code.octet-stream.net Git - m17rt/blob - m17core/src/decode.rs

   1 use crate::{
   2     bits::BitsMut,
   3     fec::{self, p_1, p_2, p_3},
   4     interleave::interleave,
   5     protocol::{
   6         LsfFrame, PacketFrame, PacketFrameCounter, StreamFrame, BERT_SYNC, LSF_SYNC, PACKET_SYNC,
   7         STREAM_SYNC,
   8     },
   9     random::random_xor,
  10 };
  11 use log::debug;
  12
  13 const PLUS_THREE: [u8; 2] = [0, 1];
  14 const PLUS_ONE: [u8; 2] = [0, 0];
  15 const MINUS_ONE: [u8; 2] = [1, 0];
  16 const MINUS_THREE: [u8; 2] = [1, 1];
  17
  18 fn decode_sample(sample: f32) -> [u8; 2] {
  19     if sample > 0.667 {
  20         PLUS_THREE
  21     } else if sample > 0.0 {
  22         PLUS_ONE
  23     } else if sample > -0.667 {
  24         MINUS_ONE
  25     } else {
  26         MINUS_THREE
  27     }
  28 }
  29
  30 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
  31 pub(crate) enum SyncBurst {
  32     Lsf,
  33     Bert,
  34     Stream,
  35     Packet,
  36 }
  37
  38 impl SyncBurst {
  39     pub(crate) fn target(&self) -> [i8; 8] {
  40         match self {
  41             Self::Lsf => LSF_SYNC,
  42             Self::Bert => BERT_SYNC,
  43             Self::Stream => STREAM_SYNC,
  44             Self::Packet => PACKET_SYNC,
  45         }
  46     }
  47 }
  48
  49 const SYNC_MIN_GAIN: f32 = 16.0;
  50 const SYNC_BIT_THRESHOLD: f32 = 0.3;
  51 pub const SYNC_THRESHOLD: f32 = 100.0;
  52
  53 pub(crate) fn sync_burst_correlation(target: [i8; 8], samples: &[f32]) -> (f32, f32, f32) {
  54     let mut pos_max: f32 = f32::MIN;
  55     let mut neg_max: f32 = f32::MAX;
  56     for i in 0..8 {
  57         pos_max = pos_max.max(samples[i * 10]);
  58         neg_max = neg_max.min(samples[i * 10]);
  59     }
  60     let gain = (pos_max - neg_max) / 2.0;
  61     let shift = pos_max + neg_max;
  62     if gain < SYNC_MIN_GAIN {
  63         return (f32::MAX, gain, shift);
  64     }
  65     let mut diff = 0.0;
  66     for i in 0..8 {
  67         let sym_diff = (((samples[i * 10] - shift) / gain) - target[i] as f32).abs();
  68         if sym_diff > SYNC_BIT_THRESHOLD {
  69             return (f32::MAX, gain, shift);
  70         }
  71         diff += sym_diff;
  72     }
  73     (diff, gain, shift)
  74 }
  75
  76 /// Decode frame and return contents after the sync burst
  77 pub(crate) fn frame_initial_decode(frame: &[f32] /* length 192 */) -> [u8; 46] {
  78     let mut decoded = [0u8; 48];
  79     let mut decoded_bits = BitsMut::new(&mut decoded);
  80     for (idx, s) in frame.iter().enumerate() {
  81         let dibits = decode_sample(*s);
  82         decoded_bits.set_bit(idx * 2, dibits[0]);
  83         decoded_bits.set_bit(idx * 2 + 1, dibits[1]);
  84     }
  85     random_xor(&mut decoded[2..]);
  86     interleave(&decoded[2..])
  87 }
  88
  89 pub(crate) fn parse_lsf(frame: &[f32] /* length 192 */) -> Option<LsfFrame> {
  90     let deinterleaved = frame_initial_decode(frame);
  91     debug!("deinterleaved: {:?}", deinterleaved);
  92     let lsf = match fec::decode(&deinterleaved, 240, p_1) {
  93         Some(lsf) => LsfFrame(lsf),
  94         None => return None,
  95     };
  96     debug!("full lsf: {:?}", lsf.0);
  97     let crc = lsf.check_crc();
  98     debug!("recv crc: {:04X}", crc);
  99     debug!("destination: {:?}", lsf.destination());
 100     debug!("source: {:?}", lsf.source());
 101     debug!("mode: {:?}", lsf.mode());
 102     debug!("data type: {:?}", lsf.data_type());
 103     debug!("encryption type: {:?}", lsf.encryption_type());
 104     debug!("can: {}", lsf.channel_access_number());
 105     debug!("meta: {:?}", lsf.meta());
 106     Some(lsf)
 107 }
 108
 109 pub(crate) fn parse_stream(frame: &[f32] /* length 192 */) -> Option<StreamFrame> {
 110     let deinterleaved = frame_initial_decode(frame);
 111     let stream_part = &deinterleaved[12..];
 112     let stream = match fec::decode(stream_part, 144, p_2) {
 113         Some(stream) => stream,
 114         None => return None,
 115     };
 116     let frame_num = u16::from_be_bytes([stream[0], stream[1]]);
 117     let eos = (frame_num & 0x8000) > 0;
 118     let frame_num = frame_num & 0x7fff; // higher layer has to handle wraparound
 119     debug!("frame number: {frame_num}, codec2: {:?}", &stream[2..18]);
 120
 121     if let Some((counter, part)) = decode_lich(&deinterleaved[0..12]) {
 122         debug!(
 123             "LICH: received part {counter} part {part:?} from raw {:?}",
 124             &deinterleaved[0..12]
 125         );
 126         Some(StreamFrame {
 127             lich_idx: counter,
 128             lich_part: part,
 129             frame_number: frame_num,
 130             end_of_stream: eos,
 131             stream_data: stream[2..18].try_into().unwrap(),
 132         })
 133     } else {
 134         None
 135     }
 136 }
 137
 138 pub(crate) fn parse_packet(frame: &[f32] /* length 192 */) -> Option<PacketFrame> {
 139     let deinterleaved = frame_initial_decode(frame);
 140     let packet = match fec::decode(&deinterleaved, 206, p_3) {
 141         Some(packet) => packet,
 142         None => return None,
 143     };
 144     // TODO: the spec is inconsistent about which bit in packet[25] is EOF
 145     // https://github.com/M17-Project/M17_spec/issues/147
 146     let final_frame = (packet[25] & 0x04) > 0;
 147     let number = packet[25] >> 3;
 148     let counter = if final_frame {
 149         PacketFrameCounter::FinalFrame {
 150             payload_len: number as usize,
 151         }
 152     } else {
 153         PacketFrameCounter::Frame {
 154             index: number as usize,
 155         }
 156     };
 157     Some(PacketFrame {
 158         payload: packet[0..25].try_into().unwrap(),
 159         counter,
 160     })
 161 }
 162
 163 pub(crate) fn decode_lich(type2_bits: &[u8]) -> Option<(u8, [u8; 5])> {
 164     let mut decoded = 0u64;
 165     for (input_idx, input_bytes) in type2_bits.chunks(3).enumerate() {
 166         let mut input: u32 = 0;
 167         for (idx, byte) in input_bytes.iter().enumerate() {
 168             input |= (*byte as u32) << (16 - (8 * idx));
 169         }
 170         let (val, _dist) = cai_golay::extended::decode(input)?;
 171         decoded |= (val as u64) << ((3 - input_idx) * 12);
 172     }
 173     let b = decoded.to_be_bytes();
 174     Some((b[7] >> 5, [b[2], b[3], b[4], b[5], b[6]]))
 175 }
 176
 177 #[cfg(test)]
 178 mod tests {
 179     use super::*;
 180
 181     #[test]
 182     fn test_lich_decode() {
 183         let input = [221, 82, 162, 16, 85, 200, 5, 14, 254, 4, 13, 153];
 184         let expected_counter = 2;
 185         let expected_part = [221, 81, 5, 5, 0];
 186         assert_eq!(decode_lich(&input), Some((expected_counter, expected_part)));
 187     }
 188 }