social_media_platform/thesisenv/lib/python3.6/site-packages/PIL/PdfParser.py

import calendar
import codecs
import collections
import mmap
import os
import re
import time
import zlib
from ._util import py3

try:
    from UserDict import UserDict  # Python 2.x
except ImportError:
    UserDict = collections.UserDict  # Python 3.x


if py3:  # Python 3.x
    def make_bytes(s):
        return s.encode("us-ascii")
else:  # Python 2.x
    def make_bytes(s):  # pragma: no cover
        return s        # pragma: no cover


# see 7.9.2.2 Text String Type on page 86 and D.3 PDFDocEncoding Character Set
# on page 656
def encode_text(s):
    return codecs.BOM_UTF16_BE + s.encode("utf_16_be")


PDFDocEncoding = {
    0x16: u"\u0017",
    0x18: u"\u02D8",
    0x19: u"\u02C7",
    0x1A: u"\u02C6",
    0x1B: u"\u02D9",
    0x1C: u"\u02DD",
    0x1D: u"\u02DB",
    0x1E: u"\u02DA",
    0x1F: u"\u02DC",
    0x80: u"\u2022",
    0x81: u"\u2020",
    0x82: u"\u2021",
    0x83: u"\u2026",
    0x84: u"\u2014",
    0x85: u"\u2013",
    0x86: u"\u0192",
    0x87: u"\u2044",
    0x88: u"\u2039",
    0x89: u"\u203A",
    0x8A: u"\u2212",
    0x8B: u"\u2030",
    0x8C: u"\u201E",
    0x8D: u"\u201C",
    0x8E: u"\u201D",
    0x8F: u"\u2018",
    0x90: u"\u2019",
    0x91: u"\u201A",
    0x92: u"\u2122",
    0x93: u"\uFB01",
    0x94: u"\uFB02",
    0x95: u"\u0141",
    0x96: u"\u0152",
    0x97: u"\u0160",
    0x98: u"\u0178",
    0x99: u"\u017D",
    0x9A: u"\u0131",
    0x9B: u"\u0142",
    0x9C: u"\u0153",
    0x9D: u"\u0161",
    0x9E: u"\u017E",
    0xA0: u"\u20AC",
}


def decode_text(b):
    if b[:len(codecs.BOM_UTF16_BE)] == codecs.BOM_UTF16_BE:
        return b[len(codecs.BOM_UTF16_BE):].decode("utf_16_be")
    elif py3:  # Python 3.x
        return "".join(PDFDocEncoding.get(byte, chr(byte)) for byte in b)
    else:  # Python 2.x
        return u"".join(PDFDocEncoding.get(ord(byte), byte) for byte in b)


class PdfFormatError(RuntimeError):
    """An error that probably indicates a syntactic or semantic error in the
    PDF file structure"""
    pass


def check_format_condition(condition, error_message):
    if not condition:
        raise PdfFormatError(error_message)


class IndirectReference(collections.namedtuple("IndirectReferenceTuple",
                        ["object_id", "generation"])):
    def __str__(self):
        return "%s %s R" % self

    def __bytes__(self):
        return self.__str__().encode("us-ascii")

    def __eq__(self, other):
        return other.__class__ is self.__class__ and \
               other.object_id == self.object_id and \
               other.generation == self.generation

    def __ne__(self, other):
        return not (self == other)

    def __hash__(self):
        return hash((self.object_id, self.generation))


class IndirectObjectDef(IndirectReference):
    def __str__(self):
        return "%s %s obj" % self


class XrefTable:
    def __init__(self):
        self.existing_entries = {}          # object ID => (offset, generation)
        self.new_entries = {}               # object ID => (offset, generation)
        self.deleted_entries = {0: 65536}   # object ID => generation
        self.reading_finished = False

    def __setitem__(self, key, value):
        if self.reading_finished:
            self.new_entries[key] = value
        else:
            self.existing_entries[key] = value
        if key in self.deleted_entries:
            del self.deleted_entries[key]

    def __getitem__(self, key):
        try:
            return self.new_entries[key]
        except KeyError:
            return self.existing_entries[key]

    def __delitem__(self, key):
        if key in self.new_entries:
            generation = self.new_entries[key][1] + 1
            del self.new_entries[key]
            self.deleted_entries[key] = generation
        elif key in self.existing_entries:
            generation = self.existing_entries[key][1] + 1
            self.deleted_entries[key] = generation
        elif key in self.deleted_entries:
            generation = self.deleted_entries[key]
        else:
            raise IndexError("object ID " + str(key) +
                             " cannot be deleted because it doesn't exist")

    def __contains__(self, key):
        return key in self.existing_entries or key in self.new_entries

    def __len__(self):
        return len(set(self.existing_entries.keys()) |
                   set(self.new_entries.keys()) |
                   set(self.deleted_entries.keys()))

    def keys(self):
        return (
            set(self.existing_entries.keys()) -
            set(self.deleted_entries.keys())
        ) | set(self.new_entries.keys())

    def write(self, f):
        keys = sorted(set(self.new_entries.keys()) |
                      set(self.deleted_entries.keys()))
        deleted_keys = sorted(set(self.deleted_entries.keys()))
        startxref = f.tell()
        f.write(b"xref\n")
        while keys:
            # find a contiguous sequence of object IDs
            prev = None
            for index, key in enumerate(keys):
                if prev is None or prev+1 == key:
                    prev = key
                else:
                    contiguous_keys = keys[:index]
                    keys = keys[index:]
                    break
            else:
                contiguous_keys = keys
                keys = None
            f.write(make_bytes("%d %d\n" %
                               (contiguous_keys[0], len(contiguous_keys))))
            for object_id in contiguous_keys:
                if object_id in self.new_entries:
                    f.write(make_bytes("%010d %05d n \n" %
                                       self.new_entries[object_id]))
                else:
                    this_deleted_object_id = deleted_keys.pop(0)
                    check_format_condition(object_id == this_deleted_object_id,
                                           "expected the next deleted object "
                                           "ID to be %s, instead found %s" %
                                           (object_id, this_deleted_object_id))
                    try:
                        next_in_linked_list = deleted_keys[0]
                    except IndexError:
                        next_in_linked_list = 0
                    f.write(make_bytes("%010d %05d f \n" %
                                       (next_in_linked_list,
                                        self.deleted_entries[object_id])))
        return startxref


class PdfName:
    def __init__(self, name):
        if isinstance(name, PdfName):
            self.name = name.name
        elif isinstance(name, bytes):
            self.name = name
        else:
            self.name = name.encode("us-ascii")

    def name_as_str(self):
        return self.name.decode("us-ascii")

    def __eq__(self, other):
        return (isinstance(other, PdfName) and other.name == self.name) or \
               other == self.name

    def __hash__(self):
        return hash(self.name)

    def __repr__(self):
        return "PdfName(%s)" % repr(self.name)

    @classmethod
    def from_pdf_stream(cls, data):
        return cls(PdfParser.interpret_name(data))

    allowed_chars = set(range(33, 127)) - set(ord(c) for c in "#%/()<>[]{}")

    def __bytes__(self):
        result = bytearray(b"/")
        for b in self.name:
            if py3:  # Python 3.x
                if b in self.allowed_chars:
                    result.append(b)
                else:
                    result.extend(make_bytes("#%02X" % b))
            else:  # Python 2.x
                if ord(b) in self.allowed_chars:
                    result.append(b)
                else:
                    result.extend(b"#%02X" % ord(b))
        return bytes(result)

    __str__ = __bytes__


class PdfArray(list):
    def __bytes__(self):
        return b"[ " + b" ".join(pdf_repr(x) for x in self) + b" ]"

    __str__ = __bytes__


class PdfDict(UserDict):
    def __setattr__(self, key, value):
        if key == "data":
            if hasattr(UserDict, "__setattr__"):
                UserDict.__setattr__(self, key, value)
            else:
                self.__dict__[key] = value
        else:
            if isinstance(key, str):
                key = key.encode("us-ascii")
            self[key] = value

    def __getattr__(self, key):
        try:
            value = self[key]
        except KeyError:
            try:
                value = self[key.encode("us-ascii")]
            except KeyError:
                raise AttributeError(key)
        if isinstance(value, bytes):
            value = decode_text(value)
        if key.endswith("Date"):
            if value.startswith("D:"):
                value = value[2:]

            relationship = 'Z'
            if len(value) > 17:
                relationship = value[14]
                offset = int(value[15:17]) * 60
                if len(value) > 20:
                    offset += int(value[18:20])

            format = '%Y%m%d%H%M%S'[:len(value) - 2]
            value = time.strptime(value[:len(format)+2], format)
            if relationship in ['+', '-']:
                offset *= 60
                if relationship == '+':
                    offset *= -1
                value = time.gmtime(calendar.timegm(value) + offset)
        return value

    def __bytes__(self):
        out = bytearray(b"<<")
        for key, value in self.items():
            if value is None:
                continue
            value = pdf_repr(value)
            out.extend(b"\n")
            out.extend(bytes(PdfName(key)))
            out.extend(b" ")
            out.extend(value)
        out.extend(b"\n>>")
        return bytes(out)

    if not py3:
        __str__ = __bytes__


class PdfBinary:
    def __init__(self, data):
        self.data = data

    if py3:  # Python 3.x
        def __bytes__(self):
            return make_bytes("<%s>" % "".join("%02X" % b for b in self.data))
    else:  # Python 2.x
        def __str__(self):
            return "<%s>" % "".join("%02X" % ord(b) for b in self.data)


class PdfStream:
    def __init__(self, dictionary, buf):
        self.dictionary = dictionary
        self.buf = buf

    def decode(self):
        try:
            filter = self.dictionary.Filter
        except AttributeError:
            return self.buf
        if filter == b"FlateDecode":
            try:
                expected_length = self.dictionary.DL
            except AttributeError:
                expected_length = self.dictionary.Length
            return zlib.decompress(self.buf, bufsize=int(expected_length))
        else:
            raise NotImplementedError(
                "stream filter %s unknown/unsupported" %
                repr(self.dictionary.Filter))


def pdf_repr(x):
    if x is True:
        return b"true"
    elif x is False:
        return b"false"
    elif x is None:
        return b"null"
    elif (isinstance(x, PdfName) or isinstance(x, PdfDict) or
          isinstance(x, PdfArray) or isinstance(x, PdfBinary)):
        return bytes(x)
    elif isinstance(x, int):
        return str(x).encode("us-ascii")
    elif isinstance(x, time.struct_time):
        return b'(D:'+time.strftime('%Y%m%d%H%M%SZ', x).encode("us-ascii")+b')'
    elif isinstance(x, dict):
        return bytes(PdfDict(x))
    elif isinstance(x, list):
        return bytes(PdfArray(x))
    elif ((py3 and isinstance(x, str)) or
          (not py3 and isinstance(x, unicode))):
        return pdf_repr(encode_text(x))
    elif isinstance(x, bytes):
        # XXX escape more chars? handle binary garbage
        x = x.replace(b"\\", b"\\\\")
        x = x.replace(b"(", b"\\(")
        x = x.replace(b")", b"\\)")
        return b"(" + x + b")"
    else:
        return bytes(x)


class PdfParser:
    """Based on https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/PDF32000_2008.pdf
    Supports PDF up to 1.4
    """

    def __init__(self, filename=None, f=None,
                 buf=None, start_offset=0, mode="rb"):
        # type: (PdfParser, str, file, Union[bytes, bytearray], int, str)
        #       -> None
        if buf and f:
            raise RuntimeError(
                "specify buf or f or filename, but not both buf and f")
        self.filename = filename
        self.buf = buf
        self.f = f
        self.start_offset = start_offset
        self.should_close_buf = False
        self.should_close_file = False
        if filename is not None and f is None:
            self.f = f = open(filename, mode)
            self.should_close_file = True
        if f is not None:
            self.buf = buf = self.get_buf_from_file(f)
            self.should_close_buf = True
            if not filename and hasattr(f, "name"):
                self.filename = f.name
        self.cached_objects = {}
        if buf:
            self.read_pdf_info()
        else:
            self.file_size_total = self.file_size_this = 0
            self.root = PdfDict()
            self.root_ref = None
            self.info = PdfDict()
            self.info_ref = None
            self.page_tree_root = {}
            self.pages = []
            self.orig_pages = []
            self.pages_ref = None
            self.last_xref_section_offset = None
            self.trailer_dict = {}
            self.xref_table = XrefTable()
        self.xref_table.reading_finished = True
        if f:
            self.seek_end()

    def __enter__(self):
        return self

    def __exit__(self, exc_type, exc_value, traceback):
        self.close()
        return False  # do not suppress exceptions

    def start_writing(self):
        self.close_buf()
        self.seek_end()

    def close_buf(self):
        try:
            self.buf.close()
        except AttributeError:
            pass
        self.buf = None

    def close(self):
        if self.should_close_buf:
            self.close_buf()
        if self.f is not None and self.should_close_file:
            self.f.close()
            self.f = None

    def seek_end(self):
        self.f.seek(0, os.SEEK_END)

    def write_header(self):
        self.f.write(b"%PDF-1.4\n")

    def write_comment(self, s):
        self.f.write(("%% %s\n" % (s,)).encode("utf-8"))

    def write_catalog(self):
        self.del_root()
        self.root_ref = self.next_object_id(self.f.tell())
        self.pages_ref = self.next_object_id(0)
        self.rewrite_pages()
        self.write_obj(self.root_ref,
                       Type=PdfName(b"Catalog"),
                       Pages=self.pages_ref)
        self.write_obj(self.pages_ref,
                       Type=PdfName(b"Pages"),
                       Count=len(self.pages),
                       Kids=self.pages)
        return self.root_ref

    def rewrite_pages(self):
        pages_tree_nodes_to_delete = []
        for i, page_ref in enumerate(self.orig_pages):
            page_info = self.cached_objects[page_ref]
            del self.xref_table[page_ref.object_id]
            pages_tree_nodes_to_delete.append(page_info[PdfName(b"Parent")])
            if page_ref not in self.pages:
                # the page has been deleted
                continue
            # make dict keys into strings for passing to write_page
            stringified_page_info = {}
            for key, value in page_info.items():
                # key should be a PdfName
                stringified_page_info[key.name_as_str()] = value
            stringified_page_info["Parent"] = self.pages_ref
            new_page_ref = self.write_page(None, **stringified_page_info)
            for j, cur_page_ref in enumerate(self.pages):
                if cur_page_ref == page_ref:
                    # replace the page reference with the new one
                    self.pages[j] = new_page_ref
        # delete redundant Pages tree nodes from xref table
        for pages_tree_node_ref in pages_tree_nodes_to_delete:
            while pages_tree_node_ref:
                pages_tree_node = self.cached_objects[pages_tree_node_ref]
                if pages_tree_node_ref.object_id in self.xref_table:
                    del self.xref_table[pages_tree_node_ref.object_id]
                pages_tree_node_ref = pages_tree_node.get(b"Parent", None)
        self.orig_pages = []

    def write_xref_and_trailer(self, new_root_ref=None):
        if new_root_ref:
            self.del_root()
            self.root_ref = new_root_ref
        if self.info:
            self.info_ref = self.write_obj(None, self.info)
        start_xref = self.xref_table.write(self.f)
        num_entries = len(self.xref_table)
        trailer_dict = {b"Root": self.root_ref, b"Size": num_entries}
        if self.last_xref_section_offset is not None:
            trailer_dict[b"Prev"] = self.last_xref_section_offset
        if self.info:
            trailer_dict[b"Info"] = self.info_ref
        self.last_xref_section_offset = start_xref
        self.f.write(b"trailer\n" + bytes(PdfDict(trailer_dict)) +
                     make_bytes("\nstartxref\n%d\n%%%%EOF" % start_xref))

    def write_page(self, ref, *objs, **dict_obj):
        if isinstance(ref, int):
            ref = self.pages[ref]
        if "Type" not in dict_obj:
            dict_obj["Type"] = PdfName(b"Page")
        if "Parent" not in dict_obj:
            dict_obj["Parent"] = self.pages_ref
        return self.write_obj(ref, *objs, **dict_obj)

    def write_obj(self, ref, *objs, **dict_obj):
        f = self.f
        if ref is None:
            ref = self.next_object_id(f.tell())
        else:
            self.xref_table[ref.object_id] = (f.tell(), ref.generation)
        f.write(bytes(IndirectObjectDef(*ref)))
        stream = dict_obj.pop("stream", None)
        if stream is not None:
            dict_obj["Length"] = len(stream)
        if dict_obj:
            f.write(pdf_repr(dict_obj))
        for obj in objs:
            f.write(pdf_repr(obj))
        if stream is not None:
            f.write(b"stream\n")
            f.write(stream)
            f.write(b"\nendstream\n")
        f.write(b"endobj\n")
        return ref

    def del_root(self):
        if self.root_ref is None:
            return
        del self.xref_table[self.root_ref.object_id]
        del self.xref_table[self.root[b"Pages"].object_id]

    @staticmethod
    def get_buf_from_file(f):
        if hasattr(f, "getbuffer"):
            return f.getbuffer()
        elif hasattr(f, "getvalue"):
            return f.getvalue()
        else:
            try:
                return mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ)
            except ValueError:  # cannot mmap an empty file
                return b""

    def read_pdf_info(self):
        self.file_size_total = len(self.buf)
        self.file_size_this = self.file_size_total - self.start_offset
        self.read_trailer()
        self.root_ref = self.trailer_dict[b"Root"]
        self.info_ref = self.trailer_dict.get(b"Info", None)
        self.root = PdfDict(self.read_indirect(self.root_ref))
        if self.info_ref is None:
            self.info = PdfDict()
        else:
            self.info = PdfDict(self.read_indirect(self.info_ref))
        check_format_condition(b"Type" in self.root, "/Type missing in Root")
        check_format_condition(self.root[b"Type"] == b"Catalog",
                               "/Type in Root is not /Catalog")
        check_format_condition(b"Pages" in self.root, "/Pages missing in Root")
        check_format_condition(isinstance(self.root[b"Pages"],
                               IndirectReference),
                               "/Pages in Root is not an indirect reference")
        self.pages_ref = self.root[b"Pages"]
        self.page_tree_root = self.read_indirect(self.pages_ref)
        self.pages = self.linearize_page_tree(self.page_tree_root)
        # save the original list of page references
        # in case the user modifies, adds or deletes some pages
        # and we need to rewrite the pages and their list
        self.orig_pages = self.pages[:]

    def next_object_id(self, offset=None):
        try:
            # TODO: support reuse of deleted objects
            reference = IndirectReference(max(self.xref_table.keys()) + 1, 0)
        except ValueError:
            reference = IndirectReference(1, 0)
        if offset is not None:
            self.xref_table[reference.object_id] = (offset, 0)
        return reference

    delimiter = br"[][()<>{}/%]"
    delimiter_or_ws = br"[][()<>{}/%\000\011\012\014\015\040]"
    whitespace = br"[\000\011\012\014\015\040]"
    whitespace_or_hex = br"[\000\011\012\014\015\0400-9a-fA-F]"
    whitespace_optional = whitespace + b"*"
    whitespace_mandatory = whitespace + b"+"
    newline_only = br"[\r\n]+"
    newline = whitespace_optional + newline_only + whitespace_optional
    re_trailer_end = re.compile(
        whitespace_mandatory + br"trailer" + whitespace_optional +
        br"\<\<(.*\>\>)" + newline + br"startxref" + newline + br"([0-9]+)" +
        newline + br"%%EOF" + whitespace_optional + br"$", re.DOTALL)
    re_trailer_prev = re.compile(
        whitespace_optional + br"trailer" + whitespace_optional +
        br"\<\<(.*?\>\>)" + newline + br"startxref" + newline + br"([0-9]+)" +
        newline + br"%%EOF" + whitespace_optional, re.DOTALL)

    def read_trailer(self):
        search_start_offset = len(self.buf) - 16384
        if search_start_offset < self.start_offset:
            search_start_offset = self.start_offset
        m = self.re_trailer_end.search(self.buf, search_start_offset)
        check_format_condition(m, "trailer end not found")
        # make sure we found the LAST trailer
        last_match = m
        while m:
            last_match = m
            m = self.re_trailer_end.search(self.buf, m.start()+16)
        if not m:
            m = last_match
        trailer_data = m.group(1)
        self.last_xref_section_offset = int(m.group(2))
        self.trailer_dict = self.interpret_trailer(trailer_data)
        self.xref_table = XrefTable()
        self.read_xref_table(xref_section_offset=self.last_xref_section_offset)
        if b"Prev" in self.trailer_dict:
            self.read_prev_trailer(self.trailer_dict[b"Prev"])

    def read_prev_trailer(self, xref_section_offset):
        trailer_offset = self.read_xref_table(
            xref_section_offset=xref_section_offset)
        m = self.re_trailer_prev.search(
            self.buf[trailer_offset:trailer_offset+16384])
        check_format_condition(m, "previous trailer not found")
        trailer_data = m.group(1)
        check_format_condition(int(m.group(2)) == xref_section_offset,
                               "xref section offset in previous trailer "
                               "doesn't match what was expected")
        trailer_dict = self.interpret_trailer(trailer_data)
        if b"Prev" in trailer_dict:
            self.read_prev_trailer(trailer_dict[b"Prev"])

    re_whitespace_optional = re.compile(whitespace_optional)
    re_name = re.compile(
        whitespace_optional + br"/([!-$&'*-.0-;=?-Z\\^-z|~]+)(?=" +
        delimiter_or_ws + br")")
    re_dict_start = re.compile(whitespace_optional + br"\<\<")
    re_dict_end = re.compile(
        whitespace_optional + br"\>\>" + whitespace_optional)

    @classmethod
    def interpret_trailer(cls, trailer_data):
        trailer = {}
        offset = 0
        while True:
            m = cls.re_name.match(trailer_data, offset)
            if not m:
                m = cls.re_dict_end.match(trailer_data, offset)
                check_format_condition(
                    m and m.end() == len(trailer_data),
                    "name not found in trailer, remaining data: " +
                    repr(trailer_data[offset:]))
                break
            key = cls.interpret_name(m.group(1))
            value, offset = cls.get_value(trailer_data, m.end())
            trailer[key] = value
        check_format_condition(
            b"Size" in trailer and isinstance(trailer[b"Size"], int),
            "/Size not in trailer or not an integer")
        check_format_condition(
            b"Root" in trailer and
            isinstance(trailer[b"Root"], IndirectReference),
            "/Root not in trailer or not an indirect reference")
        return trailer

    re_hashes_in_name = re.compile(br"([^#]*)(#([0-9a-fA-F]{2}))?")

    @classmethod
    def interpret_name(cls, raw, as_text=False):
        name = b""
        for m in cls.re_hashes_in_name.finditer(raw):
            if m.group(3):
                name += m.group(1) + \
                    bytearray.fromhex(m.group(3).decode("us-ascii"))
            else:
                name += m.group(1)
        if as_text:
            return name.decode("utf-8")
        else:
            return bytes(name)

    re_null = re.compile(
        whitespace_optional + br"null(?=" + delimiter_or_ws + br")")
    re_true = re.compile(
        whitespace_optional + br"true(?=" + delimiter_or_ws + br")")
    re_false = re.compile(
        whitespace_optional + br"false(?=" + delimiter_or_ws + br")")
    re_int = re.compile(
        whitespace_optional + br"([-+]?[0-9]+)(?=" + delimiter_or_ws + br")")
    re_real = re.compile(
        whitespace_optional + br"([-+]?([0-9]+\.[0-9]*|[0-9]*\.[0-9]+))(?=" +
        delimiter_or_ws + br")")
    re_array_start = re.compile(whitespace_optional + br"\[")
    re_array_end = re.compile(whitespace_optional + br"]")
    re_string_hex = re.compile(
        whitespace_optional + br"\<(" + whitespace_or_hex + br"*)\>")
    re_string_lit = re.compile(whitespace_optional + br"\(")
    re_indirect_reference = re.compile(
        whitespace_optional + br"([-+]?[0-9]+)" + whitespace_mandatory +
        br"([-+]?[0-9]+)" + whitespace_mandatory + br"R(?=" + delimiter_or_ws +
        br")")
    re_indirect_def_start = re.compile(
        whitespace_optional + br"([-+]?[0-9]+)" + whitespace_mandatory +
        br"([-+]?[0-9]+)" + whitespace_mandatory + br"obj(?=" +
        delimiter_or_ws + br")")
    re_indirect_def_end = re.compile(
        whitespace_optional + br"endobj(?=" + delimiter_or_ws + br")")
    re_comment = re.compile(
        br"(" + whitespace_optional + br"%[^\r\n]*" + newline + br")*")
    re_stream_start = re.compile(whitespace_optional + br"stream\r?\n")
    re_stream_end = re.compile(
        whitespace_optional + br"endstream(?=" + delimiter_or_ws + br")")

    @classmethod
    def get_value(cls, data, offset, expect_indirect=None, max_nesting=-1):
        if max_nesting == 0:
            return None, None
        m = cls.re_comment.match(data, offset)
        if m:
            offset = m.end()
        m = cls.re_indirect_def_start.match(data, offset)
        if m:
            check_format_condition(
                int(m.group(1)) > 0,
                "indirect object definition: object ID must be greater than 0")
            check_format_condition(
                int(m.group(2)) >= 0,
                "indirect object definition: generation must be non-negative")
            check_format_condition(
                expect_indirect is None or expect_indirect ==
                IndirectReference(int(m.group(1)), int(m.group(2))),
                "indirect object definition different than expected")
            object, offset = cls.get_value(
                data, m.end(), max_nesting=max_nesting-1)
            if offset is None:
                return object, None
            m = cls.re_indirect_def_end.match(data, offset)
            check_format_condition(
                m, "indirect object definition end not found")
            return object, m.end()
        check_format_condition(
            not expect_indirect, "indirect object definition not found")
        m = cls.re_indirect_reference.match(data, offset)
        if m:
            check_format_condition(
                int(m.group(1)) > 0,
                "indirect object reference: object ID must be greater than 0")
            check_format_condition(
                int(m.group(2)) >= 0,
                "indirect object reference: generation must be non-negative")
            return IndirectReference(int(m.group(1)), int(m.group(2))), m.end()
        m = cls.re_dict_start.match(data, offset)
        if m:
            offset = m.end()
            result = {}
            m = cls.re_dict_end.match(data, offset)
            while not m:
                key, offset = cls.get_value(
                    data, offset, max_nesting=max_nesting-1)
                if offset is None:
                    return result, None
                value, offset = cls.get_value(
                    data, offset, max_nesting=max_nesting-1)
                result[key] = value
                if offset is None:
                    return result, None
                m = cls.re_dict_end.match(data, offset)
            offset = m.end()
            m = cls.re_stream_start.match(data, offset)
            if m:
                try:
                    stream_len = int(result[b"Length"])
                except (TypeError, KeyError, ValueError):
                    raise PdfFormatError(
                        "bad or missing Length in stream dict (%r)" %
                        result.get(b"Length", None))
                stream_data = data[m.end():m.end() + stream_len]
                m = cls.re_stream_end.match(data, m.end() + stream_len)
                check_format_condition(m, "stream end not found")
                offset = m.end()
                result = PdfStream(PdfDict(result), stream_data)
            else:
                result = PdfDict(result)
            return result, offset
        m = cls.re_array_start.match(data, offset)
        if m:
            offset = m.end()
            result = []
            m = cls.re_array_end.match(data, offset)
            while not m:
                value, offset = cls.get_value(
                    data, offset, max_nesting=max_nesting-1)
                result.append(value)
                if offset is None:
                    return result, None
                m = cls.re_array_end.match(data, offset)
            return result, m.end()
        m = cls.re_null.match(data, offset)
        if m:
            return None, m.end()
        m = cls.re_true.match(data, offset)
        if m:
            return True, m.end()
        m = cls.re_false.match(data, offset)
        if m:
            return False, m.end()
        m = cls.re_name.match(data, offset)
        if m:
            return PdfName(cls.interpret_name(m.group(1))), m.end()
        m = cls.re_int.match(data, offset)
        if m:
            return int(m.group(1)), m.end()
        m = cls.re_real.match(data, offset)
        if m:
            # XXX Decimal instead of float???
            return float(m.group(1)), m.end()
        m = cls.re_string_hex.match(data, offset)
        if m:
            # filter out whitespace
            hex_string = bytearray([
                b for b in m.group(1)
                if b in b"0123456789abcdefABCDEF"
            ])
            if len(hex_string) % 2 == 1:
                # append a 0 if the length is not even - yes, at the end
                hex_string.append(ord(b"0"))
            return bytearray.fromhex(hex_string.decode("us-ascii")), m.end()
        m = cls.re_string_lit.match(data, offset)
        if m:
            return cls.get_literal_string(data, m.end())
        # return None, offset  # fallback (only for debugging)
        raise PdfFormatError(
            "unrecognized object: " + repr(data[offset:offset+32]))

    re_lit_str_token = re.compile(br"(\\[nrtbf()\\])|(\\[0-9]{1,3})|(\\(\r\n|\r|\n))|(\r\n|\r|\n)|(\()|(\))")
    escaped_chars = {
        b"n": b"\n",
        b"r": b"\r",
        b"t": b"\t",
        b"b": b"\b",
        b"f": b"\f",
        b"(": b"(",
        b")": b")",
        b"\\": b"\\",
        ord(b"n"): b"\n",
        ord(b"r"): b"\r",
        ord(b"t"): b"\t",
        ord(b"b"): b"\b",
        ord(b"f"): b"\f",
        ord(b"("): b"(",
        ord(b")"): b")",
        ord(b"\\"): b"\\",
        }

    @classmethod
    def get_literal_string(cls, data, offset):
        nesting_depth = 0
        result = bytearray()
        for m in cls.re_lit_str_token.finditer(data, offset):
            result.extend(data[offset:m.start()])
            if m.group(1):
                result.extend(cls.escaped_chars[m.group(1)[1]])
            elif m.group(2):
                result.append(int(m.group(2)[1:], 8))
            elif m.group(3):
                pass
            elif m.group(5):
                result.extend(b"\n")
            elif m.group(6):
                result.extend(b"(")
                nesting_depth += 1
            elif m.group(7):
                if nesting_depth == 0:
                    return bytes(result), m.end()
                result.extend(b")")
                nesting_depth -= 1
            offset = m.end()
        raise PdfFormatError("unfinished literal string")

    re_xref_section_start = re.compile(
        whitespace_optional + br"xref" + newline)
    re_xref_subsection_start = re.compile(
        whitespace_optional + br"([0-9]+)" + whitespace_mandatory +
        br"([0-9]+)" + whitespace_optional + newline_only)
    re_xref_entry = re.compile(br"([0-9]{10}) ([0-9]{5}) ([fn])( \r| \n|\r\n)")

    def read_xref_table(self, xref_section_offset):
        subsection_found = False
        m = self.re_xref_section_start.match(
            self.buf, xref_section_offset + self.start_offset)
        check_format_condition(m, "xref section start not found")
        offset = m.end()
        while True:
            m = self.re_xref_subsection_start.match(self.buf, offset)
            if not m:
                check_format_condition(
                    subsection_found, "xref subsection start not found")
                break
            subsection_found = True
            offset = m.end()
            first_object = int(m.group(1))
            num_objects = int(m.group(2))
            for i in range(first_object, first_object+num_objects):
                m = self.re_xref_entry.match(self.buf, offset)
                check_format_condition(m, "xref entry not found")
                offset = m.end()
                is_free = m.group(3) == b"f"
                generation = int(m.group(2))
                if not is_free:
                    new_entry = (int(m.group(1)), generation)
                    check_format_condition(
                        i not in self.xref_table or
                        self.xref_table[i] == new_entry,
                        "xref entry duplicated (and not identical)")
                    self.xref_table[i] = new_entry
        return offset

    def read_indirect(self, ref, max_nesting=-1):
        offset, generation = self.xref_table[ref[0]]
        check_format_condition(
            generation == ref[1],
            "expected to find generation %s for object ID %s in xref table, "
            "instead found generation %s at offset %s"
            % (ref[1], ref[0], generation, offset))
        value = self.get_value(self.buf, offset + self.start_offset,
                               expect_indirect=IndirectReference(*ref),
                               max_nesting=max_nesting)[0]
        self.cached_objects[ref] = value
        return value

    def linearize_page_tree(self, node=None):
        if node is None:
            node = self.page_tree_root
        check_format_condition(
            node[b"Type"] == b"Pages", "/Type of page tree node is not /Pages")
        pages = []
        for kid in node[b"Kids"]:
            kid_object = self.read_indirect(kid)
            if kid_object[b"Type"] == b"Page":
                pages.append(kid)
            else:
                pages.extend(self.linearize_page_tree(node=kid_object))
        return pages