code: def calcChecksum(data):
      """Calculate the checksum for an arbitrary block of data.
  
      If the data length is not a multiple of four, it assumes
      it is to be padded with null byte.
  
              >>> print(calcChecksum(b"abcd"))
              1633837924
              >>> print(calcChecksum(b"abcdxyz"))
              3655064932
      """
      remainder = len(data) % 4
      if remainder:
          data += b"\0" * (4 - remainder)
      value = 0
      blockSize = 4096
      assert blockSize % 4 == 0
      for i in range(0, len(data), blockSize):
          block = data[i : i + blockSize]
          longs = struct.unpack(">%dL" % (len(block) // 4), block)
          value = (value + sum(longs)) & 0xFFFFFFFF
      return value
  
imports: from io import BytesIO
  from types import SimpleNamespace
  from fontTools.misc.textTools import Tag
  from fontTools.misc import sstruct
  from fontTools.ttLib import TTLibError, TTLibFileIsCollectionError
  import struct
  from collections import OrderedDict
  import logging
label_desc: Use image node_id 2 for calling a global function with example usage: calcChecksum(data) and returns: value