gistfile1.rb

# Between those alternatives I'd go with this one.
#
# Ruby has open classes, there is nothing wrong about embracing them. Even Rails that used to
# create modules and mix them into the classes has decided to change it on rails3 and simply
# open the classes and add the methods.
#
# It's simple, and it works. Of course you shouldn't be adding stuff that are specific to 
# your application's business logic.
class Fixnum
  def to_bit_array
    to_s(2).reverse.split("").map(&:to_i)
  end
end

# Which is why I actually would change this method. Since a Fixnum is a sequence of bytes 
# already and you just need to check if some bits are on, then I would change the method to
# this:
class Fixnum
  # Returns true if all the given bits positions are on (set to 1)
  # Example:
  #   0b001110.bits_on?(1, 2, 3)
  def bits_on?(*bits)
    bits.all? { |n| self[n] == 1 }
  end

  # Adds a singular alias
  alias_method :bit_on?, :bits_on?
end

# If you actually need to perform more calculations over the sequence of bits, I'd create a 
# separate class for it but using composition instead of inheritance.
#
# Example:
#   m = BitMap.new(0b001110)
#   m.bits_on # => [1, 2, 3]
#   m.bits_on?(1,2,3) # => true
class BitMap
  def initialize(integer)
    @integer = integer
  end

  # Returns true if all the given bits positions are on (set to 1)
  # Example:
  #   0b001110.bits_on?(1, 2, 3)
  def bits_on?(*bits)
    bits.all? { |n| @integer[n] == 1 }
  end

  # Adds a singular alias
  alias_method :bit_on?, :bits_on? 

  # Return all the bits that are on
  # Example:
  #   0b001110.bits_on # => [1,2,3]
  def bits_on
    indexes = []
    (0...@integer.size*8).each { |n| indexes << n if @integer[n] == 1 }
    indexes
  end
end

Oh, and if the bit string was bigger then transforming the integer to a string, then to an array, then reverse, then map would definitely take longer.

Using the number 0xffffffffffffffffffffffff:

Rehearsal ---------------------------------------------------
Module method :   0.000000   0.000000   0.000000 (  0.000200)
BitMap class  :   0.000000   0.000000   0.000000 (  0.000121)
------------------------------------------ total: 0.000000sec

                      user     system      total        real
Module method :   0.000000   0.000000   0.000000 (  0.000129)
BitMap class  :   0.000000   0.000000   0.000000 (  0.000054)