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Speed up Error Correction

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Move memcopy to outside the main bit loop, and just flip the modified
bit back at the end of each loop if it didn;t work

Pass in a pointer to the mm structure being corrected, and fix-up the
crc with the value inside the function, rather than re-calculate on
return
This commit is contained in:
Malcolm Robb 2013-04-15 22:01:56 +01:00
parent bdf985a792
commit 6619d21970
1 changed files with 33 additions and 20 deletions
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53
dump1090.c
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@ -280,8 +280,8 @@ void modesSendRawOutput(struct modesMessage *mm);
void modesSendBeastOutput(struct modesMessage *mm);
void modesSendSBSOutput(struct modesMessage *mm, struct aircraft *a);
void useModesMessage(struct modesMessage *mm);
int fixSingleBitErrors(unsigned char *msg, int bits);
int fixTwoBitsErrors(unsigned char *msg, int bits);
int fixSingleBitErrors(unsigned char *msg, int bits, struct modesMessage *mm);
int fixTwoBitsErrors(unsigned char *msg, int bits, struct modesMessage *mm);
int modesMessageLenByType(int type);
/* ============================= Utility functions ========================== */
@ -675,9 +675,9 @@ void dumpRawMessage(char *descr, unsigned char *msg,
if (msgtype == 11 || msgtype == 17) {
int msgbits = (msgtype == 11) ? MODES_SHORT_MSG_BITS :
MODES_LONG_MSG_BITS;
fixable = fixSingleBitErrors(msg,msgbits);
fixable = fixSingleBitErrors(msg, msgbits, NULL);
if (fixable == -1)
fixable = fixTwoBitsErrors(msg,msgbits);
fixable = fixTwoBitsErrors(msg, msgbits, NULL);
}
if (Modes.debug & MODES_DEBUG_JS) {
@ -765,16 +765,17 @@ int modesMessageLenByType(int type) {
/* Try to fix single bit errors using the checksum. On success modifies
* the original buffer with the fixed version, and returns the position
* of the error bit. Otherwise if fixing failed -1 is returned. */
int fixSingleBitErrors(unsigned char *msg, int bits) {
int fixSingleBitErrors(unsigned char *msg, int bits, struct modesMessage *mm) {
int j;
unsigned char aux[MODES_LONG_MSG_BYTES];
memcpy(aux, msg,bits/8);
for (j = 0; j < bits; j++) {
int byte = j/8;
int bitmask = 1 << (7-(j%8));
uint32_t crc1, crc2;
memcpy(aux,msg,bits/8);
aux[byte] ^= bitmask; /* Flip j-th bit. */
crc1 = ((uint32_t)aux[(bits/8)-3] << 16) |
@ -787,8 +788,16 @@ int fixSingleBitErrors(unsigned char *msg, int bits) {
* the corrected sequence, and returns the error bit
* position. */
memcpy(msg,aux,bits/8);
if (mm)
{
mm->crc = crc2;
mm->iid = 0;
mm->crcok = 1;
}
return j;
}
aux[byte] ^= bitmask; /* Flip j-th bit back again. */
}
return -1;
}
@ -796,13 +805,16 @@ int fixSingleBitErrors(unsigned char *msg, int bits) {
/* Similar to fixSingleBitErrors() but try every possible two bit combination.
* This is very slow and should be tried only against DF17 messages that
* don't pass the checksum, and only in Aggressive Mode. */
int fixTwoBitsErrors(unsigned char *msg, int bits) {
int fixTwoBitsErrors(unsigned char *msg, int bits, struct modesMessage *mm) {
int j, i;
unsigned char aux[MODES_LONG_MSG_BYTES];
memcpy(aux,msg, bits/8);
for (j = 0; j < bits; j++) {
int byte1 = j/8;
int bitmask1 = 1 << (7-(j%8));
aux[byte1] ^= bitmask1; /* Flip j-th bit. */
/* Don't check the same pairs multiple times, so i starts from j+1 */
for (i = j+1; i < bits; i++) {
@ -810,9 +822,6 @@ int fixTwoBitsErrors(unsigned char *msg, int bits) {
int bitmask2 = 1 << (7-(i%8));
uint32_t crc1, crc2;
memcpy(aux,msg,bits/8);
aux[byte1] ^= bitmask1; /* Flip j-th bit. */
aux[byte2] ^= bitmask2; /* Flip i-th bit. */
crc1 = ((uint32_t)aux[(bits/8)-3] << 16) |
@ -825,11 +834,21 @@ int fixTwoBitsErrors(unsigned char *msg, int bits) {
* the corrected sequence, and returns the error bit
* position. */
memcpy(msg,aux,bits/8);
if (mm)
{
mm->crc = crc2;
mm->iid = 0;
mm->crcok = 1;
}
/* We return the two bits as a 16 bit integer by shifting
* 'i' on the left. This is possible since 'i' will always
* be non-zero because i starts from j+1. */
return j | (i<<8);
aux[byte2] ^= bitmask2; /* Flip i-th bit back. */
}
aux[byte1] ^= bitmask1; /* Flip j-th bit back. */
}
}
return -1;
@ -1052,16 +1071,10 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) {
else
{mm->crcok = (mm->iid == 0);}
if (!mm->crcok && Modes.fix_errors && (mm->msgtype == 17))
{
if ((mm->errorbit = fixSingleBitErrors(msg,mm->msgbits)) != -1) {
mm->crc = modesChecksum(msg,mm->msgbits);
mm->crcok = 1;
} else if (Modes.aggressive &&
(mm->errorbit = fixTwoBitsErrors(msg,mm->msgbits)) != -1)
{
mm->crc = modesChecksum(msg,mm->msgbits);
mm->crcok = 1;
if (!mm->crcok && Modes.fix_errors && (mm->msgtype == 17)){
mm->errorbit = fixSingleBitErrors(msg, mm->msgbits, mm);
if ((mm->errorbit == -1) && (Modes.aggressive)) {
mm->errorbit = fixTwoBitsErrors(msg, mm->msgbits, mm);
}
}