def __init__(self, name, lengthfield, wordsize = 1):
'''
Initialize the field with the given *name* and a
*lengthfield*. The *lengthfield* must be a numeric field
instance. *wordsize* specifies how many bytes a word contains
and it can be a numeric value or a unary function that knows
where to get the word size, it has a default value of 1. So, the
total length in bytes of a :mod:`Data` field is the length field
multiplied by the word size. If *wordsize* is a function, it
takes the top-level root :mod:`Container` field as a single
argument. This way, it is possible to provide a word size that
depends on the value of another field.
'''
Structure.__init__(self, name)
self.__wordsize = wordsize
wsizefunc = lambda root: \
lengthfield.value() * param_call(wordsize, root)
self.__length = lengthfield
self.__data = String("Data", wsizefunc)
Structure.append(self, self.__length)
Structure.append(self, self.__data)
class Data(Structure):
'''
This class lets you store strings of characters (divided by words)
and also provides a field to hold its length. It is a
:mod:`Structure` with two fields: length and data (internally
created with name *Data*). The length is a numeric field and
specifies how many words the *Data* field contains. The *Data* field
is internally a :class:`String`.
'''
def __init__(self, name, lengthfield, wordsize = 1):
'''
Initialize the field with the given *name* and a
*lengthfield*. The *lengthfield* must be a numeric field
instance. *wordsize* specifies how many bytes a word contains
and it can be a numeric value or a unary function that knows
where to get the word size, it has a default value of 1. So, the
total length in bytes of a :mod:`Data` field is the length field
multiplied by the word size. If *wordsize* is a function, it
takes the top-level root :mod:`Container` field as a single
argument. This way, it is possible to provide a word size that
depends on the value of another field.
'''
Structure.__init__(self, name)
self.__wordsize = wordsize
wsizefunc = lambda root: \
lengthfield.value() * param_call(wordsize, root)
self.__length = lengthfield
self.__data = String("Data", wsizefunc)
Structure.append(self, self.__length)
Structure.append(self, self.__data)
def value(self):
'''
Returns the value of the *Data* field as a string.
'''
return self.__data.value()
def set_value(self, value):
'''
Sets a new string to the *Data* field. The given string length
must be a mutliple of the word size and must fit in the length
field (i.e. 300 characters are too long if the length field is
:class:`UInt8`, as only 255 characters fit), otherwise a
*ValueError* exception will be raised.
'''
length = len(value)
wordsize = param_call(self.__wordsize, self.root())
if (length % wordsize) == 0:
try:
self.__length.set_value(length / wordsize)
except:
raise ValueError("Data length must be lower than length "
"field maximum size (%d given)" % length)
self.__data.set_value(value)
else:
raise ValueError("Data length must be a multiple of %d (%d given)" \
% (wordsize, length))
