120 lines
4.8 KiB
Python
120 lines
4.8 KiB
Python
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import os
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from typing import List
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from .models import Packet, Page, TeletextService
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def load_t42(file_path: str) -> TeletextService:
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service = TeletextService()
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with open(file_path, 'rb') as f:
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while True:
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chunk = f.read(42)
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if not chunk:
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break
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if len(chunk) < 42:
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# Should not happen in a valid T42 stream, or we just ignore incomplete tail
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break
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packet = Packet(chunk)
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service.all_packets.append(packet)
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# Logic to group into pages.
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# This is non-trivial because packets for a page might be interleaved or sequential.
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# Standard implementation: Packets arrive in order. Row 0 starts a new page/subpage.
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if packet.row == 0:
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# Start of a new page header.
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# Byte 2-9 of header contain Page Number, Subcode, Control bits etc.
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# We need to parse the header to identify the page.
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# Header format (after Mag/Row):
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# Bytes: P1 P2 S1 S2 S3 S4 C1 C2 ...
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# All Hamming 8/4 encoded.
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# For now, let's just create a new page entry for every Header we see,
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# or find the existing one if we want to support updates (but T42 usually is a stream capture).
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# If it's an editor file, it's likely sequential.
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p_num, sub_code = parse_header(packet.data)
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# Create new page
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new_page = Page(magazine=packet.magazine, page_number=p_num, sub_code=sub_code)
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new_page.packets.append(packet)
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service.pages.append(new_page)
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else:
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# Add to the "current" page of this magazine.
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# We need to track the current active page for each magazine.
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# A simplistic approach: add to the last page added that matches the magazine ??
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# Robust approach: Maintain a dict of current_pages_by_magazine.
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# Let's find the last page in service that matches the packet's magazine
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# This is O(N) but N (pages) is small.
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target_page = None
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for p in reversed(service.pages):
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if p.magazine == packet.magazine:
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target_page = p
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break
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if target_page:
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target_page.packets.append(packet)
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else:
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# Packet without a header? Orphaned. Just keep in all_packets
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pass
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return service
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def save_t42(file_path: str, service: TeletextService):
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with open(file_path, 'wb') as f:
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# User requirement: "without rearranging the order of the packets"
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# Implies we should iterate the original list.
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# However, if we edit data, we modify the Packet objects in place.
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# If we Add/Delete packets, we need to handle that.
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for packet in service.all_packets:
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# Reconstruct the 42 bytes from the packet fields
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# The packet.data (bytearray) should be mutable and edited by the UI.
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# packet.original_data (first 2 bytes) + packet.data
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# Note: If we changed Magazine or Row, we'd need to re-encode the first 2 bytes.
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# For now, assume we primarily edit content (bytes 2-41).
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header = packet.original_data[:2] # Keep original address for now
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# TODO: regenerating header if Mag/Row changed
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f.write(header + packet.data)
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def decode_hamming_8_4(byte_val):
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return ((byte_val >> 1) & 1) | \
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(((byte_val >> 3) & 1) << 1) | \
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(((byte_val >> 5) & 1) << 2) | \
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(((byte_val >> 7) & 1) << 3)
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def parse_header(data: bytearray):
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# Data is 40 bytes.
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# Bytes 0-7 are Page Num (2), Subcode (4), Control (2) - ALL Hamming encoded.
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# 0: Page Units (PU)
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# 1: Page Tens (PT)
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pu = decode_hamming_8_4(data[0])
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pt = decode_hamming_8_4(data[1])
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page_num = (pt & 0xF) * 10 + (pu & 0xF)
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# Subcode: S1, S2, S3, S4
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# S1 (low), S2, S3, S4 (high)
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s1 = decode_hamming_8_4(data[2])
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s2 = decode_hamming_8_4(data[3])
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s3 = decode_hamming_8_4(data[4])
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s4 = decode_hamming_8_4(data[5])
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# Subcode logic is a bit complex with specific bit mapping for "Time" vs "Subcode"
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# But usually just combining them gives the raw subcode value.
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# S1: bits 0-3
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# S2: bits 4-6 (bit 4 is C4) -> actually S2 has 3 bits of subcode + 1 control bit usually?
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# Let's simplify and just concat them for a unique identifier.
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sub_code = s1 | (s2 << 4) | (s3 << 8) | (s4 << 12)
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return page_num, sub_code
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