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# OFT Checksum calculator. def calc_checksum(buffer='',prev_checksum=0xffff,odd_start=false) checksum = (prev_checksum >> 16) & 0xffff buffer.split(//).each_with_index do |x,i| old_checksum = checksum # Even bytes are high, odd are low, unless the last chunk ended up starting # this chunk on an odd byte, in which case the reverse is true. Note that # the first chunk (and often the only chunk) is always starting on 0, thus, # even. This is the typical use case. Implementations should set the # odd_start flag and the prev_checksum value if this is the case. if odd_start value = (!(i % 2).zero? ? x[0] << 8 : x[0]) else value = ((i % 2).zero? ? x[0] << 8 : x[0]) # usually this one. end checksum -= value checksum -= 1 if checksum > old_checksum end 2.times {checksum = (checksum & 0x0000ffff) + (checksum >> 16) } checksum << 16 end

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