MAINT: Simplify file identifiers generation

pypdf/_writer.py

@@ -33,6 +33,7 @@
 import enum
 import hashlib
 import re
+import time
 import uuid
 import warnings
 from io import BytesIO, FileIO, IOBase
@@ -145,13 +146,6 @@ class ObjectDeletionFlag(enum.IntFlag):
     IMAGES = XOBJECT_IMAGES | INLINE_IMAGES | DRAWING_IMAGES
 
 
-def _rolling_checksum(stream: BytesIO, blocksize: int = 65536) -> str:
-    hash = hashlib.md5()
-    for block in iter(lambda: stream.read(blocksize), b""):
-        hash.update(block)
-    return hash.hexdigest()
-
-
 class PdfWriter:
     """
     Write a PDF file out, given pages produced by another class.
@@ -1224,10 +1218,14 @@ def cloneDocumentFromReader(
         self.clone_document_from_reader(reader, after_page_append)
 
     def _compute_document_identifier(self) -> ByteStringObject:
-        stream = BytesIO()
-        self._write_pdf_structure(stream)
-        stream.seek(0)
-        return ByteStringObject(_rolling_checksum(stream).encode("utf8"))
+        md5 = hashlib.md5()
+        md5.update(str(time.time()).encode("utf-8"))
+        md5.update(str(self.fileobj).encode("utf-8"))
+        md5.update(str(len(self._objects)).encode("utf-8"))
+        if hasattr(self, "_info"):
+            for k, v in cast(DictionaryObject, self._info.get_object()).items():
+                md5.update(f"{k}={v}".encode())
+        return ByteStringObject(md5.hexdigest().encode("utf-8"))
 
     def generate_file_identifiers(self) -> None:
         """
@@ -1246,7 +1244,7 @@ def generate_file_identifiers(self) -> None:
             id2 = self._compute_document_identifier()
         else:
             id1 = self._compute_document_identifier()
-            id2 = id1
+            id2 = ByteStringObject(id1.original_bytes)
         self._ID = ArrayObject((id1, id2))
 
     def encrypt(