Ruby bootblock checksummer

sum-bootblock.rb

#! /usr/bin/env ruby

def compute_chksum(data)
	sum = 0
	(0...256).each do |x|
		i = 4 * x
		val = data[i,i+4].unpack('N')[0]
		sum = sum + val
		if sum > 0xffffffff
			sum = (sum + 1) & 0xffffffff
		end
	end
	(~sum) & 0xffffffff
end

if not ARGV[0] or not ARGV[1]
	puts("two arguments needed")
	exit(1)
end

data = IO.read(ARGV[0], 1024)
data += "\0" * (1024 - data.length)		# pad to 1024 bytes
data[4,4] = [compute_chksum(data)].pack('N')
File.open(ARGV[1], "wb") { |f| f.write(data) }