230 lines
7.3 KiB
Python
230 lines
7.3 KiB
Python
# -*- coding: utf-8 -*-
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"""
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Physical-layer AIS frame: bit padding, CRC-16 (ITU, as in gr-aistx Build_Frame),
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HDLC-style bit stuffing, flags, NRZI — ported from gr-aistx/lib/Build_Frame_impl.cc
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"""
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from __future__ import annotations
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LEN_PREAMBLE = 24
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LEN_START = 8
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LEN_CRC = 16
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LEN_FRAME_MAX = 256
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# CRC-16-CCITT table (same as Build_frame_impl.cc)
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_CRC_ITU16_TABLE = (
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0x0000, 0x1189, 0x2312, 0x329B, 0x4624, 0x57AD, 0x6536, 0x74BF,
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0x8C48, 0x9DC1, 0xAF5A, 0xBED3, 0xCA6C, 0xDBE5, 0xE97E, 0xF8F7,
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0x1081, 0x0108, 0x3393, 0x221A, 0x56A5, 0x472C, 0x75B7, 0x643E,
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0x9CC9, 0x8D40, 0xBFDB, 0xAE52, 0xDAED, 0xCB64, 0xF9FF, 0xE876,
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0x2102, 0x308B, 0x0210, 0x1399, 0x6726, 0x76AF, 0x4434, 0x55BD,
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0xAD4A, 0xBCC3, 0x8E58, 0x9FD1, 0xEB6E, 0xFAE7, 0xC87C, 0xD9F5,
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0x3183, 0x200A, 0x1291, 0x0318, 0x77A7, 0x662E, 0x54B5, 0x453C,
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0xBDCB, 0xAC42, 0x9ED9, 0x8F50, 0xFBEF, 0xEA66, 0xD8FD, 0xC974,
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0x4204, 0x538D, 0x6116, 0x709F, 0x0420, 0x15A9, 0x2732, 0x36BB,
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0xCE4C, 0xDFC5, 0xED5E, 0xFCD7, 0x8868, 0x99E1, 0xAB7A, 0xBAF3,
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0x5285, 0x430C, 0x7197, 0x601E, 0x14A1, 0x0528, 0x37B3, 0x263A,
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0xDECD, 0xCF44, 0xFDDF, 0xEC56, 0x98E9, 0x8960, 0xBBFB, 0xAA72,
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0x6306, 0x728F, 0x4014, 0x519D, 0x2522, 0x34AB, 0x0630, 0x17B9,
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0xEF4E, 0xFEC7, 0xCC5C, 0xDDD5, 0xA96A, 0xB8E3, 0x8A78, 0x9BF1,
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0x7387, 0x620E, 0x5095, 0x411C, 0x35A3, 0x242A, 0x16B1, 0x0738,
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0xFFCF, 0xEE46, 0xDCDD, 0xCD54, 0xB9EB, 0xA862, 0x9AF9, 0x8B70,
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0x8408, 0x9581, 0xA71A, 0xB693, 0xC22C, 0xD3A5, 0xE13E, 0xF0B7,
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0x0840, 0x19C9, 0x2B52, 0x3ADB, 0x4E64, 0x5FED, 0x6D76, 0x7CFF,
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0x9489, 0x8500, 0xB79B, 0xA612, 0xD2AD, 0xC324, 0xF1BF, 0xE036,
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0x18C1, 0x0948, 0x3BD3, 0x2A5A, 0x5EE5, 0x4F6C, 0x7DF7, 0x6C7E,
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0xA50A, 0xB483, 0x8618, 0x9791, 0xE32E, 0xF2A7, 0xC03C, 0xD1B5,
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0x2942, 0x38CB, 0x0A50, 0x1BD9, 0x6F66, 0x7EEF, 0x4C74, 0x5DFD,
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0xB58B, 0xA402, 0x9699, 0x8710, 0xF3AF, 0xE226, 0xD0BD, 0xC134,
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0x39C3, 0x284A, 0x1AD1, 0x0B58, 0x7FE7, 0x6E6E, 0x5CF5, 0x4D7C,
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0xC60C, 0xD785, 0xE51E, 0xF497, 0x8028, 0x91A1, 0xA33A, 0xB2B3,
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0x4A44, 0x5BCD, 0x6956, 0x78DF, 0x0C60, 0x1DE9, 0x2F72, 0x3EFB,
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0xD68D, 0xC704, 0xF59F, 0xE416, 0x90A9, 0x8120, 0xB3BB, 0xA232,
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0x5AC5, 0x4B4C, 0x79D7, 0x685E, 0x1CE1, 0x0D68, 0x3FF3, 0x2E7A,
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0xE70E, 0xF687, 0xC41C, 0xD595, 0xA12A, 0xB0A3, 0x8238, 0x93B1,
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0x6B46, 0x7ACF, 0x4854, 0x59DD, 0x2D62, 0x3CEB, 0x0E70, 0x1FF9,
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0xF78F, 0xE606, 0xD49D, 0xC514, 0xB1AB, 0xA022, 0x92B9, 0x8330,
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0x7BC7, 0x6A4E, 0x58D5, 0x495C, 0x3DE3, 0x2C6A, 0x1EF1, 0x0F78,
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)
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def bits_from_payload_string(s: str) -> list[int]:
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"""ASCII '0'/'1' string -> list of 0/1 (same as Build_Frame ctor)."""
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s = s.strip().replace(" ", "").replace("\n", "")
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out: list[int] = []
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for c in s:
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if c == "0":
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out.append(0)
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elif c == "1":
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out.append(1)
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else:
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raise ValueError("Payload must be only 0 and 1 characters")
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return out
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def _pad_to_multiple_of_8(bits: list[int]) -> tuple[list[int], int]:
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r = len(bits) % 8
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if r == 0:
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return bits, 0
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pad = 8 - r
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return bits + [0] * pad, pad
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def reverse_bit_order(bits: list[int]) -> None:
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"""In-place: reverse bit order within each byte (8-bit group)."""
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n = len(bits)
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assert n % 8 == 0
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for i in range(n // 8):
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base = i * 8
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for j in range(4):
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a = base + j
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b = base + 7 - j
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bits[a], bits[b] = bits[b], bits[a]
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def _compute_crc_bits(buffer_bits: list[int]) -> list[int]:
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"""
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Match Build_Frame_impl::compute_crc + int2bin/reverse/swap (16-bit FCS bits).
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buffer_bits length must be multiple of 8.
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"""
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datalen = len(buffer_bits) // 8
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data = []
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for j in range(datalen):
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v = 0
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for k in range(8):
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v = (v << 1) | (buffer_bits[j * 8 + k] & 1)
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data.append(v)
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crc = 0xFFFF
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for b in data:
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crc = ((crc >> 8) ^ _CRC_ITU16_TABLE[(crc ^ b) & 0xFF]) & 0xFFFF
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crc = (crc & 0xFFFF) ^ 0xFFFF
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ret = ["0"] * 16
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buf_idx = 15
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a = crc & 0xFFFF
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for _ in range(16):
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ret[buf_idx] = "1" if (a & 1) else "0"
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a >>= 1
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buf_idx -= 1
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rb = [1 if c == "1" else 0 for c in ret]
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reverse_bit_order(rb)
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ret = ["1" if x else "0" for x in rb]
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buf_idx = 15
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a = crc & 0xFFFF
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for _ in range(16):
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ret[buf_idx] = "1" if (a & 1) else "0"
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a >>= 1
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buf_idx -= 1
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temp = ret[8:16]
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ret[8:16] = ret[0:8]
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ret[0:8] = temp
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return [1 if c == "1" else 0 for c in ret]
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def bit_stuff(bits: list[int]) -> list[int]:
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"""HDLC-style: after five consecutive 1s insert a 0."""
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out: list[int] = []
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consecutive = 0
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for b in bits:
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out.append(b)
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if b & 1:
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consecutive += 1
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if consecutive == 5:
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out.append(0)
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consecutive = 0
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else:
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consecutive = 0
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return out
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def nrz_to_nrzi(data: list[int]) -> None:
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"""In-place NRZI (same rule as nrz_to_nrzi_impl.cc / Build_Frame_impl)."""
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prev = 0
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for i in range(len(data)):
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nrz = data[i] & 1
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if nrz == 0:
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nrzi = prev ^ 1
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else:
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nrzi = prev
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data[i] = nrzi
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prev = nrzi
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def _preamble_bits() -> list[int]:
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return [1, 0] * (LEN_PREAMBLE // 2)
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def _start_flag_bits() -> list[int]:
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return [0, 1, 1, 1, 1, 1, 1, 0]
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def build_nrzi_frame(payload_ascii: str, enable_nrzi: bool = True) -> list[int]:
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"""
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:param payload_ascii: AIS PDU as '0'/'1' string (output style of AIVDM_Encoder.py)
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:param enable_nrzi: if False, return NRZ frame bits (before NRZI), for debugging
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:return: list of 0/1 (length 256 for short PDUs, or dynamic for long)
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"""
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payload = bits_from_payload_string(payload_ascii)
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payload, _ = _pad_to_multiple_of_8(payload)
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len_payload = len(payload)
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crc_bits = _compute_crc_bits(payload)
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full = payload + crc_bits
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reverse_bit_order(full)
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stuffed = bit_stuff(full)
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preamble = _preamble_bits()
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start = _start_flag_bits()
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end = _start_flag_bits()
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if len_payload <= 168:
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frame = [0] * LEN_FRAME_MAX
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idx = 0
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frame[idx:idx + LEN_PREAMBLE] = preamble
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idx += LEN_PREAMBLE
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frame[idx:idx + LEN_START] = start
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idx += LEN_START
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frame[idx:idx + len(stuffed)] = stuffed
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idx += len(stuffed)
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frame[idx:idx + 8] = end
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idx += 8
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# padding to 256
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assert idx <= LEN_FRAME_MAX
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if enable_nrzi:
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nrz_to_nrzi(frame)
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return frame
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len_frame = LEN_PREAMBLE + LEN_START * 2 + len(stuffed)
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while len_frame % 8 != 0:
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len_frame += 1
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frame = [0] * len_frame
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idx = 0
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frame[idx:idx + LEN_PREAMBLE] = preamble
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idx += LEN_PREAMBLE
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frame[idx:idx + LEN_START] = start
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idx += LEN_START
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frame[idx:idx + len(stuffed)] = stuffed
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idx += len(stuffed)
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frame[idx:idx + 8] = end
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if enable_nrzi:
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nrz_to_nrzi(frame)
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return frame
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def nrzi_bits_to_bytes(bits: list[int]) -> bytes:
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"""Pack MSB-first within each byte (same as byte_packing in Build_Frame)."""
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assert len(bits) % 8 == 0
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out = bytearray()
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for i in range(len(bits) // 8):
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b = 0
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for k in range(8):
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b = b * 2 + (bits[i * 8 + k] & 1)
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out.append(b & 0xFF)
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return bytes(out)
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