def _parse_header(inf):
"""
@param inf: an object which I can call read(1) on to get another byte
@return: tuple of (m, k, pad, sh,); side-effect: the first one to four
bytes of inf will be read
"""
# The first 8 bits always encode m.
ch = inf.read(1)
if not ch:
raise CorruptedShareFilesError("Share files were corrupted -- share file %r didn't have a complete metadata header at the front. Perhaps the file was truncated." % (inf.name,))
byte = ord(ch)
m = byte + 1
# The next few bits encode k.
kbits = log_ceil(m, 2) # num bits needed to store all possible values of k
b2_bits_left = 8-kbits
kbitmask = MASK(kbits) << b2_bits_left
ch = inf.read(1)
if not ch:
raise CorruptedShareFilesError("Share files were corrupted -- share file %r didn't have a complete metadata header at the front. Perhaps the file was truncated." % (inf.name,))
byte = ord(ch)
k = ((byte & kbitmask) >> b2_bits_left) + 1
shbits = log_ceil(m, 2) # num bits needed to store all possible values of shnum
padbits = log_ceil(k, 2) # num bits needed to store all possible values of pad
val = byte & (~kbitmask)
needed_padbits = padbits - b2_bits_left
if needed_padbits > 0:
ch = inf.read(1)
if not ch:
raise CorruptedShareFilesError("Share files were corrupted -- share file %r didn't have a complete metadata header at the front. Perhaps the file was truncated." % (inf.name,))
byte = struct.unpack(">B", ch)[0]
val <<= 8
val |= byte
needed_padbits -= 8
assert needed_padbits <= 0
extrabits = -needed_padbits
pad = val >> extrabits
val &= MASK(extrabits)
needed_shbits = shbits - extrabits
if needed_shbits > 0:
ch = inf.read(1)
if not ch:
raise CorruptedShareFilesError("Share files were corrupted -- share file %r didn't have a complete metadata header at the front. Perhaps the file was truncated." % (inf.name,))
byte = struct.unpack(">B", ch)[0]
val <<= 8
val |= byte
needed_shbits -= 8
assert needed_shbits <= 0
gotshbits = -needed_shbits
sh = val >> gotshbits
return (m, k, pad, sh,)
def _parse_header(inf):
"""
@param inf: an object which I can call read(1) on to get another byte
@return: tuple of (m, k, pad, sh,); side-effect: the first one to four
bytes of inf will be read
"""
# The first 8 bits always encode m.
ch = inf.read(1)
if not ch:
raise CorruptedShareFilesError("Share files were corrupted -- share file %r didn't have a complete metadata header at the front. Perhaps the file was truncated." % (inf.name,))
byte = ord(ch)
m = byte + 1
# The next few bits encode k.
kbits = log_ceil(m, 2) # num bits needed to store all possible values of k
b2_bits_left = 8-kbits
kbitmask = MASK(kbits) << b2_bits_left
ch = inf.read(1)
if not ch:
raise CorruptedShareFilesError("Share files were corrupted -- share file %r didn't have a complete metadata header at the front. Perhaps the file was truncated." % (inf.name,))
byte = ord(ch)
k = ((byte & kbitmask) >> b2_bits_left) + 1
shbits = log_ceil(m, 2) # num bits needed to store all possible values of shnum
padbits = log_ceil(k, 2) # num bits needed to store all possible values of pad
val = byte & (~kbitmask)
needed_padbits = padbits - b2_bits_left
if needed_padbits > 0:
ch = inf.read(1)
if not ch:
raise CorruptedShareFilesError("Share files were corrupted -- share file %r didn't have a complete metadata header at the front. Perhaps the file was truncated." % (inf.name,))
byte = struct.unpack(">B", ch)[0]
val <<= 8
val |= byte
needed_padbits -= 8
assert needed_padbits <= 0
extrabits = -needed_padbits
pad = val >> extrabits
val &= MASK(extrabits)
needed_shbits = shbits - extrabits
if needed_shbits > 0:
ch = inf.read(1)
if not ch:
raise CorruptedShareFilesError("Share files were corrupted -- share file %r didn't have a complete metadata header at the front. Perhaps the file was truncated." % (inf.name,))
byte = struct.unpack(">B", ch)[0]
val <<= 8
val |= byte
needed_shbits -= 8
assert needed_shbits <= 0
gotshbits = -needed_shbits
sh = val >> gotshbits
return (m, k, pad, sh,)
def num_chars_that_this_many_octets_encode_to(numos):
return log_ceil(256**numos, 62)
def _build_header(m, k, pad, sh):
"""
@param m: the total number of shares; 1 <= m <= 256
@param k: the number of shares required to reconstruct; 1 <= k <= m
@param pad: the number of bytes of padding added to the file before encoding; 0 <= pad < k
@param sh: the shnum of this share; 0 <= k < m
@return: a compressed string encoding m, k, pad, and sh
"""
assert m >= 1
assert m <= 2**8
assert k >= 1
assert k <= m
assert pad >= 0
assert pad < k
assert sh >= 0
assert sh < m
bitsused = 0
val = 0
val |= (m - 1)
bitsused += 8 # the first 8 bits always encode m
kbits = log_ceil(m, 2) # num bits needed to store all possible values of k
val <<= kbits
bitsused += kbits
val |= (k - 1)
padbits = log_ceil(k, 2) # num bits needed to store all possible values of pad
val <<= padbits
bitsused += padbits
val |= pad
shnumbits = log_ceil(m, 2) # num bits needed to store all possible values of shnum
val <<= shnumbits
bitsused += shnumbits
val |= sh
assert bitsused >= 8, bitsused
assert bitsused <= 32, bitsused
if bitsused <= 16:
val <<= (16-bitsused)
cs = struct.pack('>H', val)
assert cs[:-2] == '\x00' * (len(cs)-2)
return cs[-2:]
if bitsused <= 24:
val <<= (24-bitsused)
cs = struct.pack('>I', val)
assert cs[:-3] == '\x00' * (len(cs)-3)
return cs[-3:]
else:
val <<= (32-bitsused)
cs = struct.pack('>I', val)
assert cs[:-4] == '\x00' * (len(cs)-4)
return cs[-4:]
def _build_header(m, k, pad, sh):
"""
@param m: the total number of shares; 1 <= m <= 256
@param k: the number of shares required to reconstruct; 1 <= k <= m
@param pad: the number of bytes of padding added to the file before encoding; 0 <= pad < k
@param sh: the shnum of this share; 0 <= k < m
@return: a compressed string encoding m, k, pad, and sh
"""
assert m >= 1
assert m <= 2**8
assert k >= 1
assert k <= m
assert pad >= 0
assert pad < k
assert sh >= 0
assert sh < m
bitsused = 0
val = 0
val |= (m - 1)
bitsused += 8 # the first 8 bits always encode m
kbits = log_ceil(m, 2) # num bits needed to store all possible values of k
val <<= kbits
bitsused += kbits
val |= (k - 1)
padbits = log_ceil(k, 2) # num bits needed to store all possible values of pad
val <<= padbits
bitsused += padbits
val |= pad
shnumbits = log_ceil(m, 2) # num bits needed to store all possible values of shnum
val <<= shnumbits
bitsused += shnumbits
val |= sh
assert bitsused >= 8, bitsused
assert bitsused <= 32, bitsused
if bitsused <= 16:
val <<= (16-bitsused)
cs = struct.pack('>H', val)
assert cs[:-2] == b'\x00' * (len(cs)-2)
return cs[-2:]
if bitsused <= 24:
val <<= (24-bitsused)
cs = struct.pack('>I', val)
assert cs[:-3] == b'\x00' * (len(cs)-3)
return cs[-3:]
else:
val <<= (32-bitsused)
cs = struct.pack('>I', val)
assert cs[:-4] == b'\x00' * (len(cs)-4)
return cs[-4:]