Module: Fit4Ruby::CRC16

Included in:
FitFile, FitHeader
Defined in:
lib/fit4ruby/CRC16.rb

Instance Method Summary collapse

Instance Method Details

#compute_crc(io, start_pos, end_pos) ⇒ Object 



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# File 'lib/fit4ruby/CRC16.rb', line 29

def compute_crc(io, start_pos, end_pos)
  crc_table = [
    0x0000, 0xCC01, 0xD801, 0x1400, 0xF001, 0x3C00, 0x2800, 0xE401,
    0xA001, 0x6C00, 0x7800, 0xB401, 0x5000, 0x9C01, 0x8801, 0x4400
  ]

  io.seek(start_pos)

  crc = 0
  while io.pos < end_pos
    if io.eof?
      raise IOError, "Premature end of file"
    end

    byte = io.readbyte

    0.upto(1) do |i|
      tmp = crc_table[crc & 0xF]
      crc = (crc >> 4) & 0x0FFF
      crc = crc ^ tmp ^ crc_table[(byte >> (4 * i)) & 0xF]
    end
  end

  crc
end

#write_crc(io, start_pos, end_pos) ⇒ Object 



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# File 'lib/fit4ruby/CRC16.rb', line 18

def write_crc(io, start_pos, end_pos)
  # Compute the checksum over the data section of the file and append it
  # to the file. Ideally, we should compute the CRC from data in memory
  # instead of the file data.
  crc = compute_crc(io, start_pos, end_pos)
  io.seek(end_pos)
  BinData::Uint16le.new(crc).write(io)

  crc
end