wspr-beacon/src/wspr.rs

// Implemented by following http://www.g4jnt.com/Coding/WSPR_Coding_Process.pdf

const WSPR_LENGTH: usize = 162;
const WSPR_POWER_LOC_BITS: usize = 22;

const WSPR_SYNC: [u8; WSPR_LENGTH] = [
    1, 1, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 1, 1, 0, 0, 0, 1, 0, 0, 1, 0, 1, 1, 1, 1, 0, 0, 0, 0, 0,
    0, 0, 1, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 0, 1, 1, 0, 1, 0,
    0, 0, 0, 1, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 0, 1, 0, 0, 1, 0, 1, 1, 0, 0, 0, 1, 1, 0, 1, 0, 1, 0,
    0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 1, 1, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 0, 0, 1, 1, 1,
    0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 1, 1, 0, 1, 0, 1, 1, 0, 0, 0, 1, 1, 0,
    0, 0,
];

fn encode_call_char(call_sign: &arrayvec::ArrayString<6>, idx: usize) -> u32 {
    let c = call_sign.chars().nth(idx).unwrap().to_ascii_uppercase();

    if c >= '0' && c <= '9' {
        return (c as u32) - ('0' as u32);
    } else if c >= 'A' && c <= 'Z' {
        return 10 + (c as u32) - ('A' as u32);
    }
    // Turn everything else to space
    else {
        return 36;
    }
}

fn encode_loc_char(locator: &arrayvec::ArrayString<6>, idx: usize) -> u32 {
    let c = locator.chars().nth(idx).unwrap().to_ascii_uppercase();

    if c >= '0' && c <= '9' {
        return (c as u32) - ('0' as u32);
    } else if c >= 'A' && c <= 'R' {
        return (c as u32) - ('A' as u32);
    }
    // Everthing else becomes 9/I
    else {
        return 9;
    }
}

fn encode_power(power: u8) -> u32 {
    if power > 60 {
        return 60 as u32;
    }
    return power as u32;
}

fn message_to_bits(
    call_sign: &arrayvec::ArrayString<6>,
    locator: &arrayvec::ArrayString<6>,
    power: u8,
) -> u64 {
    let mut n = encode_call_char(call_sign, 0);
    n = n * 36 + encode_call_char(call_sign, 1);
    n = n * 10 + encode_call_char(call_sign, 2);
    n = n * 27 + encode_call_char(call_sign, 3) - 10;
    n = n * 27 + encode_call_char(call_sign, 4) - 10;
    n = n * 27 + encode_call_char(call_sign, 5) - 10;
    // lowest 28 bits of n contain the encoded callsign

    let mut m = 179 - 10 * encode_loc_char(locator, 0) - encode_loc_char(locator, 2);
    m = m * 180 + 10 * encode_loc_char(locator, 1) + encode_loc_char(locator, 3);
    m = m * 128 + encode_power(power) + 64;
    // lowest 22 bits of m contain the encoded locator (15bit) and the encoded power (7 bits)

    // Concactenate to 50bit wspr message
    return (n as u64) << WSPR_POWER_LOC_BITS | (m as u64);
}

const FEC_TAPS1: u32 = 0xF2D05351;
const FEC_TAPS2: u32 = 0xE4613C47;

fn parity32(mut x: u32) -> u8 {
    x ^= x >> 16;
    x ^= x >> 8;
    x ^= x >> 4;
    x ^= x >> 2;
    x ^= x >> 1;

    return (x & 1) as u8;
}

fn generate_fec(msg: u64) -> [u8; WSPR_LENGTH] {
    let mut output: [u8; WSPR_LENGTH] = [0; WSPR_LENGTH];

    let mut reg: u32 = 0;
    for i in 0..81 {
        let bit = if i < 50 {
            ((msg >> (50 - i - 1)) & 1) as u32
        } else {
            0
        };

        reg = (reg << 1) | bit;

        let tabs1 = reg & FEC_TAPS1;
        let tabs2 = reg & FEC_TAPS2;

        output[i * 2] = parity32(tabs1);
        output[i * 2 + 1] = parity32(tabs2);
    }

    return output;
}

fn interleave_message(input: [u8; WSPR_LENGTH]) -> [u8; WSPR_LENGTH] {
    let mut output: [u8; WSPR_LENGTH] = [0; WSPR_LENGTH];

    let mut p: u8 = 0;
    let mut i: u8 = 0;
    while p < WSPR_LENGTH as u8 {
        let rev_i = i.reverse_bits();
        if rev_i < WSPR_LENGTH as u8 {
            output[rev_i as usize] = input[p as usize];
            p += 1;
        }
        i += 1;
    }

    return output;
}

fn add_sync(symbols: &mut [u8; WSPR_LENGTH]) {
    for i in 0..WSPR_LENGTH {
        symbols[i] = WSPR_SYNC[i] | symbols[i] << 1;
    }
}

pub fn encode_message(
    call_sign: &arrayvec::ArrayString<6>,
    locator: &arrayvec::ArrayString<6>,
    power: u8,
) -> [u8; WSPR_LENGTH] {
    let bits = message_to_bits(call_sign, locator, power);
    let msg_fec = generate_fec(bits);
    let mut msg = interleave_message(msg_fec);
    add_sync(&mut msg);

    return msg;
}