feat: implement Packet 26 enhancement support with Hamming 24/18 decoding

src/teletext/io.py

@@ -1,6 +1,6 @@
 import os
 from typing import List, Callable, Optional
-from .models import Packet, Page, TeletextService, decode_hamming_8_4
+from .models import Packet, Page, TeletextService, decode_hamming_8_4, decode_hamming_24_18, Packet26Enhancement
 
 def load_t42(file_path: str, progress_callback: Optional[Callable[[int, int], None]] = None) -> TeletextService:
     service = TeletextService()
@@ -70,6 +70,35 @@ def load_t42(file_path: str, progress_callback: Optional[Callable[[int, int], No
                 # Update tracking
                 current_pages_by_mag[mag] = new_page
                 last_row_by_mag[mag] = 0
+            elif row == 26:
+                # Enhancement packet
+                target_page = current_pages_by_mag.get(mag)
+                if target_page:
+                    target_page.packets.append(packet)
+                    # Track active row within this packet/page for enhancements
+                    # Default active row is usually the last one? 
+                    # Spec: "The active row is set to 0 at the beginning of each page."
+                    # We should probably keep state in the Page object during loading.
+                    if not hasattr(target_page, '_active_row'):
+                        target_page._active_row = 0
+                    
+                    for i in range(13):
+                        b1 = packet.data[1 + i*3]
+                        b2 = packet.data[2 + i*3]
+                        b3 = packet.data[3 + i*3]
+                        triplet = decode_hamming_24_18(b1, b2, b3)
+                        if triplet is not None:
+                            address = triplet & 0x3F
+                            mode = (triplet >> 6) & 0x1F
+                            data = (triplet >> 11) & 0x7F
+                            
+                            if 0x01 <= mode <= 0x07:
+                                # Set Active Row (bits 0-4 of data = row number)
+                                target_page._active_row = data & 0x1F
+                            elif mode == 0x00:
+                                # Overwrite character at (active_row, address)
+                                enh = Packet26Enhancement(row=target_page._active_row, col=address, mode=mode, data=data)
+                                target_page.packet26_enhancements.append(enh)
             else:
                 # Add to the "current" page of this magazine.
                 target_page = current_pages_by_mag.get(mag)

src/teletext/models.py

@@ -39,6 +39,52 @@ def decode_hamming_8_4(byte_val):
     # Return 4 data bits: D1, D2, D3, D4
     return b[1] | (b[3] << 1) | (b[5] << 2) | (b[7] << 3)
 
+def decode_hamming_24_18(b1, b2, b3):
+    """
+    Decodes a 24/18 Hamming triplet.
+    Returns 18 bits of data or None if uncorrectable error.
+    """
+    v = b1 | (b2 << 8) | (b3 << 16)
+    
+    syndrome = 0
+    for i in range(5):
+        check = 0
+        for j in range(23):
+            if ((j + 1) >> i) & 1:
+                check ^= (v >> j) & 1
+        if check:
+            syndrome |= (1 << i)
+            
+    overall_parity = 0
+    for j in range(24):
+        overall_parity ^= (v >> j) & 1
+        
+    if overall_parity == 0:
+        if syndrome != 0:
+            return None # Double error
+    else:
+        if syndrome != 0:
+            v ^= (1 << (syndrome - 1))
+            
+    # Extract 18 data bits
+    d_indices = [2, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14, 16, 17, 18, 19, 20, 21, 22]
+    res = 0
+    for i, idx in enumerate(d_indices):
+        if (v >> idx) & 1:
+            res |= (1 << i)
+    return res
+
+@dataclass
+class Packet26Enhancement:
+    """
+    Represents an enhancement from Packet 26.
+    Usually a character replacement or attribute at a specific (row, col).
+    """
+    row: int
+    col: int
+    mode: int
+    data: int
+
 @dataclass
 class Packet:
     """
@@ -100,6 +146,7 @@ class Page:
     sub_code: int = 0 # Subpage code (0000 to 3F7F hex usually, simplest is 0-99 equivalent)
     language: int = 0 # National Option (0-7)
     packets: List[Packet] = field(default_factory=list)
+    packet26_enhancements: List[Packet26Enhancement] = field(default_factory=list)
     
     @property
     def full_page_number(self):

src/teletext/renderer.py

@@ -336,6 +336,25 @@ class TeletextCanvas(QWidget):
             
             byte_val &= 0x7F # Strip parity
             
+            # Packet 26 Overwrite?
+            # Check if there is an enhancement for this (row, col)
+            p26_data = None
+            if self.page:
+                for enh in self.page.packet26_enhancements:
+                    if enh.row == row and enh.col == c:
+                        if enh.mode == 0x00:
+                            # Overwrite character
+                            p26_data = enh.data
+                            break
+            
+            # If we have P26 overwrite, we use it for display, but it doesn't change 
+            # the serial attribute processing of the 'original' byte_val if it was a control code?
+            # Actually, P26 overwrites a displayable character. 
+            # If the original byte_val was a control code, does P26 replace it? 
+            # Usually yes, it's a "display at" operation.
+            
+            display_byte = p26_data if p26_data is not None else byte_val
+            
             # Control Codes
             is_control = False
             
@@ -460,6 +479,20 @@ class TeletextCanvas(QWidget):
 
                         if drcs_char:
                             self.draw_drcs(painter, x, y, drcs_char, fg, double_height)
+                        elif p26_data is not None:
+                            # Level 2.5 character from G2 supplementary set
+                            # (Technically P26 data can point to other sets, but G2 is common)
+                            from .charsets import get_char_g2
+                            char = get_char_g2(p26_data)
+                            painter.setPen(fg)
+                            if double_height:
+                                painter.save()
+                                painter.translate(x, y)
+                                painter.scale(1, 2)
+                                painter.drawText(QRect(0, 0, self.cell_w, self.cell_h), Qt.AlignmentFlag.AlignCenter, char)
+                                painter.restore()
+                            else:
+                                painter.drawText(QRect(x, y, self.cell_w, self.cell_h), Qt.AlignmentFlag.AlignCenter, char)
                         elif graphics_mode:
                             # Mosaic Graphics
                             h_mos = self.cell_h * 2 if double_height else self.cell_h