[pdurrant]

Oh - very useful stuff. And it turns out that the Mobipocket decoder will need some fixes for cases where bit position 1 is set. I can only suppose that very few commercial DRMed eBooks are out there with that bit set.

Happily easy to fix given this code, once such a book turns up.

Originally Posted by llasram

This is Calibre's current code for find the total size of the trailing entries:

def sizeof_trailing_entries(self, data):
    def sizeof_trailing_entry(ptr, psize):
        bitpos, result = 0, 0
        while True:
            v = ord(ptr[psize-1])
            result |= (v & 0x7F) << bitpos
            bitpos += 7
            psize -= 1
            if (v & 0x80) != 0 or (bitpos >= 28) or (psize == 0):
                return result
    
    num = 0
    size = len(data)
    flags = self.book_header.extra_flags >> 1
    while flags:
        if flags & 1:
            num += sizeof_trailing_entry(data, size - num)
        flags >>= 1
    if self.book_header.extra_flags & 1:
        num += (ord(data[size - num - 1]) & 0x3) + 1
    return num

HTH!