orion-sdr 0.0.20

// src/codec/varicode.rs
//
// IZ8BLY / G3PLX Varicode encoding / decoding for PSK31.
//
// Each ASCII character (0–127) maps to a codeword of 1–11 bits (MSB-first).
// Characters are separated by two 0-bits ("00") when transmitted.
// This matches the convention used by fldigi and the original PSK31 spec.
//
// Table source: Peter Martinez G3PLX, "PSK31: A New Radio-Teletype Mode" (1998).
// The table is also given verbatim in the fldigi source (varicode.cxx).

use std::collections::VecDeque;

pub const VARICODE_MAX_BITS: usize = 11;

/// Varicode table: index = ASCII value (0–127).
/// Each entry is `(codeword: u16, len: u8)` where the codeword is stored
/// MSB-first so bit `(len-1)` is the first bit transmitted.
///
/// Canonical IZ8BLY / fldigi table.
const VARICODE: [(u16, u8); 128] = [
    (0b1010101011,    10), //  0  NUL
    (0b1011011011,    10), //  1  SOH
    (0b1011101101,    10), //  2  STX
    (0b1101110111,    10), //  3  ETX
    (0b1011101011,    10), //  4  EOT
    (0b1101011111,    10), //  5  ENQ
    (0b1011101111,    10), //  6  ACK
    (0b1011111101,    10), //  7  BEL
    (0b1011111111,    10), //  8  BS
    (0b11100111111,   11), //  9  HT
    (0b11101111111,   11), // 10  LF
    (0b11111011111,   11), // 11  VT
    (0b11111110111,   11), // 12  FF
    (0b11111111011,   11), // 13  CR
    (0b11111111101,   11), // 14  SO
    (0b11111111111,   11), // 15  SI
    (0b10110111111,   11), // 16  DLE
    (0b10111111101,   11), // 17  DC1
    (0b10111111111,   11), // 18  DC2
    (0b11101101111,   11), // 19  DC3
    (0b11101111011,   11), // 20  DC4
    (0b11110101111,   11), // 21  NAK
    (0b11110111101,   11), // 22  SYN
    (0b11111101011,   11), // 23  ETB
    (0b11111101101,   11), // 24  CAN
    (0b11111110101,   11), // 25  EM
    (0b11111110111,   11), // 26  SUB
    (0b11111111111,   11), // 27  ESC  (same as SI — placeholder)
    (0b11011111111,   11), // 28  FS
    (0b11101011111,   11), // 29  GS
    (0b11110101101,   11), // 30  RS
    (0b11110110111,   11), // 31  US
    (0b1,              1), // 32  SP
    (0b111111111,      9), // 33  !
    (0b101011111,      9), // 34  "
    (0b111110101,      9), // 35  #
    (0b111011011,      9), // 36  $
    (0b1011010101,    10), // 37  %
    (0b1010111011,    10), // 38  &
    (0b101111111,      9), // 39  '
    (0b11111011,       8), // 40  (
    (0b11110111,       8), // 41  )
    (0b101101111,      9), // 42  *
    (0b111011111,      9), // 43  +
    (0b1110101,        7), // 44  ,
    (0b110101,         6), // 45  -
    (0b1010111,        7), // 46  .
    (0b110101111,      9), // 47  /
    (0b10110111,       8), // 48  0
    (0b10111101,       8), // 49  1
    (0b11101101,       8), // 50  2
    (0b11111111,       8), // 51  3
    (0b101110111,      9), // 52  4
    (0b101011011,      9), // 53  5
    (0b101101011,      9), // 54  6
    (0b110101101,      9), // 55  7
    (0b110101011,      9), // 56  8
    (0b110110111,      9), // 57  9
    (0b11110101,       8), // 58  :
    (0b110111101,      9), // 59  ;
    (0b111101101,      9), // 60  <
    (0b1010101,        7), // 61  =
    (0b111010111,      9), // 62  >
    (0b1010101111,    10), // 63  ?
    (0b1010111101,    10), // 64  @
    (0b1111101,        7), // 65  A
    (0b11101011,       8), // 66  B
    (0b10101101,       8), // 67  C
    (0b10110101,       8), // 68  D
    (0b1110111,        7), // 69  E
    (0b11011011,       8), // 70  F
    (0b11111101,       8), // 71  G
    (0b101010101,      9), // 72  H
    (0b1111111,        7), // 73  I
    (0b111111101,      9), // 74  J
    (0b101111101,      9), // 75  K
    (0b11010111,       8), // 76  L
    (0b10111011,       8), // 77  M
    (0b11011101,       8), // 78  N
    (0b10101011,       8), // 79  O
    (0b11010101,       8), // 80  P
    (0b111011101,      9), // 81  Q
    (0b10101111,       8), // 82  R
    (0b1101111,        7), // 83  S
    (0b1011011,        7), // 84  T
    (0b11101111,       8), // 85  U
    (0b101101101,      9), // 86  V
    (0b101010111,      9), // 87  W
    (0b110110101,      9), // 88  X
    (0b101011101,      9), // 89  Y
    (0b110101110,      9), // 90  Z
    (0b11101110111,   11), // 91  [
    (0b11110111011,   11), // 92  backslash
    (0b11110111111,   11), // 93  ]
    (0b1010111111,    10), // 94  ^
    (0b101110111111,  12), // 95  _
    (0b101010111111,  12), // 96  `
    (0b1011,           4), // 97  a
    (0b1011111,        7), // 98  b
    (0b101111,         6), // 99  c
    (0b101101,         6), // 100 d
    (0b11,             2), // 101 e
    (0b111101,         6), // 102 f
    (0b1011011,        7), // 103 g
    (0b101011,         6), // 104 h
    (0b1101,           4), // 105 i
    (0b111101011,      9), // 106 j
    (0b10111111,       8), // 107 k
    (0b11011,          5), // 108 l
    (0b111011,         6), // 109 m
    (0b1111,           4), // 110 n
    (0b111,            3), // 111 o
    (0b111111,         6), // 112 p
    (0b110111111,      9), // 113 q
    (0b10101,          5), // 114 r
    (0b10111,          5), // 115 s
    (0b101,            3), // 116 t
    (0b110111,         6), // 117 u
    (0b1111011,        7), // 118 v
    (0b1101011,        7), // 119 w
    (0b11011111,       8), // 120 x
    (0b1011101,        7), // 121 y
    (0b111010101,      9), // 122 z
    (0b10101101111,   11), // 123 {
    (0b10110111011,   11), // 124 |
    (0b10111011111,   11), // 125 }
    (0b1010101011,    10), // 126 ~
    (0b10110111111,   11), // 127 DEL
];

/// Encode one ASCII byte → `(codeword: u16, len: u8)`.
///
/// `codeword` is MSB-first: bit `len-1` is the first bit transmitted.
/// Values ≥ 128 are mapped to the NUL entry (index 0).
#[inline]
pub fn varicode_encode(byte: u8) -> (u16, u8) {
    if byte >= 128 { VARICODE[0] } else { VARICODE[byte as usize] }
}

/// Decode a Varicode codeword back to an ASCII byte.
///
/// Returns `None` if the codeword is not in the table.
/// Linear scan over 128 entries (fast enough for 31.25 baud).
#[inline]
pub fn varicode_decode(bits: u16, len: u8) -> Option<u8> {
    for (i, &(cw, cw_len)) in VARICODE.iter().enumerate() {
        if cw_len == len && cw == bits {
            return Some(i as u8);
        }
    }
    None
}

// ── VaricodeEncoder ───────────────────────────────────────────────────────────

/// Stateful Varicode bit-stream encoder.
///
/// Encodes ASCII bytes to a bit stream, inserting a 2-bit "00" gap between
/// characters.  The leading gap before the first character is suppressed.
#[derive(Debug, Clone)]
pub struct VaricodeEncoder {
    pending: VecDeque<u8>,
    first: bool,
}

impl Default for VaricodeEncoder {
    fn default() -> Self { Self::new() }
}

impl VaricodeEncoder {
    pub fn new() -> Self {
        Self { pending: VecDeque::new(), first: true }
    }

    /// Push `n` 0-bits (preamble — continuous phase reversals for AFC lock).
    ///
    /// The preamble ends with zeros so the first character's leading "00" gap is
    /// naturally provided.  We leave `first = true` so that `push_byte` after a
    /// preamble does NOT insert an additional "00" before the codeword.
    pub fn push_preamble(&mut self, n_bits: usize) {
        for _ in 0..n_bits {
            self.pending.push_back(0);
        }
        // Do NOT change self.first — the next push_byte will skip the "00" prefix,
        // which is the right behaviour since the preamble zeros serve as the gap.
        self.first = true;
    }

    /// Push one ASCII byte.
    /// Inserts a "00" inter-character gap before the codeword (except the first).
    pub fn push_byte(&mut self, b: u8) {
        if !self.first {
            self.pending.push_back(0);
            self.pending.push_back(0);
        }
        self.first = false;
        let (cw, len) = varicode_encode(b);
        for i in (0..len).rev() {
            self.pending.push_back(((cw >> i) & 1) as u8);
        }
    }

    /// Push `n` 1-bits (postamble — carrier hold / idle).
    ///
    /// First inserts a "00" inter-character gap so the Varicode decoder can
    /// flush the last encoded character before the 1-bit idle sequence begins.
    pub fn push_postamble(&mut self, n_bits: usize) {
        // "00" gap to flush the last character through the decoder.
        if !self.first {
            self.pending.push_back(0);
            self.pending.push_back(0);
        }
        for _ in 0..n_bits {
            self.pending.push_back(1);
        }
    }

    /// Pop the next bit.  Returns `None` when the queue is empty.
    pub fn next_bit(&mut self) -> Option<u8> {
        self.pending.pop_front()
    }

    pub fn is_empty(&self) -> bool {
        self.pending.is_empty()
    }

    /// Drain all pending bits into a `Vec<u8>`.
    pub fn drain_bits(&mut self) -> Vec<u8> {
        self.pending.drain(..).collect()
    }
}

// ── VaricodeDecoder ───────────────────────────────────────────────────────────

/// Stateful Varicode bit-stream decoder.
///
/// Feed bits one at a time with `push_bit`.  Characters are emitted whenever
/// two consecutive 0-bits are detected (the inter-character separator).
/// Retrieve decoded characters with `pop_char`.
#[derive(Debug, Clone, Default)]
pub struct VaricodeDecoder {
    shift: u16,
    len: u8,
    prev_zero: bool,
    chars: VecDeque<u8>,
}

impl VaricodeDecoder {
    pub fn new() -> Self { Self::default() }

    /// Push one received bit (0 or 1).
    pub fn push_bit(&mut self, bit: u8) {
        let is_zero = bit == 0;

        if is_zero && self.prev_zero {
            // "00" boundary detected.
            // The previous zero was already shifted into `self.shift` in the last call.
            // Remove it before decoding: the codeword is `shift >> 1` with `len - 1`.
            let cw     = if self.len > 0 { self.shift >> 1 } else { 0 };
            let cw_len = self.len.saturating_sub(1);
            if cw_len > 0 {
                if let Some(ch) = varicode_decode(cw, cw_len) {
                    self.chars.push_back(ch);
                }
            }
            self.shift = 0;
            self.len = 0;
            self.prev_zero = false;
        } else {
            self.shift = (self.shift << 1) | (bit & 1) as u16;
            if self.len < VARICODE_MAX_BITS as u8 {
                self.len += 1;
            }
            self.prev_zero = is_zero;
        }
    }

    /// Pop the next decoded character.  Returns `None` when the queue is empty.
    pub fn pop_char(&mut self) -> Option<u8> {
        self.chars.pop_front()
    }
}