Add CRC-32 checksum implementation

hashes/crc32.py

@@ -0,0 +1,71 @@
+"""
+CRC-32 is a cyclic redundancy check algorithm that produces a 32-bit checksum.
+It is widely used for error detection in network transmissions and file integrity
+checks (ZIP, PNG, gzip, etc.). This implementation uses the reflected polynomial
+0xEDB88320 with a precomputed 256-entry lookup table.
+
+Reference: https://en.wikipedia.org/wiki/Cyclic_redundancy_check
+"""
+
+
+def _generate_crc32_table() -> list[int]:
+    """
+    Build a 256-entry lookup table for CRC-32 using the reflected
+    polynomial 0xEDB88320.
+
+    >>> table = _generate_crc32_table()
+    >>> len(table)
+    256
+    >>> hex(table[0])
+    '0x0'
+    >>> hex(table[1])
+    '0x77073096'
+    """
+    table: list[int] = []
+    for byte in range(256):
+        crc = byte
+        for _ in range(8):
+            if crc & 1:
+                crc = (crc >> 1) ^ 0xEDB88320
+            else:
+                crc >>= 1
+        table.append(crc)
+    return table
+
+
+_CRC32_TABLE = _generate_crc32_table()
+
+
+def crc32(data: bytes) -> int:
+    """
+    Compute the CRC-32 checksum for the given bytes.
+
+    The output matches Python's ``zlib.crc32()`` for the same input.
+
+    >>> crc32(b'')
+    0
+    >>> hex(crc32(b'hello'))
+    '0x3610a686'
+    >>> crc32(b'hello world')
+    222957957
+    >>> hex(crc32(b'The quick brown fox jumps over the lazy dog'))
+    '0x414fa339'
+
+    Verify against zlib:
+    >>> import zlib
+    >>> crc32(b'hello world') == zlib.crc32(b'hello world')
+    True
+    >>> crc32(b'\\x00\\xff\\x55\\xaa') == zlib.crc32(b'\\x00\\xff\\x55\\xaa')
+    True
+    """
+    crc = 0xFFFFFFFF
+    for byte in data:
+        lookup_index = (crc ^ byte) & 0xFF
+        crc = (crc >> 8) ^ _CRC32_TABLE[lookup_index]
+    return crc ^ 0xFFFFFFFF
+
+
+if __name__ == "__main__":
+    import doctest
+
+    doctest.testmod()