gh-134635: add zlib.{adler32,crc32}_combine to combine checksums

Doc/library/zlib.rst

@@ -44,6 +44,16 @@ The available exception and functions in this module are:
    .. versionchanged:: 3.0
       The result is always unsigned.
 
+.. function:: adler32_combine(adler1, adler2, len2, /)
+
+   Combine two Adler-32 checksums into one.
+
+   Given an Adler-32 check value *adler1* of a sequence A and an Adler-32 check
+   value *adler2* of a sequence B of length *len2*, the returned checksum
+   is the Adler-32 check value of A and B concatenated.
+
+   .. versionadded:: next
+
 .. function:: compress(data, /, level=-1, wbits=MAX_WBITS)
 
    Compresses the bytes in *data*, returning a bytes object containing compressed data.
@@ -136,6 +146,16 @@ The available exception and functions in this module are:
    .. versionchanged:: 3.0
       The result is always unsigned.
 
+.. function:: crc32_combine(crc1, crc2, len2, /)
+
+   Combine two CRC-32 checksums into one.
+
+   Given a CRC-32 check value *crc1* of a sequence A and a CRC-32 check
+   value *crc2* of a sequence B of length *len2*, the returned checksum
+   is the CRC-32 check value of A and B concatenated.
+
+   .. versionadded:: next
+
 .. function:: decompress(data, /, wbits=MAX_WBITS, bufsize=DEF_BUF_SIZE)
 
    Decompresses the bytes in *data*, returning a bytes object containing the

Doc/whatsnew/3.15.rst

@@ -97,6 +97,16 @@ ssl
   (Contributed by Will Childs-Klein in :gh:`133624`.)
 
 
+zlib
+----
+
+* Allow to combine two Adler-32 checksums via :func:`~zlib.adler32_combine`.
+  (Contributed by Callum Attryde and Bénédikt Tran in :gh:`134635`.)
+
+* Allow to combine two CRC-32 checksums via :func:`~zlib.crc32_combine`.
+  (Contributed by Bénédikt Tran in :gh:`134635`.)
+
+
 .. Add improved modules above alphabetically, not here at the end.
 
 Optimizations

Lib/test/test_zlib.py

@@ -119,6 +119,104 @@ def test_same_as_binascii_crc32(self):
         self.assertEqual(binascii.crc32(b'spam'), zlib.crc32(b'spam'))
 
 
+class ChecksumCombineMixin:
+    """Mixin class for testing checksum combination."""
+
+    N = 1000
+    default_iv: int
+
+    def parse_iv(self, iv):
+        """Parse an IV value.
+
+        - The default IV is returned if *iv* is None.
+        - A random IV is returned if *iv* is -1.
+        - Otherwise, *iv* is returned as is.
+        """
+        if iv is None:
+            return self.default_iv
+        if iv == -1:
+            return random.randint(1, 0x80000000)
+        return iv
+
+    def checksum(self, data, init=None):
+        """Compute the checksum of data with a given initial value.
+
+        The *init* value is parsed by ``parse_iv``.
+        """
+        iv = self.parse_iv(init)
+        return self._checksum(data, iv)
+
+    def _checksum(self, data, init):
+        raise NotImplementedError
+
+    def combine(self, a, b, blen):
+        """Combine two checksums together."""
+        raise NotImplementedError
+
+    def get_random_data(self, data_len, *, iv=None):
+        """Get a triplet (data, iv, checksum)."""
+        data = random.randbytes(data_len)
+        init = self.parse_iv(iv)
+        checksum = self.checksum(data, init)
+        return data, init, checksum
+
+    def test_combine_empty(self):
+        for _ in range(self.N):
+            a, iv, checksum = self.get_random_data(32, iv=-1)
+            res = self.combine(iv, self.checksum(a), len(a))
+            self.assertEqual(res, checksum)
+
+    def test_combine_no_iv(self):
+        for _ in range(self.N):
+            a, _, chk_a = self.get_random_data(32)
+            b, _, chk_b = self.get_random_data(64)
+            res = self.combine(chk_a, chk_b, len(b))
+            self.assertEqual(res, self.checksum(a + b))
+
+    def test_combine_with_iv(self):
+        for _ in range(self.N):
+            a, iv_a, chk_a_with_iv = self.get_random_data(32, iv=-1)
+            chk_a_no_iv = self.checksum(a)
+            b, iv_b, chk_b_with_iv = self.get_random_data(64, iv=-1)
+            chk_b_no_iv = self.checksum(b)
+
+            # We can represent c = COMBINE(CHK(a, iv_a), CHK(b, iv_b)) as:
+            #
+            #   c = CHK(CHK(b'', iv_a) + CHK(a) + CHK(b'', iv_b) + CHK(b))
+            #     = COMBINE(
+            #           COMBINE(CHK(b'', iv_a), CHK(a)),
+            #           COMBINE(CHK(b'', iv_b), CHK(b)),
+            #       )
+            #     = COMBINE(COMBINE(iv_a, CHK(a)), COMBINE(iv_b, CHK(b)))
+            tmp0 = self.combine(iv_a, chk_a_no_iv, len(a))
+            tmp1 = self.combine(iv_b, chk_b_no_iv, len(b))
+            expected = self.combine(tmp0, tmp1, len(b))
+            checksum = self.combine(chk_a_with_iv, chk_b_with_iv, len(b))
+            self.assertEqual(checksum, expected)
+
+
+class CRC32CombineTestCase(ChecksumCombineMixin, unittest.TestCase):
+
+    default_iv = 0
+
+    def _checksum(self, data, init):
+        return zlib.crc32(data, init)
+
+    def combine(self, a, b, blen):
+        return zlib.crc32_combine(a, b, blen)
+
+
+class Adler32CombineTestCase(ChecksumCombineMixin, unittest.TestCase):
+
+    default_iv = 1
+
+    def _checksum(self, data, init):
+        return zlib.adler32(data, init)
+
+    def combine(self, a, b, blen):
+        return zlib.adler32_combine(a, b, blen)
+
+
 # Issue #10276 - check that inputs >=4 GiB are handled correctly.
 class ChecksumBigBufferTestCase(unittest.TestCase):
 

Misc/NEWS.d/next/Library/2025-05-25-13-46-37.gh-issue-134635.ZlPrlX.rst

@@ -0,0 +1,3 @@
+:mod:`zlib`: Allow to combine Adler-32 and CRC-32 checksums via
+:func:`~zlib.adler32_combine` and :func:`~zlib.crc32_combine`. Patch by
+Callum Attryde and Bénédikt Tran.