File content as of revision 1:2f2f793fcf4f:

#include <math.h>
#include <string.h>

#define NOTFOUND_MARK (-65536)

struct apinfo registered_ap[] = {
    { "ap-game-432afd", {0, 0x22, 0xcf, 0x43, 0x2a, 0xfd}, 0 },
    { "ap-pc-432afc", {0, 0x22, 0xcf, 0x43, 0x2a, 0xfc}, 0 },
    { "ap-game-433f85", {0, 0x22, 0xcf, 0x43, 0x3f, 0x85}, 0 },
    { "ap-pc-433f84", {0, 0x22, 0xcf, 0x43, 0x3f, 0x84}, 0 },
    { "ap-game-433223", {0, 0x22, 0xcf, 0x43, 0x32, 0x23}, 0 },
    { "ap-pc-433222", {0, 0x22, 0xcf, 0x43, 0x32, 0x22}, 0 },
    { "ap-game-4334c9", {0, 0x22, 0xcf, 0x43, 0x34, 0xc9}, 0 },
    { "ap-pc-4334c8", {0, 0x22, 0xcf, 0x43, 0x34, 0xc8}, 0 }
};  // this structure must be writable

struct pos {
    float x, y;
};

const struct pos registered_pos[] = {
    {0.0, 0.0}, {0.0, 0.0}, {0.0, 100.0}, {0.0, 100.0}, 
    {100.0, 0.0}, {100.0, 0.0}, {100.0, 100.0}, {100.0, 100.0}
};

struct pos result_pos;

void LocationManagerCalcPos(int apinfo_count, struct apinfo* searched_ap)
{
    int i, j, k;
    int match_count;
    const int length_of_registered_ap = sizeof(registered_ap) / sizeof(struct apinfo);
    float proportion;
    float sum;

    for ( i = 0; i < length_of_registered_ap; i++ ) {
        registered_ap[i].power = NOTFOUND_MARK;
    }
    match_count = 0;
    for ( i = 0; i < apinfo_count; i++ ) {
        for ( j = 0; j < length_of_registered_ap; j++ ) {
            for ( k = 0; k < ESSID_LEN; k++ ) {
                if ( registered_ap[j].essid[k] == '\0' ) {
                    break;
                }
                if ( searched_ap[i].essid[k] != registered_ap[j].essid[k] ) {
                    goto no_match;
                }
            }
            if ( memcmp( registered_ap[j].bssid, searched_ap[i].bssid, MACADDR_LEN) != 0 ) {
                continue;
            }
            registered_ap[j].power = searched_ap[i].power;
            match_count++;
            break;
no_match:;
        }
    }
    
    if ( match_count == 0 ) {
        lcd.cls();
        lcd.printf("location lost\nAP count %d", apinfo_count);
        return;
    }
    
    result_pos.x = result_pos.y = 0.0;
    sum = 0.0;
    for ( i = 0; i < length_of_registered_ap; i++ ) {
        proportion = pow(10, registered_ap[i].power / 40.0f);   // calc dB to linear number
        sum += proportion;
        result_pos.x += registered_pos[i].x * proportion;
        result_pos.y += registered_pos[i].y * proportion;
    }
    result_pos.x /= sum;
    result_pos.y /= sum;
    
    lcd.cls();
    lcd.printf("AP %d/%d, location x=%.1f, y=%.1f", match_count, apinfo_count, result_pos.x, result_pos.y);
}