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Improve DF field error correction speed

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Use bitmasks to speed up the detection of valid and invalid ICAO DF
field types.
This commit is contained in:
Malcolm Robb 2013-04-30 10:35:52 +01:00
parent 8d65abd317
commit c81498a0d0
1 changed files with 17 additions and 10 deletions
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dump1090.c
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@ -1977,18 +1977,25 @@ void detectModeS(uint16_t *m, uint32_t mlen) {
if (msglen > i) {msglen = i;} if (msglen > i) {msglen = i;}
else if (msglen < i) {msglen = 0;} else if (msglen < i) {msglen = 0;}
//
// If we guessed at any of the bits in the DF type field, then look to see if our guess was sensible.
// Do this by looking to see if the original guess results in the DF type being one of the ICAO defined
// message types. If it isn't then toggle the guessed bit and see if this new value is ICAO defined.
// if the new value is ICAO defined, then update it in our message.
if ((msglen) && (errorsTy == 1) && (theErrs & 0x78)) { if ((msglen) && (errorsTy == 1) && (theErrs & 0x78)) {
// We guessed at one of the message type bits. See if our guess is "likely" // We guessed at one (and only one) of the message type bits. See if our guess is "likely"
// to be correct by comparing the DF against a list of known good DF's // to be correct by comparing the DF against a list of known good DF's
int DF = ((theByte = msg[0]) >> 3) & 0x1f; int thisDF = ((theByte = msg[0]) >> 3) & 0x1f;
if ( (DF != 0) && (DF != 4) && (DF != 5) && (DF != 11) uint32_t validDFbits = 0x017F0831; // One bit per 32 possible DF's. Set bits 0,4,5,11,16.17.18.19,20,21,22,24
&& (DF != 16) && (DF != 17) && (DF != 18) && (DF != 19) && (DF != 20) && (DF != 21) && (DF != 22) && (DF != 24) ) { uint32_t thisDFbit = (1 << thisDF);
// Other DF values are probably errors. Toggle the bit we guessed at and see if the resultant DF is more likely if (0 == (validDFbits & thisDFbit)) {
theByte ^= theErrs; // The current DF is not ICAO defined, so is probably an errors.
DF = (theByte >> 3) & 0x1f; // Toggle the bit we guessed at and see if the resultant DF is more likely
// if this DF any more likely?? theByte ^= theErrs;
if ( (DF == 0) || (DF == 4) || (DF == 5) || (DF == 11) thisDF = (theByte >> 3) & 0x1f;
|| (DF == 16) || (DF == 17) || (DF == 18) || (DF == 19) || (DF == 20) || (DF == 21) || (DF == 22) || (DF == 24) ) { thisDFbit = (1 << thisDF);
// if this DF any more likely?
if (validDFbits & thisDFbit) {
// Yep, more likely, so update the main message // Yep, more likely, so update the main message
msg[0] = theByte; msg[0] = theByte;
Modes.stat_DF_Type_Corrected++; Modes.stat_DF_Type_Corrected++;