needed to test a class which had a certain number of 'binary' inputs (ie. each input 
takes either 0 or 1 (or false or true) ).  In order to do this I needed to try out every 
possible combination of inputs - essentially a binary count, like:

0,0,0,0
0,0,0,1
....
1,1,1,0
1,1,1,1

I figured I could just increment an integer and, then convert it to binary, but:
  1) there doesn't seem to be a builtin to_bin method on FixNum.
  2) I'd still have to deal with indexing, etc.
  (OK, it wouldn't be that hard to implement #1 and #2 is not that hard either, but I 
decided to go a different route for fun ;-)

So I made a BinCounter class and I present it here in case anyone else has need for it:

class BinaryCounter
  attr_reader :bin
  def initialize(num_bits, tv=1,fv=0)
    @trueval  = tv 
    @falseval = fv 
    @bin      = Array.new(num_bits,@falseval)
  end
  def bincount(array=@bin,&block)
    recurse(array,0,@falseval,&block)
    recurse(array,0,@trueval,&block)
  end
  private
  def recurse(array,idx,val,&block)
    if idx == array.length
      yield array if val == @falseval && block_given?
      return
    end
    array[idx] = val 
    recurse(array,idx+1,@falseval,&block)
    recurse(array,idx+1,@trueval,&block)
  end
end

bc = BinaryCounter.new(4)
bc.bincount { |array| p array }
#end

Which produces:

[0, 0, 0, 0]
[0, 0, 0, 1]
....
[1, 1, 1, 0]
[1, 1, 1, 1]

....it's also possible to change the values, like:

bc = BinaryCounter.new(2,'cat','dog')
bc.bincount { |array| p array }

which produces:

  ["dog", "dog"]
  ["dog", "cat"]
  ["cat", "dog"]
  ["cat", "cat"]

....well, I suppose if you're a 'dog person' you'll want to swap it:

  ["cat", "cat"]
  ["cat", "dog"]
  ["dog", "cat"]
  ["dog", "dog"]


Hmmm.... Crazy idea:  One could use code generation to create N'ary counter 
classes where N is the number of values that each slot could take.

Phil