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
  66     let mut diff = 0.0;
  67     for i in 0..8 {
  68         let sym_diff = (((samples[i * 10] - shift) / gain) - target[i] as f32).abs();
  69         if sym_diff > SYNC_BIT_THRESHOLD {
  70             return (f32::MAX, gain, shift);
  71         }
  72         diff += sym_diff;
  73     }
  74     (diff, gain, shift)
  75 }
  76
  77 /// Decode frame and return contents after the sync burst
  78 pub(crate) fn frame_initial_decode(frame: &[f32] /* length 192 */) -> [u8; 46] {
  79     let mut decoded = [0u8; 48];
  80     let mut decoded_bits = BitsMut::new(&mut decoded);
  81     for (idx, s) in frame.iter().enumerate() {
  82         let dibits = decode_sample(*s);
  83         decoded_bits.set_bit(idx * 2, dibits[0]);
  84         decoded_bits.set_bit(idx * 2 + 1, dibits[1]);
  85     }
  86     random_xor(&mut decoded[2..]);
  87     interleave(&decoded[2..])
  88 }
  89
  90 pub(crate) fn parse_lsf(frame: &[f32] /* length 192 */) -> Option<LsfFrame> {
  91     let deinterleaved = frame_initial_decode(frame);
  92     debug!("deinterleaved: {:?}", deinterleaved);
  93     let lsf = match fec::decode(&deinterleaved, 240, p_1) {
  94         Some(lsf) => LsfFrame(lsf),
  95         None => return None,
  96     };
  97     debug!("full lsf: {:?}", lsf.0);
  98     let crc = lsf.check_crc();
  99     debug!("recv crc: {:04X}", crc);
 100     debug!("destination: {:?}", lsf.destination());
 101     debug!("source: {:?}", lsf.source());
 102     debug!("mode: {:?}", lsf.mode());
 103     debug!("data type: {:?}", lsf.data_type());
 104     debug!("encryption type: {:?}", lsf.encryption_type());
 105     debug!("can: {}", lsf.channel_access_number());
 106     debug!("meta: {:?}", lsf.meta());
 107     Some(lsf)
 108 }
 109
 110 pub(crate) fn parse_stream(frame: &[f32] /* length 192 */) -> Option<StreamFrame> {
 111     let deinterleaved = frame_initial_decode(frame);
 112     let stream_part = &deinterleaved[12..];
 113     let stream = match fec::decode(stream_part, 144, p_2) {
 114         Some(stream) => stream,
 115         None => return None,
 116     };
 117     let frame_num = u16::from_be_bytes([stream[0], stream[1]]);
 118     let eos = (frame_num & 0x8000) > 0;
 119     let frame_num = frame_num & 0x7fff; // higher layer has to handle wraparound
 120     debug!("frame number: {frame_num}, codec2: {:?}", &stream[2..18]);
 121
 122     if let Some((counter, part)) = decode_lich(&deinterleaved[0..12]) {
 123         debug!(
 124             "LICH: received part {counter} part {part:?} from raw {:?}",
 125             &deinterleaved[0..12]
 126         );
 127         Some(StreamFrame {
 128             lich_idx: counter,
 129             lich_part: part,
 130             frame_number: frame_num,
 131             end_of_stream: eos,
 132             stream_data: stream[2..18].try_into().unwrap(),
 133         })
 134     } else {
 135         None
 136     }
 137 }
 138
 139 pub(crate) fn parse_packet(frame: &[f32] /* length 192 */) -> Option<PacketFrame> {
 140     let deinterleaved = frame_initial_decode(frame);
 141     let packet = match fec::decode(&deinterleaved, 206, p_3) {
 142         Some(packet) => packet,
 143         None => return None,
 144     };
 145     // TODO: the spec is inconsistent about which bit in packet[25] is EOF
 146     // https://github.com/M17-Project/M17_spec/issues/147
 147     let final_frame = (packet[25] & 0x04) > 0;
 148     let number = packet[25] >> 3;
 149     let counter = if final_frame {
 150         PacketFrameCounter::FinalFrame {
 151             payload_len: number as usize,
 152         }
 153     } else {
 154         PacketFrameCounter::Frame {
 155             index: number as usize,
 156         }
 157     };
 158     Some(PacketFrame {
 159         payload: packet[0..25].try_into().unwrap(),
 160         counter,
 161     })
 162 }
 163
 164 pub(crate) fn decode_lich(type2_bits: &[u8]) -> Option<(u8, [u8; 5])> {
 165     let mut decoded = 0u64;
 166     for (input_idx, input_bytes) in type2_bits.chunks(3).enumerate() {
 167         let mut input: u32 = 0;
 168         for (idx, byte) in input_bytes.iter().enumerate() {
 169             input |= (*byte as u32) << (16 - (8 * idx));
 170         }
 171         let (val, _dist) = cai_golay::extended::decode(input)?;
 172         decoded |= (val as u64) << ((3 - input_idx) * 12);
 173     }
 174     let b = decoded.to_be_bytes();
 175     Some((b[7] >> 5, [b[2], b[3], b[4], b[5], b[6]]))
 176 }
 177
 178 #[cfg(test)]
 179 mod tests {
 180     use super::*;
 181
 182     #[test]
 183     fn test_lich_decode() {
 184         let input = [221, 82, 162, 16, 85, 200, 5, 14, 254, 4, 13, 153];
 185         let expected_counter = 2;
 186         let expected_part = [221, 81, 5, 5, 0];
 187         assert_eq!(decode_lich(&input), Some((expected_counter, expected_part)));
 188     }
 189 }