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1import struct
2import sys
3
4def readU32(contents, offset):
5  wordBytes = contents[offset:offset + 4]
6  return struct.unpack('>I', wordBytes)[0]
7
8def readU16(contents, offset):
9  wordBytes = contents[offset:offset + 2]
10  return struct.unpack('>H', wordBytes)[0]
11
12def checkChecksum(infile):
13  contents = infile.read()
14  if len(contents) % 4:
15    print 'File length is not a multiple of 4'
16
17  sum = 0
18  for offset in range(0, len(contents), 4):
19    sum += readU32(contents, offset)
20    while sum >= 2**32:
21      sum -= 2**32
22  print 'Sum of whole file: %x' % sum
23
24  numTables = readU16(contents, 4)
25
26  for offset in range(12, 12 + numTables * 16, 16):
27    tag = contents[offset:offset + 4]
28    chksum = readU32(contents, offset + 4)
29    toffset = readU32(contents, offset + 8)
30    tlength = readU32(contents, offset + 12)
31
32    sum = 0
33    for offset2 in range(toffset, toffset + tlength, 4):
34      sum += readU32(contents, offset2)
35      while sum >= 2**32:
36        sum -= 2**32
37    if sum != chksum:
38      print 'Bad chksum: %s' % tag
39
40if __name__ == '__main__':
41  if len(sys.argv) != 2:
42    print 'Usage: %s <ttf filename>' % sys.argv[0]
43  else:
44    checkChecksum(file(sys.argv[1], 'r'))
45