feat: Add CRC checksum calculation and display

src/teletext/models.py

@@ -1,6 +1,13 @@
 from dataclasses import dataclass, field
 from typing import List, Optional
 
+def decode_hamming_8_4(byte_val):
+    # Extract data bits: bits 1, 3, 5, 7
+    return ((byte_val >> 1) & 1) | \
+           (((byte_val >> 3) & 1) << 1) | \
+           (((byte_val >> 5) & 1) << 2) | \
+           (((byte_val >> 7) & 1) << 3)
+
 @dataclass
 class Packet:
     """
@@ -27,22 +34,6 @@ class Packet:
         b2 = self.original_data[1]
         
         # De-interleave Hamming bits to get M (3 bits) and R (5 bits)
-        # This is the "basic" interpretation. 
-        # For a robust editor we assume the input T42 is valid or we just store bytes.
-        # But we need Mag/Row to organize pages.
-        
-        # Decode Hamming 8/4 logic is complex to implementation from scratch correctly 
-        # without a reference, but usually D1, D2, D3, D4 are at bit positions 1, 3, 5, 7 
-        # (0-indexed, where 0 is LSB). 
-        # Let's perform a simple extraction assuming no bit errors for now.
-        
-        def decode_hamming_8_4(byte_val):
-            # Extract data bits: bits 1, 3, 5, 7
-            return ((byte_val >> 1) & 1) | \
-                   (((byte_val >> 3) & 1) << 1) | \
-                   (((byte_val >> 5) & 1) << 2) | \
-                   (((byte_val >> 7) & 1) << 3)
-
         d1 = decode_hamming_8_4(b1)
         d2 = decode_hamming_8_4(b2)
         
@@ -84,6 +75,78 @@ class Page:
         # Format as Hex to support A-F pages
         return f"{self.magazine}{self.page_number:02X}"
 
+    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).
+        """
+        crc = 0xFFFF
+        poly = 0x1021
+        
+        # Helper to update CRC with a byte
+        def update_crc(c, val):
+            v = (val << 8) & 0xFFFF
+            for _ in range(8):
+                if (c ^ v) & 0x8000:
+                    c = (c << 1) ^ poly
+                else:
+                    c = c << 1
+                v <<= 1
+                c &= 0xFFFF
+            return c
+
+        # Organize packets by row
+        rows = {}
+        for p in self.packets:
+            rows[p.row] = p
+            
+        for r in range(24): # 0 to 23
+            if r in rows:
+                data = rows[r].data
+                start_col = 8 if r == 0 else 0
+                
+                for i in range(start_col, 40):
+                    byte_val = data[i] & 0x7F # Strip parity
+                    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):
+                    crc = update_crc(crc, 0x20)
+                    
+        return crc
+
+    def get_stored_crc(self) -> Optional[int]:
+        """
+        Attempts to retrieve the stored CRC from Packet 27/0 if present.
+        Returns None if not found.
+        """
+        # Look for Packet 27
+        for p in self.packets:
+            if p.row == 27:
+                # Check Designation Code (Byte 0)
+                try:
+                    b0 = p.data[0]
+                    # Decode Hamming 8/4
+                    designation = decode_hamming_8_4(b0)
+                    
+                    if designation == 0:
+                        # Packet 27/0
+                        # Checksum is in bytes 38 and 39
+                        if len(p.data) >= 40:
+                            hi = p.data[38]
+                            lo = p.data[39]
+                            
+                            # Strip parity
+                            crc = ((hi & 0x7F) << 8) | (lo & 0x7F)
+                            return crc
+                except:
+                    pass
+        return None
+
 @dataclass
 class TeletextService:
     """