Teletext-Editor/src/teletext/io.py

import os
from typing import List
from .models import Packet, Page, TeletextService

def load_t42(file_path: str) -> TeletextService:
    service = TeletextService()
    
    with open(file_path, 'rb') as f:
        while True:
            chunk = f.read(42)
            if not chunk:
                break
            if len(chunk) < 42:
                # Should not happen in a valid T42 stream, or we just ignore incomplete tail
                break
                
            packet = Packet(chunk)
            service.all_packets.append(packet)
            
            # Logic to group into pages.
            # This is non-trivial because packets for a page might be interleaved or sequential.
            # Standard implementation: Packets arrive in order. Row 0 starts a new page/subpage.
            
            if packet.row == 0:
                # Start of a new page header.
                # Byte 2-9 of header contain Page Number, Subcode, Control bits etc.
                # We need to parse the header to identify the page.
                
                # Header format (after Mag/Row):
                # Bytes: P1 P2 S1 S2 S3 S4 C1 C2 ...
                # All Hamming 8/4 encoded.
                
                # For now, let's just create a new page entry for every Header we see,
                # or find the existing one if we want to support updates (but T42 usually is a stream capture).
                # If it's an editor file, it's likely sequential.
                
                p_num, sub_code = parse_header(packet.data)
                
                # Create new page
                new_page = Page(magazine=packet.magazine, page_number=p_num, sub_code=sub_code)
                new_page.packets.append(packet)
                service.pages.append(new_page)
            else:
                # Add to the "current" page of this magazine.
                # We need to track the current active page for each magazine.
                # A simplistic approach: add to the last page added that matches the magazine ??
                # Robust approach: Maintain a dict of current_pages_by_magazine.
                
                # Let's find the last page in service that matches the packet's magazine
                # This is O(N) but N (pages) is small.
                target_page = None
                for p in reversed(service.pages):
                    if p.magazine == packet.magazine:
                        target_page = p
                        break
                
                if target_page:
                    target_page.packets.append(packet)
                else:
                    # Packet without a header? Orphaned. Just keep in all_packets
                    pass

    return service


def encode_hamming_8_4(value):
    # Value is 4 bits (0-15)
    d1 = (value >> 0) & 1
    d2 = (value >> 1) & 1
    d3 = (value >> 2) & 1
    d4 = (value >> 3) & 1
    
    # Parity bits (Odd parity default? Or standard Hamming?)
    # Teletext spec: 
    # P1 = 1 + D1 + D2 + D4 (mod 2) -> Inverse of even parity check?
    # Actually, simpler to look up or calculate. 
    # Let's match typical implementation:
    # P1 (b0) covers 1,3,7 (D1, D2, D4)
    # P2 (b2) covers 1,5,7 (D1, D3, D4)
    # P3 (b4) covers 3,5,7 (D2, D3, D4)
    # P4 (b6) covers all.
    # Teletext uses ODD parity for the hamming bits usually?
    # "Hamming 8/4 with odd parity"
    
    p1 = 1 ^ d1 ^ d2 ^ d4
    p2 = 1 ^ d1 ^ d3 ^ d4
    p3 = 1 ^ d2 ^ d3 ^ d4
    
    res = (p1 << 0) | (d1 << 1) | \
          (p2 << 2) | (d2 << 3) | \
          (p3 << 4) | (d3 << 5) | \
          (d4 << 7)
          
    # P4 (bit 6) makes total bits odd
    # Count set bits so far
    set_bits = bin(res).count('1')
    p4 = 1 if (set_bits % 2 == 0) else 0
    
    res |= (p4 << 6)
    
    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:
            # 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
                
                # 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):
    return ((byte_val >> 1) & 1) | \
           (((byte_val >> 3) & 1) << 1) | \
           (((byte_val >> 5) & 1) << 2) | \
           (((byte_val >> 7) & 1) << 3)

def parse_header(data: bytearray):
    # Data is 40 bytes.
    # Bytes 0-7 are Page Num (2), Subcode (4), Control (2) - ALL Hamming encoded.
    
    # 0: Page Units (PU)
    # 1: Page Tens (PT)
    
    pu = decode_hamming_8_4(data[0])
    pt = decode_hamming_8_4(data[1])
    
    page_num = (pt & 0xF) * 10 + (pu & 0xF)
    
    # Subcode: S1, S2, S3, S4
    # S1 (low), S2, S3, S4 (high)
    
    s1 = decode_hamming_8_4(data[2])
    s2 = decode_hamming_8_4(data[3])
    s3 = decode_hamming_8_4(data[4])
    s4 = decode_hamming_8_4(data[5])
    
    # Subcode logic is a bit complex with specific bit mapping for "Time" vs "Subcode"
    # But usually just combining them gives the raw subcode value.
    # S1: bits 0-3
    # S2: bits 4-6 (bit 4 is C4) -> actually S2 has 3 bits of subcode + 1 control bit usually?
    # Let's simplify and just concat them for a unique identifier.
    
    sub_code = s1 | (s2 << 4) | (s3 << 8) | (s4 << 12)
    
    return page_num, sub_code