from lib import StringIO,Packer
from lib import Container,ListContainer,AttrDict,LazyContainer
#===============================================================================
# exceptions
#===============================================================================
class ConstructError(Exception):
__slots__ = []
class FieldError(ConstructError):
__slots__ = []
class SizeofError(ConstructError):
__slots__ = []
class AdaptationError(ConstructError):
__slots__ = []
class ArrayError(ConstructError):
__slots__ = []
class RangeError(ConstructError):
__slots__ = []
class SwitchError(ConstructError):
__slots__ = []
class SelectError(ConstructError):
__slots__ = []
class TerminatorError(ConstructError):
__slots__ = []
#===============================================================================
# abstract constructs
#===============================================================================
class Construct(object):
"""
The mother of all constructs!
User API:
* parse(buf) - parses an in-memory buffer (usually a string)
* parse_stream(stream) - parses a stream (in-memory, file, pipe, ...)
* build(obj) - builds the object into an in-memory buffer (a string)
* build_stream(obj, stream) - builds the object into the given stream
* sizeof(context) - calculates the size of the construct, if possible,
based on the context
Overriable methods for subclassing:
* _parse(stream, context) - low-level parse from stream
* _build(obj, stream, context) - low-level build to stream
* _sizeof(context) - low-level compute size
Flags API:
* _set_flag(flag) - sets the given flag/flags
* _clear_flag(flag) - clears the given flag/flags
* _inherit_flags(*subcons) - inherits the flag of subcons
* _is_flag(flag) - is the flag set? (predicate)
Overridable methods for the copy-API:
* __getstate__() - returns a dict of the attributes of self
* __setstate__(attrs) - sets the attrs to self
Attributes:
All constructs have a name and flags. The name is used for naming
struct-members and context dicts. Note that the name must be a string or
None (if the name is not needed). A single underscore ("_") is a reserved
name, and so are names starting with a less-than character ("<"). The name
should be descriptive, short, and valid as a python identifier (although
these rules are not enforced).
The flags specify additional behavioral information about this construct.
The flags are used by enclosing constructs to determine a proper course
of action. Usually, flags are "inherited", i.e., an enclosing construct
inherits the flags of its subconstruct. The enclosing construct may
set new flags or clear existing ones, as necessary.
For example, if FLAG_COPY_CONTEXT is set, repeaters will pass a copy of
the context for each iteration, which is necessary for OnDemand parsing.
"""
FLAG_COPY_CONTEXT = 0x0001
FLAG_DYNAMIC = 0x0002
FLAG_EMBED = 0x0004
FLAG_NESTING = 0x0008
__slots__ = ["name", "conflags"]
def __init__(self, name, flags = 0):
if name is not None:
if type(name) is not str:
raise TypeError("name must be a string or None", name)
if name == "_" or name.startswith("<"):
raise ValueError("reserved name", name)
self.name = name
self.conflags = flags
def __repr__(self):
return "%s(%r)" % (self.__class__.__name__, self.name)
def _set_flag(self, flag):
self.conflags |= flag
def _clear_flag(self, flag):
self.conflags &= ~flag
def _inherit_flags(self, *subcons):
for sc in subcons:
self._set_flag(sc.conflags)
def _is_flag(self, flag):
return bool(self.conflags & flag)
def __getstate__(self):
attrs = {}
if hasattr(self, "__dict__"):
attrs.update(self.__dict__)
slots = []
c = self.__class__
while c is not None:
if hasattr(c, "__slots__"):
slots.extend(c.__slots__)
c = c.__base__
for name in slots:
if hasattr(self, name):
attrs[name] = getattr(self, name)
return attrs
def __setstate__(self, attrs):
for name, value in attrs.iteritems():
setattr(self, name, value)
def __copy__(self):
"""returns a copy of this construct"""
self2 = object.__new__(self.__class__)
self2.__setstate__(self.__getstate__())
return self2
def parse(self, data):
"""parses data given as a buffer or a string (in-memory)"""
return self.parse_stream(StringIO(data))
def parse_stream(self, stream):
"""parses data read directly from a stream"""
return self._parse(stream, AttrDict())
def _parse(self, stream, context):
raise NotImplementedError()
def build(self, obj):
"""builds an object in a string (in memory)"""
stream = StringIO()
self.build_stream(obj, stream)
return stream.getvalue()
def build_stream(self, obj, stream):
"""builds an object into a stream"""
self._build(obj, stream, AttrDict())
def _build(self, obj, stream, context):
raise NotImplementedError()
def sizeof(self, context = None):
"""calculates the size of the construct (if possible) using the
given context"""
if context is None:
context = AttrDict()
return self._sizeof(context)
def _sizeof(self, context):
raise SizeofError("can't calculate size")
class Subconstruct(Construct):
"""
Abstract subconstruct (wraps an inner construct, inheriting it's
name and flags).
Parameters:
* subcon - the construct to wrap
"""
__slots__ = ["subcon"]
def __init__(self, subcon):
Construct.__init__(self, subcon.name, subcon.conflags)
self.subcon = subcon
def _parse(self, stream, context):
return self.subcon._parse(stream, context)
def _build(self, obj, stream, context):
self.subcon._build(obj, stream, context)
def _sizeof(self, context):
return self.subcon._sizeof(context)
class Adapter(Subconstruct):
"""
Abstract adapter: calls _decode for parsing and _encode for building.
Parameters:
* subcon - the construct to wrap
"""
__slots__ = []
def _parse(self, stream, context):
return self._decode(self.subcon._parse(stream, context), context)
def _build(self, obj, stream, context):
self.subcon._build(self._encode(obj, context), stream, context)
def _decode(self, obj, context):
raise NotImplementedError()
def _encode(self, obj, context):
raise NotImplementedError()
#===============================================================================
# primitives
#===============================================================================
def _read_stream(stream, length):
if length < 0:
raise ValueError("length must be >= 0", length)
data = stream.read(length)
if len(data) != length:
raise FieldError("expected %d, found %d" % (length, len(data)))
return data
def _write_stream(stream, length, data):
if length < 0:
raise ValueError("length must be >= 0", length)
if len(data) != length:
raise FieldError("expected %d, found %d" % (length, len(data)))
stream.write(data)
class StaticField(Construct):
"""
A field of a fixed size
Parameters:
* name - the name of the field
* length - the length (an integer)
Example:
StaticField("foo", 5)
"""
__slots__ = ["length"]
def __init__(self, name, length):
Construct.__init__(self, name)
self.length = length
def _parse(self, stream, context):
return _read_stream(stream, self.length)
def _build(self, obj, stream, context):
_write_stream(stream, self.length, obj)
def _sizeof(self, context):
return self.length
class FormatField(StaticField):
"""
A field that uses python's built-in struct module to pack/unpack data
according to a format string.
Note: this field has been originally implemented as an Adapter, but it
was made a construct for performance reasons.
Parameters:
* name - the name
* endianity - "<" for little endian, ">" for big endian, or "=" for native
* format - a single format character
Example:
FormatField("foo", ">", "L")
"""
__slots__ = ["packer"]
def __init__(self, name, endianity, format):
if endianity not in (">", "<", "="):
raise ValueError("endianity must be be '=', '<', or '>'",
endianity)
if len(format) != 1:
raise ValueError("must specify one and only one format char")
self.packer = Packer(endianity + format)
StaticField.__init__(self, name, self.packer.size)
def __getstate__(self):
attrs = StaticField.__getstate__(self)
attrs["packer"] = attrs["packer"].format
return attrs
def __setstate__(self, attrs):
attrs["packer"] = Packer(attrs["packer"])
return StaticField.__setstate__(attrs)
def _parse(self, stream, context):
try:
return self.packer.unpack(_read_stream(stream, self.length))[0]
except Exception, ex:
raise FieldError(ex)
def _build(self, obj, stream, context):
try:
_write_stream(stream, self.length, self.packer.pack(obj))
except Exception, ex:
raise FieldError(ex)
class MetaField(Construct):
"""
A field of a meta-length. The length is computed at runtime based on
the context.
Parameters:
* name - the name of the field
* lengthfunc - a function that takes the context as a parameter and return
the length of the field
Example:
MetaField("foo", lambda ctx: 5)
"""
__slots__ = ["lengthfunc"]
def __init__(self, name, lengthfunc):
Construct.__init__(self, name)
self.lengthfunc = lengthfunc
self._set_flag(self.FLAG_DYNAMIC)
def _parse(self, stream, context):
return _read_stream(stream, self.lengthfunc(context))
def _build(self, obj, stream, context):
_write_stream(stream, self.lengthfunc(context), obj)
def _sizeof(self, context):
return self.lengthfunc(context)
#===============================================================================
# arrays and repeaters
#===============================================================================
class MetaArray(Subconstruct):
"""
An array (repeater) of a meta-count. The array will iterate exactly
`countfunc()` times. Will raise ArrayError if less elements are found.
See also Array, Range and RepeatUntil.
Parameters:
* countfunc - a function that takes the context as a parameter and returns
the number of elements of the array (count)
* subcon - the subcon to repeat `countfunc()` times
Example:
MetaArray(lambda ctx: 5, UBInt8("foo"))
"""
__slots__ = ["countfunc"]
def __init__(self, countfunc, subcon):
Subconstruct.__init__(self, subcon)
self.countfunc = countfunc
self._clear_flag(self.FLAG_COPY_CONTEXT)
self._set_flag(self.FLAG_DYNAMIC)
def _parse(self, stream, context):
obj = ListContainer()
c = 0
count = self.countfunc(context)
try:
if self.subcon.conflags & self.FLAG_COPY_CONTEXT:
while c < count:
obj.append(self.subcon._parse(stream, context.__copy__()))
c += 1
else:
while c < count:
obj.append(self.subcon._parse(stream, context))
c += 1
except ConstructError, ex:
raise ArrayError("expected %d, found %d" % (count, c), ex)
return obj
def _build(self, obj, stream, context):
count = self.countfunc(context)
if len(obj) != count:
raise ArrayError("expected %d, found %d" % (count, len(obj)))
if self.subcon.conflags & self.FLAG_COPY_CONTEXT:
for subobj in obj:
self.subcon._build(subobj, stream, context.__copy__())
else:
for subobj in obj:
self.subcon._build(subobj, stream, context)
def _sizeof(self, context):
return self.subcon._sizeof(context) * self.countfunc(context)
class Range(Subconstruct):
"""
A range-array. The subcon will iterate between `mincount` to `maxcount`
times. If less than `mincount` elements are found, raises RangeError.
See also GreedyRange and OptionalGreedyRange.
Notes:
* requires a seekable stream.
Parameters:
* mincount - the minimal count (an integer)
* maxcount - the maximal count (an integer)
* subcon - the subcon to repeat
Example:
Range(5, 8, UBInt8("foo"))
"""
__slots__ = ["mincount", "maxcout"]
def __init__(self, mincount, maxcout, subcon):
Subconstruct.__init__(self, subcon)
self.mincount = mincount
self.maxcout = maxcout
self._clear_flag(self.FLAG_COPY_CONTEXT)
self._set_flag(self.FLAG_DYNAMIC)
def _parse(self, stream, context):
obj = ListContainer()
c = 0
try:
if self.subcon.conflags & self.FLAG_COPY_CONTEXT:
while c < self.maxcout:
pos = stream.tell()
obj.append(self.subcon._parse(stream, context.__copy__()))
c += 1
else:
while c < self.maxcout:
pos = stream.tell()
obj.append(self.subcon._parse(stream, context))
c += 1
except ConstructError:
if c < self.mincount:
raise RangeError("expected %d to %d, found %d" %
(self.mincount, self.maxcout, c))
stream.seek(pos)
return obj
def _build(self, obj, stream, context):
if len(obj) < self.mincount or len(obj) > self.maxcout:
raise RangeError("expected %d to %d, found %d" %
(self.mincount, self.maxcout, len(obj)))
cnt = 0
try:
if self.subcon.conflags & self.FLAG_COPY_CONTEXT:
for subobj in obj:
self.subcon._build(subobj, stream, context.__copy__())
cnt += 1
else:
for subobj in obj:
self.subcon._build(subobj, stream, context)
cnt += 1
except ConstructError:
if cnt < self.mincount:
raise RangeError("expected %d to %d, found %d" %
(self.mincount, self.maxcout, len(obj)))
def _sizeof(self, context):
raise SizeofError("can't calculate size")
class RepeatUntil(Subconstruct):
"""
An array that repeat until the predicate indicates it to stop. Note that
the last element (which caused the repeat to exit) is included in the
return value.
Parameters:
* predicate - a predicate function that takes (obj, context) and returns
True if the stop-condition is met, or False to continue.
* subcon - the subcon to repeat.
Example:
# will read chars until \x00 (inclusive)
RepeatUntil(lambda obj, ctx: obj == "\x00",
Field("chars", 1)
)
"""
__slots__ = ["predicate"]
def __init__(self, predicate, subcon):
Subconstruct.__init__(self, subcon)
self.predicate = predicate
self._clear_flag(self.FLAG_COPY_CONTEXT)
self._set_flag(self.FLAG_DYNAMIC)
def _parse(self, stream, context):
obj = []
try:
if self.subcon.conflags & self.FLAG_COPY_CONTEXT:
while True:
subobj = self.subcon._parse(stream, context.__copy__())
obj.append(subobj)
if self.predicate(subobj, context):
break
else:
while True:
subobj = self.subcon._parse(stream, context)
obj.append(subobj)
if self.predicate(subobj, context):
break
except ConstructError, ex:
raise ArrayError("missing terminator", ex)
return obj
def _build(self, obj, stream, context):
terminated = False
if self.subcon.conflags & self.FLAG_COPY_CONTEXT:
for subobj in obj:
self.subcon._build(subobj, stream, context.__copy__())
if self.predicate(subobj, context):
terminated = True
break
else:
for subobj in obj:
self.subcon._build(subobj, stream, context.__copy__())
if self.predicate(subobj, context):
terminated = True
break
if not terminated:
raise ArrayError("missing terminator")
def _sizeof(self, context):
raise SizeofError("can't calculate size")
#===============================================================================
# structures and sequences
#===============================================================================
class Struct(Construct):
"""
A sequence of named constructs, similar to structs in C. The elements are
parsed and built in the order they are defined.
See also Embedded.
Parameters:
* name - the name of the structure
* subcons - a sequence of subconstructs that make up this structure.
* nested - a keyword-only argument that indicates whether this struct
creates a nested context. The default is True. This parameter is
considered "advanced usage", and may be removed in the future.
Example:
Struct("foo",
UBInt8("first_element"),
UBInt16("second_element"),
Padding(2),
UBInt8("third_element"),
)
"""
__slots__ = ["subcons", "nested"]
def __init__(self, name, *subcons, **kw):
self.nested = kw.pop("nested", True)
if kw:
raise TypeError("the only keyword argument accepted is 'nested'", kw)
Construct.__init__(self, name)
self.subcons = subcons
self._inherit_flags(*subcons)
self._clear_flag(self.FLAG_EMBED)
def _parse(self, stream, context):
if "<obj>" in context:
obj = context["<obj>"]
del context["<obj>"]
else:
obj = Container()
if self.nested:
context = AttrDict(_ = context)
for sc in self.subcons:
if sc.conflags & self.FLAG_EMBED:
context["<obj>"] = obj
sc._parse(stream, context)
else:
subobj = sc._parse(stream, context)
if sc.name is not None:
obj[sc.name] = subobj
context[sc.name] = subobj
return obj
def _build(self, obj, stream, context):
if "<unnested>" in context:
del context["<unnested>"]
elif self.nested:
context = AttrDict(_ = context)
for sc in self.subcons:
if sc.conflags & self.FLAG_EMBED:
context["<unnested>"] = True
subobj = obj
elif sc.name is None:
subobj = None
else:
subobj = getattr(obj, sc.name)
context[sc.name] = subobj
sc._build(subobj, stream, context)
def _sizeof(self, context):
if self.nested:
context = AttrDict(_ = context)
return sum(sc._sizeof(context) for sc in self.subcons)
class Sequence(Struct):
"""
A sequence of unnamed constructs. The elements are parsed and built in the
order they are defined.
See also Embedded.
Parameters:
* name - the name of the structure
* subcons - a sequence of subconstructs that make up this structure.
* nested - a keyword-only argument that indicates whether this struct
creates a nested context. The default is True. This parameter is
considered "advanced usage", and may be removed in the future.
Example:
Sequence("foo",
UBInt8("first_element"),
UBInt16("second_element"),
Padding(2),
UBInt8("third_element"),
)
"""
__slots__ = []
def _parse(self, stream, context):
if "<obj>" in context:
obj = context["<obj>"]
del context["<obj>"]
else:
obj = ListContainer()
if self.nested:
context = AttrDict(_ = context)
for sc in self.subcons:
if sc.conflags & self.FLAG_EMBED:
context["<obj>"] = obj
sc._parse(stream, context)
else:
subobj = sc._parse(stream, context)
if sc.name is not None:
obj.append(subobj)
context[sc.name] = subobj
return obj
def _build(self, obj, stream, context):
if "<unnested>" in context:
del context["<unnested>"]
elif self.nested:
context = AttrDict(_ = context)
objiter = iter(obj)
for sc in self.subcons:
if sc.conflags & self.FLAG_EMBED:
context["<unnested>"] = True
subobj = objiter
elif sc.name is None:
subobj = None
else:
subobj = objiter.next()
context[sc.name] = subobj
sc._build(subobj, stream, context)
class Union(Construct):
"""
a set of overlapping fields (like unions in C). when parsing,
all fields read the same data; when building, only the first subcon
(called "master") is used.
Parameters:
* name - the name of the union
* master - the master subcon, i.e., the subcon used for building and
calculating the total size
* subcons - additional subcons
Example:
Union("what_are_four_bytes",
UBInt32("one_dword"),
Struct("two_words", UBInt16("first"), UBInt16("second")),
Struct("four_bytes",
UBInt8("a"),
UBInt8("b"),
UBInt8("c"),
UBInt8("d")
),
)
"""
__slots__ = ["parser", "builder"]
def __init__(self, name, master, *subcons, **kw):
Construct.__init__(self, name)
args = [Peek(sc) for sc in subcons]
args.append(MetaField(None, lambda ctx: master._sizeof(ctx)))
self.parser = Struct(name, Peek(master, perform_build = True), *args)
self.builder = Struct(name, master)
def _parse(self, stream, context):
return self.parser._parse(stream, context)
def _build(self, obj, stream, context):
return self.builder._build(obj, stream, context)
def _sizeof(self, context):
return self.builder._sizeof(context)
#===============================================================================
# conditional
#===============================================================================
class Switch(Construct):
"""
A conditional branch. Switch will choose the case to follow based on
the return value of keyfunc. If no case is matched, and no default value
is given, SwitchError will be raised.
See also Pass.
Parameters:
* name - the name of the construct
* keyfunc - a function that takes the context and returns a key, which
will ne used to choose the relevant case.
* cases - a dictionary mapping keys to constructs. the keys can be any
values that may be returned by keyfunc.
* default - a default value to use when the key is not found in the cases.
if not supplied, an exception will be raised when the key is not found.
You can use the builtin construct Pass for 'do-nothing'.
* include_key - whether or not to include the key in the return value
of parsing. defualt is False.
Example:
Struct("foo",
UBInt8("type"),
Switch("value", lambda ctx: ctx.type, {
1 : UBInt8("spam"),
2 : UBInt16("spam"),
3 : UBInt32("spam"),
4 : UBInt64("spam"),
}
),
)
"""
class NoDefault(Construct):
def _parse(self, stream, context):
raise SwitchError("no default case defined")
def _build(self, obj, stream, context):
raise SwitchError("no default case defined")
def _sizeof(self, context):
raise SwitchError("no default case defined")
NoDefault = NoDefault("NoDefault")
__slots__ = ["subcons", "keyfunc", "cases", "default", "include_key"]
def __init__(self, name, keyfunc, cases, default = NoDefault,
include_key = False):
Construct.__init__(self, name)
self._inherit_flags(*cases.values())
self.keyfunc = keyfunc
self.cases = cases
self.default = default
self.include_key = include_key
self._inherit_flags(*cases.values())
self._set_flag(self.FLAG_DYNAMIC)
def _parse(self, stream, context):
key = self.keyfunc(context)
obj = self.cases.get(key, self.default)._parse(stream, context)
if self.include_key:
return key, obj
else:
return obj
def _build(self, obj, stream, context):
if self.include_key:
key, obj = obj
else:
key = self.keyfunc(context)
case = self.cases.get(key, self.default)
case._build(obj, stream, context)
def _sizeof(self, context):
case = self.cases.get(self.keyfunc(context), self.default)
return case._sizeof(context)
class Select(Construct):
"""
Selects the first matching subconstruct. It will literally try each of
the subconstructs, until one matches.
Notes:
* requires a seekable stream.
Parameters:
* name - the name of the construct
* subcons - the subcons to try (order-sensitive)
* include_name - a keyword only argument, indicating whether to include
the name of the selected subcon in the return value of parsing. default
is false.
Example:
Select("foo",
UBInt64("large"),
UBInt32("medium"),
UBInt16("small"),
UBInt8("tiny"),
)
"""
__slots__ = ["subcons", "include_name"]
def __init__(self, name, *subcons, **kw):
include_name = kw.pop("include_name", False)
if kw:
raise TypeError("the only keyword argument accepted "
"is 'include_name'", kw)
Construct.__init__(self, name)
self.subcons = subcons
self.include_name = include_name
self._inherit_flags(*subcons)
self._set_flag(self.FLAG_DYNAMIC)
def _parse(self, stream, context):
for sc in self.subcons:
pos = stream.tell()
context2 = context.__copy__()
try:
obj = sc._parse(stream, context2)
except ConstructError:
stream.seek(pos)
else:
context.__update__(context2)
if self.include_name:
return sc.name, obj
else:
return obj
raise SelectError("no subconstruct matched")
def _build(self, obj, stream, context):
if self.include_name:
name, obj = obj
for sc in self.subcons:
if sc.name == name:
sc._build(obj, stream, context)
return
else:
for sc in self.subcons:
stream2 = StringIO()
context2 = context.__copy__()
try:
sc._build(obj, stream2, context2)
except Exception:
pass
else:
context.__update__(context2)
stream.write(stream2.getvalue())
return
raise SelectError("no subconstruct matched", obj)
def _sizeof(self, context):
raise SizeofError("can't calculate size")
#===============================================================================
# stream manipulation
#===============================================================================
class Pointer(Subconstruct):
"""
Changes the stream position to a given offset, where the construction
should take place, and restores the stream position when finished.
See also Anchor, OnDemand and OnDemandPointer.
Notes:
* requires a seekable stream.
Parameters:
* offsetfunc: a function that takes the context and returns an absolute
stream position, where the construction would take place
* subcon - the subcon to use at `offsetfunc()`
Example:
Struct("foo",
UBInt32("spam_pointer"),
Pointer(lambda ctx: ctx.spam_pointer,
Array(5, UBInt8("spam"))
)
)
"""
__slots__ = ["offsetfunc"]
def __init__(self, offsetfunc, subcon):
Subconstruct.__init__(self, subcon)
self.offsetfunc = offsetfunc
def _parse(self, stream, context):
newpos = self.offsetfunc(context)
origpos = stream.tell()
stream.seek(newpos)
obj = self.subcon._parse(stream, context)
stream.seek(origpos)
return obj
def _build(self, obj, stream, context):
newpos = self.offsetfunc(context)
origpos = stream.tell()
stream.seek(newpos)
self.subcon._build(obj, stream, context)
stream.seek(origpos)
def _sizeof(self, context):
return 0
class Peek(Subconstruct):
"""
Peeks at the stream: parses without changing the stream position.
See also Union. If the end of the stream is reached when peeking,
returns None.
Notes:
* requires a seekable stream.
Parameters:
* subcon - the subcon to peek at
* perform_build - whether or not to perform building. by default this
parameter is set to False, meaning building is a no-op.
Example:
Peek(UBInt8("foo"))
"""
__slots__ = ["perform_build"]
def __init__(self, subcon, perform_build = False):
Subconstruct.__init__(self, subcon)
self.perform_build = perform_build
def _parse(self, stream, context):
pos = stream.tell()
try:
try:
return self.subcon._parse(stream, context)
except FieldError:
pass
finally:
stream.seek(pos)
def _build(self, obj, stream, context):
if self.perform_build:
self.subcon._build(obj, stream, context)
def _sizeof(self, context):
return 0
class OnDemand(Subconstruct):
"""
Allows for on-demand (lazy) parsing. When parsing, it will return a
LazyContainer that represents a pointer to the data, but does not actually
parses it from stream until it's "demanded".
By accessing the 'value' property of LazyContainers, you will demand the
data from the stream. The data will be parsed and cached for later use.
You can use the 'has_value' property to know whether the data has already
been demanded.
See also OnDemandPointer.
Notes:
* requires a seekable stream.
Parameters:
* subcon -
* advance_stream - whether or not to advance the stream position. by
default this is True, but if subcon is a pointer, this should be False.
* force_build - whether or not to force build. If set to False, and the
LazyContainer has not been demaned, building is a no-op.
Example:
OnDemand(Array(10000, UBInt8("foo"))
"""
__slots__ = ["advance_stream", "force_build"]
def __init__(self, subcon, advance_stream = True, force_build = True):
Subconstruct.__init__(self, subcon)
self.advance_stream = advance_stream
self.force_build = force_build
def _parse(self, stream, context):
obj = LazyContainer(self.subcon, stream, stream.tell(), context)
if self.advance_stream:
stream.seek(self.subcon._sizeof(context), 1)
return obj
def _build(self, obj, stream, context):
if not isinstance(obj, LazyContainer):
self.subcon._build(obj, stream, context)
elif self.force_build or obj.has_value:
self.subcon._build(obj.value, stream, context)
elif self.advance_stream:
stream.seek(self.subcon._sizeof(context), 1)
class Buffered(Subconstruct):
"""
Creates an in-memory buffered stream, which can undergo encoding and
decoding prior to being passed on to the subconstruct.
See also Bitwise.
Note:
* Do not use pointers inside Buffered
Parameters:
* subcon - the subcon which will operate on the buffer
* encoder - a function that takes a string and returns an encoded
string (used after building)
* decoder - a function that takes a string and returns a decoded
string (used before parsing)
* resizer - a function that takes the size of the subcon and "adjusts"
or "resizes" it according to the encoding/decoding process.
Example:
Buffered(BitField("foo", 16),
encoder = decode_bin,
decoder = encode_bin,
resizer = lambda size: size / 8,
)
"""
__slots__ = ["encoder", "decoder", "resizer"]
def __init__(self, subcon, decoder, encoder, resizer):
Subconstruct.__init__(self, subcon)
self.encoder = encoder
self.decoder = decoder
self.resizer = resizer
def _parse(self, stream, context):
data = _read_stream(stream, self._sizeof(context))
stream2 = StringIO(self.decoder(data))
return self.subcon._parse(stream2, context)
def _build(self, obj, stream, context):
size = self._sizeof(context)
stream2 = StringIO()
self.subcon._build(obj, stream2, context)
data = self.encoder(stream2.getvalue())
assert len(data) == size
_write_stream(stream, self._sizeof(context), data)
def _sizeof(self, context):
return self.resizer(self.subcon._sizeof(context))
class Restream(Subconstruct):
"""
Wraps the stream with a read-wrapper (for parsing) or a
write-wrapper (for building). The stream wrapper can buffer the data
internally, reading it from- or writing it to the underlying stream
as needed. For example, BitStreamReader reads whole bytes from the
underlying stream, but returns them as individual bits.
See also Bitwise.
When the parsing or building is done, the stream's close method
will be invoked. It can perform any finalization needed for the stream
wrapper, but it must not close the underlying stream.
Note:
* Do not use pointers inside Restream
Parameters:
* subcon - the subcon
* stream_reader - the read-wrapper
* stream_writer - the write wrapper
* resizer - a function that takes the size of the subcon and "adjusts"
or "resizes" it according to the encoding/decoding process.
Example:
Restream(BitField("foo", 16),
stream_reader = BitStreamReader,
stream_writer = BitStreamWriter,
resizer = lambda size: size / 8,
)
"""
__slots__ = ["stream_reader", "stream_writer", "resizer"]
def __init__(self, subcon, stream_reader, stream_writer, resizer):
Subconstruct.__init__(self, subcon)
self.stream_reader = stream_reader
self.stream_writer = stream_writer
self.resizer = resizer
def _parse(self, stream, context):
stream2 = self.stream_reader(stream)
obj = self.subcon._parse(stream2, context)
stream2.close()
return obj
def _build(self, obj, stream, context):
stream2 = self.stream_writer(stream)
self.subcon._build(obj, stream2, context)
stream2.close()
def _sizeof(self, context):
return self.resizer(self.subcon._sizeof(context))
#===============================================================================
# miscellaneous
#===============================================================================
class Reconfig(Subconstruct):
"""
Reconfigures a subconstruct. Reconfig can be used to change the name and
set and clear flags of the inner subcon.
Parameters:
* name - the new name
* subcon - the subcon to reconfigure
* setflags - the flags to set (default is 0)
* clearflags - the flags to clear (default is 0)
Example:
Reconfig("foo", UBInt8("bar"))
"""
__slots__ = []
def __init__(self, name, subcon, setflags = 0, clearflags = 0):
Construct.__init__(self, name, subcon.conflags)
self.subcon = subcon
self._set_flag(setflags)
self._clear_flag(clearflags)
class Anchor(Construct):
"""
Returns the "anchor" (stream position) at the point where it's inserted.
Useful for adjusting relative offsets to absolute positions, or to measure
sizes of constructs.
absolute pointer = anchor + relative offset
size = anchor_after - anchor_before
See also Pointer.
Notes:
* requires a seekable stream.
Parameters:
* name - the name of the anchor
Example:
Struct("foo",
Anchor("base"),
UBInt8("relative_offset"),
Pointer(lambda ctx: ctx.relative_offset + ctx.base,
UBInt8("data")
)
)
"""
__slots__ = []
def _parse(self, stream, context):
return stream.tell()
def _build(self, obj, stream, context):
context[self.name] = stream.tell()
def _sizeof(self, context):
return 0
class Value(Construct):
"""
A computed value.
Parameters:
* name - the name of the value
* func - a function that takes the context and return the computed value
Example:
Struct("foo",
UBInt8("width"),
UBInt8("height"),
Value("total_pixels", lambda ctx: ctx.width * ctx.height),
)
"""
__slots__ = ["func"]
def __init__(self, name, func):
Construct.__init__(self, name)
self.func = func
self._set_flag(self.FLAG_DYNAMIC)
def _parse(self, stream, context):
return self.func(context)
def _build(self, obj, stream, context):
context[self.name] = self.func(context)
def _sizeof(self, context):
return 0
#class Dynamic(Construct):
# """
# Dynamically creates a construct and uses it for parsing and building.
# This allows you to create change the construction tree on the fly.
# Deprecated.
#
# Parameters:
# * name - the name of the construct
# * factoryfunc - a function that takes the context and returns a new
# construct object which will be used for parsing and building.
#
# Example:
# def factory(ctx):
# if ctx.bar == 8:
# return UBInt8("spam")
# if ctx.bar == 9:
# return String("spam", 9)
#
# Struct("foo",
# UBInt8("bar"),
# Dynamic("spam", factory),
# )
# """
# __slots__ = ["factoryfunc"]
# def __init__(self, name, factoryfunc):
# Construct.__init__(self, name, self.FLAG_COPY_CONTEXT)
# self.factoryfunc = factoryfunc
# self._set_flag(self.FLAG_DYNAMIC)
# def _parse(self, stream, context):
# return self.factoryfunc(context)._parse(stream, context)
# def _build(self, obj, stream, context):
# return self.factoryfunc(context)._build(obj, stream, context)
# def _sizeof(self, context):
# return self.factoryfunc(context)._sizeof(context)
class LazyBound(Construct):
"""
Lazily bound construct, useful for constructs that need to make cyclic
references (linked-lists, expression trees, etc.).
Parameters:
Example:
foo = Struct("foo",
UBInt8("bar"),
LazyBound("next", lambda: foo),
)
"""
__slots__ = ["bindfunc", "bound"]
def __init__(self, name, bindfunc):
Construct.__init__(self, name)
self.bound = None
self.bindfunc = bindfunc
def _parse(self, stream, context):
if self.bound is None:
self.bound = self.bindfunc()
return self.bound._parse(stream, context)
def _build(self, obj, stream, context):
if self.bound is None:
self.bound = self.bindfunc()
self.bound._build(obj, stream, context)
def _sizeof(self, context):
if self.bound is None:
self.bound = self.bindfunc()
return self.bound._sizeof(context)
class Pass(Construct):
"""
A do-nothing construct, useful as the default case for Switch, or
to indicate Enums.
See also Switch and Enum.
Notes:
* this construct is a singleton. do not try to instatiate it, as it
will not work :)
Example:
Pass
"""
__slots__ = []
def _parse(self, stream, context):
pass
def _build(self, obj, stream, context):
assert obj is None
def _sizeof(self, context):
return 0
Pass = Pass(None)
class Terminator(Construct):
"""
Asserts the end of the stream has been reached at the point it's placed.
You can use this to ensure no more unparsed data follows.
Notes:
* this construct is a singleton. do not try to instatiate it, as it
will not work :)
Example:
Terminator
"""
__slots__ = []
def _parse(self, stream, context):
if stream.read(1):
raise TerminatorError("expected end of stream")
def _build(self, obj, stream, context):
assert obj is None
def _sizeof(self, context):
return 0
Terminator = Terminator(None)
|