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

src/teletext/io.py

@@ -100,36 +100,63 @@ 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
             
-            mag = packet.magazine
-            if mag == 8: mag = 0 # 0 encoded as 8
+            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
             
-            # Bits:
-            # B1 data: M1(0) M2(1) M3(2) R1(3)
-            m1 = (mag >> 0) & 1
-            m2 = (mag >> 1) & 1
-            m3 = (mag >> 2) & 1
-            r1 = (packet.row >> 0) & 1
-            
-            b1_val = m1 | (m2 << 1) | (m3 << 2) | (r1 << 3)
-            b1_enc = encode_hamming_8_4(b1_val)
-            
-            # B2 data: R2(0) R3(1) R4(2) R5(3)
-            r2 = (packet.row >> 1) & 1
-            r3 = (packet.row >> 2) & 1
-            r4 = (packet.row >> 3) & 1
-            r5 = (packet.row >> 4) & 1
-            
-            b2_val = r2 | (r3 << 1) | (r4 << 2) | (r5 << 3)
-            b2_enc = encode_hamming_8_4(b2_val)
-            
-            header = bytes([b1_enc, b2_enc])
+            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
+                
+                # Bits:
+                # B1 data: M1(0) M2(1) M3(2) R1(3)
+                m1 = (mag >> 0) & 1
+                m2 = (mag >> 1) & 1
+                m3 = (mag >> 2) & 1
+                r1 = (packet.row >> 0) & 1
+                
+                b1_val = m1 | (m2 << 1) | (m3 << 2) | (r1 << 3)
+                b1_enc = encode_hamming_8_4(b1_val)
+                
+                # B2 data: R2(0) R3(1) R4(2) R5(3)
+                r2 = (packet.row >> 1) & 1
+                r3 = (packet.row >> 2) & 1
+                r4 = (packet.row >> 3) & 1
+                r5 = (packet.row >> 4) & 1
+                
+                b2_val = r2 | (r3 << 1) | (r4 << 2) | (r5 << 3)
+                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):