fix: Align CRC calculation with ETSI EN 300 706 standard

src/teletext/models.py

@@ -78,11 +78,12 @@ class Page:
     def calculate_crc(self) -> int:
         """
         Calculates the CRC-16 (CCITT) checksum for the page.
-        Covers Rows 0 to 23.
-        Row 0: Skips first 8 bytes (Header/Control). Uses bytes 8-39.
-        Rows 1-23: Uses all 40 bytes.
-        Data is 7-bit (stripped parity).
-        Initial value: 0 (ETSI EN 300 706 9.4.1.2).
+        According to ETSI EN 300 706 (Section 9.4.1.2 & Figure 13):
+        - Covers Row 0 columns 8-31 (Bytes 14-37 in packet, excluding header and clock).
+        - Covers Rows 1-25 columns 0-39 (Bytes 6-45 in packet).
+        - Total 1024 bytes (8192 bits).
+        - Data is 7-bit (stripped parity).
+        - Initial value: 0.
         """
         crc = 0
         poly = 0x1021
@@ -104,18 +105,27 @@ class Page:
         for p in self.packets:
             rows[p.row] = p
             
-        for r in range(24): # 0 to 23
+        for r in range(26): # 0 to 25
+            # Determine column range
+            if r == 0:
+                start_col, end_col = 8, 32 # Cols 8-31 (24 bytes)
+            else:
+                start_col, end_col = 0, 40 # Cols 0-39 (40 bytes)
+                
             if r in rows:
                 data = rows[r].data
-                start_col = 8 if r == 0 else 0
+                # Ensure data is long enough (should be 40)
+                d_len = len(data)
                 
-                for i in range(start_col, 40):
-                    byte_val = data[i] & 0x7F # Strip parity
+                for i in range(start_col, end_col):
+                    if i < d_len:
+                        byte_val = data[i] & 0x7F # Strip parity
+                    else:
+                        byte_val = 0x20 # Pad with space if short
                     crc = update_crc(crc, byte_val)
             else:
-                # Missing row? Usually treated as spaces (0x20)
-                start_col = 8 if r == 0 else 0
-                for i in range(start_col, 40):
+                # Missing row treated as spaces
+                for i in range(start_col, end_col):
                     crc = update_crc(crc, 0x20)
                     
         return crc