Fix: Improve file integrity on save by preserving original packet headers

src/teletext/io.py

@@ -100,13 +100,39 @@ def encode_hamming_8_4(value):
     
     return res
 
+
+def decode_packet_header(b1, b2):
+    """
+    Decodes the Magazine and Row from the first 2 bytes of a T42 packet.
+    """
+    d1 = decode_hamming_8_4(b1)
+    d2 = decode_hamming_8_4(b2)
+    
+    mag = (d1 & 0b0111)
+    if mag == 0: mag = 8
+    
+    row = (d2 << 1) | ((d1 >> 3) & 1)
+    return mag, row
+
 def save_t42(file_path: str, service: TeletextService):
     with open(file_path, 'wb') as f:
         for packet in service.all_packets:
-            # Reconstruct header bytes from packet.magazine and packet.row
-            # Byte 1: M1 M2 M3 R1
-            # Byte 2: R2 R3 R4 R5
+            # Check if we can reuse the original header (preserving parity/integrity)
+            use_original_header = False
             
+            if hasattr(packet, 'original_data') and len(packet.original_data) >= 2:
+                # Try to decode the original header
+                try:
+                    orig_mag, orig_row = decode_packet_header(packet.original_data[0], packet.original_data[1])
+                    if orig_mag == packet.magazine and orig_row == packet.row:
+                        use_original_header = True
+                except:
+                    pass
+            
+            if use_original_header:
+                header = packet.original_data[:2]
+            else:
+                # Reconstruct header bytes
                 mag = packet.magazine
                 if mag == 8: mag = 0 # 0 encoded as 8
                 
@@ -130,6 +156,7 @@ def save_t42(file_path: str, service: TeletextService):
                 b2_enc = encode_hamming_8_4(b2_val)
                 
                 header = bytes([b1_enc, b2_enc])
+                
             f.write(header + packet.data)
 
 def decode_hamming_8_4(byte_val):