core.py

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.

    The general-case repeater. Repeats the given unit for at least mincount
    times, and up to maxcount times. If an exception occurs (EOF, validation
    error), the repeater exits. If less than mincount units have been
    successfully parsed, a RepeaterError is raised.

    .. note::
       This object requires a seekable stream for parsing.

    Parameters:

     * mincount - the minimal count (an integer)
     * maxcount - the maximal count (an integer)
     * subcon - the subcon to repeat

    Example:
    Range(5, 8, UBInt8("foo"))

    >>> c = Repeater(3, 7, UBInt8("foo"))
    >>> c.parse("\\x01\\x02")
    Traceback (most recent call last):
      ...
    construct.core.RepeaterError: expected 3..7, found 2
    >>> c.parse("\\x01\\x02\\x03")
    [1, 2, 3]
    >>> c.parse("\\x01\\x02\\x03\\x04\\x05\\x06")
    [1, 2, 3, 4, 5, 6]
    >>> c.parse("\\x01\\x02\\x03\\x04\\x05\\x06\\x07")
    [1, 2, 3, 4, 5, 6, 7]
    >>> c.parse("\\x01\\x02\\x03\\x04\\x05\\x06\\x07\\x08\\x09")
    [1, 2, 3, 4, 5, 6, 7]
    >>> c.build([1,2])
    Traceback (most recent call last):
      ...
    construct.core.RepeaterError: expected 3..7, found 2
    >>> c.build([1,2,3,4])
    '\\x01\\x02\\x03\\x04'
    >>> c.build([1,2,3,4,5,6,7,8])
    Traceback (most recent call last):
      ...
    construct.core.RepeaterError: expected 3..7, found 8
    """

    __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, ex:
            if c < self.mincount:
                raise RangeError("expected %d to %d, found %d" % 
                    (self.mincount, self.maxcout, c), ex)
            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, ex:
            if cnt < self.mincount:
                raise RangeError("expected %d to %d, found %d" % 
                    (self.mincount, self.maxcout, len(obj)), ex)
    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("No default value specified")
    
    __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:
            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 only meaningful for parsing. for building, it's 
      a no-op.
    * 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)