#!/usr/bin/env ruby
# encoding: utf-8
# ruby: 1.9.3
=begin
this script verifies and writes the checksum for NXP LPC13xx firmwares.
for more information about how it is calculated refer to document UM10375 section 21.7 Criterion for Valid User Code
=end

def help
  puts "usage: #{__FILE__} [operation] file"
  puts "operations:"
  puts "\t-v,--verify\tverify checksum (default)"
  puts "\t-w,--write\twrite checksum"
  puts "file: LPC13xx firmware"
end

puts "LPC13xx firmware checksum verifier and writer"

# should the checksum be written
write = false
file = nil

if ARGV.size==1 then
  file = ARGV[0]
elsif ARGV.size==2 then
  case ARGV[0]
  when "-h","--help"
    help
    exit 0
  when "-v","--verify"
    # this is done per default
  when "-w","--write"
    write = true
  else
    help
    exit 0
  end
  file = ARGV[1]
else
  help
  exit 0
end

raise "#{file} is not a file" unless File.file? file
raise "#{file} is too short" if File.size(file)<4*8

firmware = File.open(file,"r+") # open file
header = firmware.read(4*7).unpack("l*") # read the 7 first words (of 32 bits) to checksum
checksum = firmware.read(4).unpack("l*")[0]&0xffffffff # read the current checksum (8th word)

# calculate the correct checksum
sum = 0
header.each do |word|
  sum += word
  sum &= 0xffffffff
end
correct = ((~sum)+1)&0xffffffff # the 2's complement

puts "current checksum: 0x%08x" % checksum
puts "correct checksum: 0x%08x" % correct
if checksum==correct then
  puts "checksum matches"
else
  puts "checksum does not match"
  if write then
    firmware.seek(4*7,:SET)
    firmware.write([correct].pack("l"))
    puts "checksum written"
  end
end
firmware.close