Counting bits without testing every bit

countbits.rb

# Counts bits without testing every single bit, using a simple lookup table. 
# Assumes a maximum size of 32bits.

$blookup = (0..255).map do |i| 
  count = 0
  (0..7).each { |j| count += 1 if (i>>j) & 0x1 != 0 }
  count
end

# Count
def countbits(number)
	count = 0
	num = number
	count += $blookup[num & 0xFF]; num >>= 8
	count += $blookup[num & 0xFF]; num >>= 8
	count += $blookup[num & 0xFF]; num >>= 8
	count += $blookup[num & 0xFF]; num >>= 8
	count
end

puts "Bits set: #{countbits(ARGV[0].to_i)}"