ft8_lib/ft8/unpack.cpp

#include "unpack.h"
#include "text.h"

#include <string.h>

constexpr uint32_t NBASE = 37L*36L*10L*27L*27L*27L;


// convert packed character to ASCII character
// 0..9 a..z space +-./?
char charn(uint8_t c) {
    if (c >= 0 && c <= 9)
        return '0' + c;
    if (c >= 10 && c < 36)
        return 'A' + c - 10;

    if (c < 42)
        return " +-./?" [c - 36];

    return ' ';
}


// nc is a 28-bit integer, e.g. nc1 or nc2, containing all the
// call sign bits from a packed message.
void unpackcall(uint32_t nc, char *callsign) {
    callsign[5] = charn((nc % 27) + 10); // + 10 b/c only alpha+space
    nc /= 27;
    callsign[4] = charn((nc % 27) + 10);
    nc /= 27;
    callsign[3] = charn((nc % 27) + 10);
    nc /= 27;
    callsign[2] = charn(nc % 10); // digit only
    nc /= 10;
    callsign[1] = charn(nc % 36); // letter or digit
    nc /= 36;
    callsign[0] = charn(nc);

    callsign[6] = 0;
}


// extract maidenhead locator
void unpackgrid(uint16_t ng, char *grid) {
    // start of special grid locators for sig strength &c.
    constexpr uint16_t NGBASE = 180*180;

    if (ng == NGBASE + 1) {
        // Empty grid is allowed
        grid[0] = 0;
        return;
    }

    // Check for signal strength reports (-01 to -30 dB)
    if (ng >= NGBASE + 1 && ng < NGBASE + 31) {
        int_to_dd(grid, ((NGBASE + 1) - ng), 2);
        return;
    }
    if (ng >= NGBASE + 31 && ng < NGBASE + 62) {
        grid[0] = 'R';
        int_to_dd(grid + 1, ((NGBASE + 31) - ng), 2); 
        return;
    }

    // Check for special cases
    if (ng == NGBASE + 62) {
        strcpy(grid, "RO");
        return;
    }
    if (ng == NGBASE + 63) {
        strcpy(grid, "RRR");
        return;
    }
    if (ng == NGBASE + 64) {
        strcpy(grid, "73");
        return;        
    }

    // Decode 4-symbol grid
    int16_t lat = (int16_t)(ng % 180) - 90;
    int16_t lng = ((int16_t)(ng / 180) * 2) - 180;

    grid[0] = 'A' + ((179 - lng) / 20);
    grid[1] = 'A' + ((90 + lat) / 10);
    grid[2] = '0' + (((179 - lng) % 20) / 2);
    grid[3] = '0' + ((90 + lat) % 10);
    grid[4] = 0;

    // Check for extended range signal reports
    if ((grid[0] == 'K') && (grid[1] == 'A')) {
        int sig = dd_to_int(grid + 2, 2) - 50;
        int_to_dd(grid, sig, 2);
        return;
    }
    if ((grid[0] == 'L') && (grid[1] == 'A')) {
        int sig = dd_to_int(grid + 2, 2) - 50;
        grid[0] = 'R';
        int_to_dd(grid + 1, sig, 2);
        return;
    }
}


void unpacktext(uint32_t nc1, uint32_t nc2, uint16_t ng, char *text) {
    uint32_t nc3 = (ng & 0x7FFF);

    // Check for bit 0 and copy it to nc3
    if ((nc1 & 1) != 0)
        nc3 |= 0x08000;

    if ((nc2 & 1) != 0)
        nc3 |= 0x10000;

    nc1 >>= 1;
    nc2 >>= 1;

    for (int i = 4; i >= 0; --i) {
        text[i] = charn(nc1 % 42);
        nc1 /= 42;
    }

    for (int i = 9; i >= 5; --i) {
        text[i] = charn(nc2 % 42);
        nc2 /= 42;
    }

    for (int i = 12; i >= 10; --i) {
        text[i] = charn(nc3 % 42);
        nc3 /= 42;
    }

    text[13] = 0;
}


// message should have at least 19 bytes allocated (18 characters + zero terminator)
int unpack(const uint8_t *a72, char *message) {
    uint32_t nc1, nc2;
    uint16_t ng;

    nc1 = (a72[0] << 20);
    nc1 |= (a72[1] << 12);
    nc1 |= (a72[2] << 4);
    nc1 |= (a72[3] >> 4);
    
    nc2 = ((a72[3] & 0x0F) << 24);
    nc2 |= (a72[4] << 16);
    nc2 |= (a72[5] << 8);
    nc2 |= (a72[6]);

    ng = (a72[7] << 8);
    ng |= (a72[8]);

    if (ng & 0x8000) {
        unpacktext(nc1, nc2, ng, message);
        return 0;
    }

    char c2[7];
    char grid[5];

    if (nc1 == NBASE+1) {
        // CQ with standard callsign
        unpackcall(nc2, c2);
        unpackgrid(ng, grid);
        strcpy(message, "CQ ");
        strcat(message, c2);
        strcat(message, " ");
        strcat(message, grid);
        return 0;
    }

    if (nc1 >= 267649090L && nc1 <= 267698374L) {
        // CQ with suffix (e.g. /QRP)
        uint32_t n = nc1 - 267649090L;
        char sf[4];
        sf[0] = charn(n % 37);
        n /= 37;
        sf[1] = charn(n % 37);
        n /= 37;
        sf[2] = charn(n % 37);

        unpackcall(nc2, c2);
        unpackgrid(ng, grid);

        strcpy(message, "CQ ");
        strcat(message, c2);
        strcat(message, "/");
        strcat(message, sf);
        strcat(message, " ");
        strcat(message, grid);
        return 0;
    }

    // Standard two-call exchange
    char c1[7];
    
    unpackcall(nc1, c1);
    if (equals(c1, "CQ9DX ")) {
        strcpy(c1, "CQ DX");
    }
    else if (starts_with(c1, " E9") && is_letter(c1[3]) && is_letter(c1[4]) && is_space(c1[5])) {
        strcpy(c1, "CQ ");
        c1[5] = 0;
    }
    unpackcall(nc2, c2);
    unpackgrid(ng, grid);

    strcpy(message, c1);
    strcat(message, " ");
    strcat(message, c2);
    strcat(message, " ");
    strcat(message, grid);

    return 0;
}