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CPR relative decoding: avoid truncation when finding the fractional

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position of the reference position within a lat/lng zone.
This commit is contained in:
Oliver Jowett 2015-01-12 16:16:32 +00:00
parent 2febe75ce7
commit e1d2af42bb
1 changed files with 13 additions and 7 deletions
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20
mode_s.c
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@ -2398,11 +2398,17 @@ void useModesMessage(struct modesMessage *mm) {
//
// Always positive MOD operation, used for CPR decoding.
//
int cprModFunction(int a, int b) {
int cprModInt(int a, int b) {
int res = a % b;
if (res < 0) res += b;
return res;
}
double cprModDouble(double a, double b) {
double res = fmod(a,b);
if (res < 0) res += b;
return res;
}
//
//=========================================================================
//
@ -2503,8 +2509,8 @@ int decodeCPR(struct aircraft *a, int fflag, int surface) {
// Compute the Latitude Index "j"
int j = (int) floor(((59*lat0 - 60*lat1) / 131072) + 0.5);
double rlat0 = AirDlat0 * (cprModFunction(j,60) + lat0 / 131072);
double rlat1 = AirDlat1 * (cprModFunction(j,59) + lat1 / 131072);
double rlat0 = AirDlat0 * (cprModInt(j,60) + lat0 / 131072);
double rlat1 = AirDlat1 * (cprModInt(j,59) + lat1 / 131072);
time_t now = time(NULL);
double surface_rlat = MODES_USER_LATITUDE_DFLT;
@ -2542,13 +2548,13 @@ int decodeCPR(struct aircraft *a, int fflag, int surface) {
int ni = cprNFunction(rlat1,1);
int m = (int) floor((((lon0 * (cprNLFunction(rlat1)-1)) -
(lon1 * cprNLFunction(rlat1))) / 131072.0) + 0.5);
a->lon = cprDlonFunction(rlat1, 1, surface) * (cprModFunction(m, ni)+lon1/131072);
a->lon = cprDlonFunction(rlat1, 1, surface) * (cprModInt(m, ni)+lon1/131072);
a->lat = rlat1;
} else { // Use even packet.
int ni = cprNFunction(rlat0,0);
int m = (int) floor((((lon0 * (cprNLFunction(rlat0)-1)) -
(lon1 * cprNLFunction(rlat0))) / 131072) + 0.5);
a->lon = cprDlonFunction(rlat0, 0, surface) * (cprModFunction(m, ni)+lon0/131072);
a->lon = cprDlonFunction(rlat0, 0, surface) * (cprModInt(m, ni)+lon0/131072);
a->lat = rlat0;
}
@ -2608,7 +2614,7 @@ int decodeCPRrelative(struct aircraft *a, int fflag, int surface) {
// Compute the Latitude Index "j"
j = (int) (floor(latr/AirDlat) +
trunc(0.5 + cprModFunction((int)latr, (int)AirDlat)/AirDlat - lat/131072));
trunc(0.5 + cprModDouble(latr, AirDlat)/AirDlat - lat/131072));
rlat = AirDlat * (j + lat/131072);
if (rlat >= 270) rlat -= 360;
@ -2627,7 +2633,7 @@ int decodeCPRrelative(struct aircraft *a, int fflag, int surface) {
// Compute the Longitude Index "m"
AirDlon = cprDlonFunction(rlat, fflag, surface);
m = (int) (floor(lonr/AirDlon) +
trunc(0.5 + cprModFunction((int)lonr, (int)AirDlon)/AirDlon - lon/131072));
trunc(0.5 + cprModDouble(lonr, AirDlon)/AirDlon - lon/131072));
rlon = AirDlon * (m + lon/131072);
if (rlon > 180) rlon -= 360;