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VK1ET : Bugfix in Error correction table

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vk1etI believe there is an error in the way the CRC error syndromes are
being created in the two bit case. The inner loop was introducing an
extra error bit each time through rather than moving it. Below is the
modified code fragment for syndrome creation collapsedSun 07:26
19 May 2013 07:26vk1et [notifications@github.com]Actions
To:Mantirez/dump1090 ‎[dump1090@noreply.github.com]‎Cc:MMalcolmRobb
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Bugfix first noted by VK1ET, and posted here :
https://github.com/antirez/dump1090/pull/23#issuecomment-18113094

I believe there is an error in the way the CRC error syndromes are being
created in the two bit case. The inner loop was introducing an extra
error bit each time through rather than moving it.
This commit is contained in:
Malcolm Robb 2013-05-21 11:28:56 +01:00
parent bd39479e70
commit 92fd06bcd6
1 changed files with 14 additions and 18 deletions
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30
dump1090.c
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@ -56,7 +56,7 @@
// MinorVer changes when additional features are added, but not for bug fixes (range 00-99)
// DayDate & Year changes for all changes, including for bug fixes. It represent the release date of the update
//
#define MODES_DUMP1090_VERSION "1.06.1405.13"
#define MODES_DUMP1090_VERSION "1.06.2105.13"
#define MODES_USER_LATITUDE_DFLT (0.0)
#define MODES_USER_LONGITUDE_DFLT (0.0)
@ -1258,28 +1258,22 @@ void modesInitErrorInfo() {
uint32_t crc;
n = 0;
memset(bitErrorTable, 0, sizeof(bitErrorTable));
/* First, insert infos about all possible single bit errors */
for (i = 0; i < MODES_LONG_MSG_BITS; i++) {
int bytepos = (i >> 3);
int mask = 1 << (7 - (i & 7));
memset(msg, 0, MODES_LONG_MSG_BYTES);
msg[bytepos] ^= mask;
crc = modesChecksum(msg, MODES_LONG_MSG_BITS);
bitErrorTable[n].syndrome = crc;
bitErrorTable[n].pos0 = i;
bitErrorTable[n].pos1 = -1;
n += 1;
}
/* Add also all double bit errors */
/* Add all possible single and double bit errors */
for (i = 0; i < MODES_LONG_MSG_BITS; i++) {
int bytepos0 = (i >> 3);
int mask0 = 1 << (7 - (i & 7));
memset(msg, 0, MODES_LONG_MSG_BYTES);
msg[bytepos0] ^= mask0;
msg[bytepos0] ^= mask0; // create error0
crc = modesChecksum(msg, MODES_LONG_MSG_BITS);
bitErrorTable[n].syndrome = crc; // single bit error case
bitErrorTable[n].pos0 = i;
bitErrorTable[n].pos1 = -1;
n += 1;
for (j = i+1; j < MODES_LONG_MSG_BITS; j++) {
int bytepos1 = (j >> 3);
int mask1 = 1 << (7 - (j & 7));
msg[bytepos1] ^= mask1;
msg[bytepos1] ^= mask1; // create error1
crc = modesChecksum(msg, MODES_LONG_MSG_BITS);
if (n >= NERRORINFO) {
/*
@ -1289,11 +1283,13 @@ void modesInitErrorInfo() {
*/
break;
}
bitErrorTable[n].syndrome = crc;
bitErrorTable[n].syndrome = crc; // two bit error case
bitErrorTable[n].pos0 = i;
bitErrorTable[n].pos1 = j;
n += 1;
msg[bytepos1] ^= mask1; // revert error1
}
msg[bytepos0] ^= mask0; // revert error0
}
qsort(bitErrorTable, NERRORINFO,
sizeof(struct errorinfo), cmpErrorInfo);