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CRC reimplementation to be byte-at-a-time table-driven.

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Also be smarter about error correction, avoiding ambiguous cases
when --aggressive is specified.
This commit is contained in:
Oliver Jowett 2015-01-20 23:04:05 +00:00
parent 0d3fcfff2e
commit 270a22e06a
8 changed files with 438 additions and 237 deletions
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7
Makefile
View file

@ -28,10 +28,13 @@ view1090: view1090.o anet.o interactive.o mode_ac.o mode_s.o net_io.o crc.o stat
$(CC) -g -o $@ $^ $(LIBS) $(LDFLAGS)
clean:
rm -f *.o dump1090 view1090 cprtests
rm -f *.o dump1090 view1090 cprtests crctests
test: cprtests
$(PWD)/cprtests
cprtests: cpr.o cprtests.o
$(CC) -g -o $@ $^ $(LIBS) $(LDFLAGS)
$(CC) $(CPPFLAGS) $(CFLAGS) $(EXTRACFLAGS) -g -o $@ $^ -lm
crctests: crc.c crc.h
$(CC) $(CPPFLAGS) $(CFLAGS) $(EXTRACFLAGS) -g -DCRCDEBUG -o $@ $<

627
crc.c
View file

@ -1,245 +1,434 @@
// dump1090, a Mode S messages decoder for RTLSDR devices.
// Part of dump1090, a Mode S message decoder for RTLSDR devices.
//
// Copyright (C) 2012 by Salvatore Sanfilippo <antirez@gmail.com>
// crc.h: Mode S CRC calculation and error correction.
//
// All rights reserved.
// Copyright (c) 2014,2015 Oliver Jowett <oliver@mutability.co.uk>
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
// This file is free software: you may copy, redistribute and/or modify it
// under the terms of the GNU General Public License as published by the
// Free Software Foundation, either version 2 of the License, or (at your
// option) any later version.
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//
// * Redistributions in binary form must reproduce the above copyright
// notice, this list of conditions and the following disclaimer in the
// documentation and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
// This file is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program. If not, see <http://www.gnu.org/licenses/>.
#include "dump1090.h"
// Parity table for MODE S Messages.
// The table contains 112 elements, every element corresponds to a bit set
// in the message, starting from the first bit of actual data after the
// preamble.
//
// For messages of 112 bit, the whole table is used.
// For messages of 56 bits only the last 56 elements are used.
//
// The algorithm is as simple as xoring all the elements in this table
// for which the corresponding bit on the message is set to 1.
//
// The latest 24 elements in this table are set to 0 as the checksum at the
// end of the message should not affect the computation.
//
// Note: this function can be used with DF11 and DF17, other modes have
// the CRC xored with the sender address as they are reply to interrogations,
// but a casual listener can't split the address from the checksum.
//
uint32_t modes_checksum_table[112] = {
0x3935ea, 0x1c9af5, 0xf1b77e, 0x78dbbf, 0xc397db, 0x9e31e9, 0xb0e2f0, 0x587178,
0x2c38bc, 0x161c5e, 0x0b0e2f, 0xfa7d13, 0x82c48d, 0xbe9842, 0x5f4c21, 0xd05c14,
0x682e0a, 0x341705, 0xe5f186, 0x72f8c3, 0xc68665, 0x9cb936, 0x4e5c9b, 0xd8d449,
0x939020, 0x49c810, 0x24e408, 0x127204, 0x093902, 0x049c81, 0xfdb444, 0x7eda22,
0x3f6d11, 0xe04c8c, 0x702646, 0x381323, 0xe3f395, 0x8e03ce, 0x4701e7, 0xdc7af7,
0x91c77f, 0xb719bb, 0xa476d9, 0xadc168, 0x56e0b4, 0x2b705a, 0x15b82d, 0xf52612,
0x7a9309, 0xc2b380, 0x6159c0, 0x30ace0, 0x185670, 0x0c2b38, 0x06159c, 0x030ace,
0x018567, 0xff38b7, 0x80665f, 0xbfc92b, 0xa01e91, 0xaff54c, 0x57faa6, 0x2bfd53,
0xea04ad, 0x8af852, 0x457c29, 0xdd4410, 0x6ea208, 0x375104, 0x1ba882, 0x0dd441,
0xf91024, 0x7c8812, 0x3e4409, 0xe0d800, 0x706c00, 0x383600, 0x1c1b00, 0x0e0d80,
0x0706c0, 0x038360, 0x01c1b0, 0x00e0d8, 0x00706c, 0x003836, 0x001c1b, 0xfff409,
0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000,
0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000,
0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000
};
#include <assert.h>
uint32_t modesChecksum(unsigned char *msg, int bits) {
uint32_t crc = 0;
uint32_t rem = 0;
int offset = (bits == 112) ? 0 : (112-56);
uint8_t theByte = *msg;
uint32_t * pCRCTable = &modes_checksum_table[offset];
// Generator polynomial for the Mode S CRC:
#define MODES_GENERATOR_POLY 0xfff409U
// CRC values for all single-byte messages;
// used to speed up CRC calculation.
static uint32_t crc_table[256];
// Syndrome values for all single-bit errors;
// used to speed up construction of error-
// correction tables.
static uint32_t single_bit_syndrome[112];
static void initLookupTables()
{
int i;
uint8_t msg[112/8];
for (i = 0; i < 256; ++i) {
uint32_t c = i << 16;
int j;
// We don't really need to include the checksum itself
bits -= 24;
for(j = 0; j < bits; j++) {
if ((j & 7) == 0)
theByte = *msg++;
// If bit is set, xor with corresponding table entry.
if (theByte & 0x80) {crc ^= *pCRCTable;}
pCRCTable++;
theByte = theByte << 1;
for (j = 0; j < 8; ++j) {
if (c & 0x800000)
c = (c<<1) ^ MODES_GENERATOR_POLY;
else
c = (c<<1);
}
rem = (msg[0] << 16) | (msg[1] << 8) | msg[2]; // message checksum
return ((crc ^ rem) & 0x00FFFFFF); // 24 bit checksum syndrome.
crc_table[i] = c & 0x00ffffff;
}
memset(msg, 0, sizeof(msg));
for (i = 0; i < 112; ++i) {
msg[i/8] ^= 1 << (7 - (i & 7));
single_bit_syndrome[i] = modesChecksum(msg, 112);
msg[i/8] ^= 1 << (7 - (i & 7));
}
}
//=========================================================================
//
// Code for introducing a less CPU-intensive method of correcting
// single bit errors.
//
// Makes use of the fact that the crc checksum is linear with respect to
// the bitwise xor operation, i.e.
// crc(m^e) = (crc(m)^crc(e)
// where m and e are the message resp. error bit vectors.
//
// Call crc(e) the syndrome.
//
// The code below works by precomputing a table of (crc(e), e) for all
// possible error vectors e (here only single bit and double bit errors),
// search for the syndrome in the table, and correct the then known error.
// The error vector e is represented by one or two bit positions that are
// changed. If a second bit position is not used, it is -1.
//
// Run-time is binary search in a sorted table, plus some constant overhead,
// instead of running through all possible bit positions (resp. pairs of
// bit positions).
//
struct errorinfo {
uint32_t syndrome; // CRC syndrome
int bits; // Number of bit positions to fix
int pos[MODES_MAX_BITERRORS]; // Bit positions corrected by this syndrome
};
uint32_t modesChecksum(uint8_t *message, int bits)
{
uint32_t rem = 0;
int i;
int n = bits/8;
#define NERRORINFO \
(MODES_LONG_MSG_BITS+MODES_LONG_MSG_BITS*(MODES_LONG_MSG_BITS-1)/2)
struct errorinfo bitErrorTable[NERRORINFO];
assert(bits % 8 == 0);
assert(n >= 3);
// Compare function as needed for stdlib's qsort and bsearch functions
static int cmpErrorInfo(const void *p0, const void *p1) {
struct errorinfo *e0 = (struct errorinfo*)p0;
struct errorinfo *e1 = (struct errorinfo*)p1;
if (e0->syndrome == e1->syndrome) {
return 0;
} else if (e0->syndrome < e1->syndrome) {
return -1;
} else {
for (i = 0; i < n-3; ++i) {
rem = (rem << 8) ^ crc_table[message[i] ^ ((rem & 0xff0000) >> 16)];
rem = rem & 0xffffff;
}
rem = rem ^ (message[n-3] << 16) ^ (message[n-2] << 8) ^ (message[n-1]);
return rem;
}
static struct errorinfo *bitErrorTable_short;
static int bitErrorTableSize_short;
static struct errorinfo *bitErrorTable_long;
static int bitErrorTableSize_long;
// compare two errorinfo structures
static int syndrome_compare(const void *x, const void *y) {
struct errorinfo *ex = (struct errorinfo*)x;
struct errorinfo *ey = (struct errorinfo*)y;
return (int)ex->syndrome - (int)ey->syndrome;
}
// (n k), the number of ways of selecting k distinct items from a set of n items
static int combinations(int n, int k)
{
int result = 1, i;
if (k == 0 || k == n)
return 1;
}
}
//
//=========================================================================
//
// Compute the table of all syndromes for 1-bit and 2-bit error vectors
void modesInitErrorInfo() {
unsigned char msg[MODES_LONG_MSG_BYTES];
int i, j, n;
uint32_t crc;
n = 0;
memset(bitErrorTable, 0, sizeof(bitErrorTable));
memset(msg, 0, MODES_LONG_MSG_BYTES);
// Add all possible single and double bit errors
// don't include errors in first 5 bits (DF type)
for (i = 5; i < MODES_LONG_MSG_BITS; i++) {
int bytepos0 = (i >> 3);
int mask0 = 1 << (7 - (i & 7));
msg[bytepos0] ^= mask0; // create error0
crc = modesChecksum(msg, MODES_LONG_MSG_BITS);
bitErrorTable[n].syndrome = crc; // single bit error case
bitErrorTable[n].bits = 1;
bitErrorTable[n].pos[0] = i;
bitErrorTable[n].pos[1] = -1;
n += 1;
if (Modes.nfix_crc > 1) {
for (j = i+1; j < MODES_LONG_MSG_BITS; j++) {
int bytepos1 = (j >> 3);
int mask1 = 1 << (7 - (j & 7));
msg[bytepos1] ^= mask1; // create error1
crc = modesChecksum(msg, MODES_LONG_MSG_BITS);
if (n >= NERRORINFO) {
//fprintf(stderr, "Internal error, too many entries, fix NERRORINFO\n");
if (k > n)
return 0;
for (i = 1; i <= k; ++i) {
result = result * n / i;
n = n - 1;
}
return result;
}
// Recursively populates an errorinfo table with error syndromes
//
// in:
// table: the table to fill
// n: first entry to fill
// maxSize: max size of table
// offset: start bit offset for checksum calculation
// startbit: first bit to introduce errors into
// endbit: (one past) last bit to introduce errors info
// base_entry: template entry to start from
// error_bit: how many error bits have already been set
// max_errors: maximum total error bits to set
// out:
// returns: the next free entry in the table
// table: has been populated between [n, return value)
static int prepareSubtable(struct errorinfo *table, int n, int maxsize, int offset, int startbit, int endbit, struct errorinfo *base_entry, int error_bit, int max_errors)
{
int i = 0;
if (error_bit >= max_errors)
return n;
for (i = startbit; i < endbit; ++i) {
assert(n < maxsize);
table[n] = *base_entry;
table[n].syndrome ^= single_bit_syndrome[i + offset];
table[n].errors = error_bit+1;
table[n].bit[error_bit] = i;
++n;
n = prepareSubtable(table, n, maxsize, offset, i + 1, endbit, &table[n-1], error_bit + 1, max_errors);
}
return n;
}
static int flagCollisions(struct errorinfo *table, int tablesize, int offset, int startbit, int endbit, uint32_t base_syndrome, int error_bit, int first_error, int last_error)
{
int i = 0;
int count = 0;
if (error_bit > last_error)
return 0;
for (i = startbit; i < endbit; ++i) {
struct errorinfo ei;
ei.syndrome = base_syndrome ^ single_bit_syndrome[i + offset];
if (error_bit >= first_error) {
struct errorinfo *collision = bsearch(&ei, table, tablesize, sizeof(struct errorinfo), syndrome_compare);
if (collision != NULL && collision->errors != -1) {
++count;
collision->errors = -1;
}
}
count += flagCollisions(table, tablesize, offset, i+1, endbit, ei.syndrome, error_bit + 1, first_error, last_error);
}
return count;
}
// Allocate and build an error table for messages of length "bits" (max 112)
// returns a pointer to the new table and sets *size_out to the table length
static struct errorinfo *prepareErrorTable(int bits, int max_correct, int max_detect, int *size_out)
{
int maxsize, usedsize;
struct errorinfo *table;
struct errorinfo base_entry;
int i, j;
assert (bits >= 0 && bits <= 112);
assert (max_correct >=0 && max_correct <= MODES_MAX_BITERRORS);
assert (max_detect >= max_correct);
if (!max_correct) {
*size_out = 0;
return NULL;
}
maxsize = 0;
for (i = 1; i <= max_correct; ++i) {
maxsize += combinations(bits - 5, i); // space needed for all i-bit errors
}
#ifdef CRCDEBUG
fprintf(stderr, "Preparing syndrome table to correct up to %d-bit errors (detecting %d-bit errors) in a %d-bit message (max %d entries)\n", max_correct, max_detect, bits, maxsize);
#endif
table = malloc(maxsize * sizeof(struct errorinfo));
base_entry.syndrome = 0;
base_entry.errors = 0;
for (i = 0; i < MODES_MAX_BITERRORS; ++i)
base_entry.bit[i] = -1;
// ignore the first 5 bits (DF type)
usedsize = prepareSubtable(table, 0, maxsize, 112 - bits, 5, bits, &base_entry, 0, max_correct);
#ifdef CRCDEBUG
fprintf(stderr, "%d syndromes (expected %d).\n", usedsize, maxsize);
fprintf(stderr, "Sorting syndromes..\n");
#endif
qsort(table, usedsize, sizeof(struct errorinfo), syndrome_compare);
#ifdef CRCDEBUG
{
// Show the table stats
fprintf(stderr, "Undetectable errors:\n");
for (i = 1; i <= max_correct; ++i) {
int j, count;
count = 0;
for (j = 0; j < usedsize; ++j)
if (table[j].errors == i && table[j].syndrome == 0)
++count;
fprintf(stderr, " %d undetectable %d-bit errors\n", count, i);
}
}
#endif
// Handle ambiguous cases, where there is more than one possible error pattern
// that produces a given syndrome (this happens with >2 bit errors).
#ifdef CRCDEBUG
fprintf(stderr, "Finding collisions..\n");
#endif
for (i = 0, j = 0; i < usedsize; ++i) {
if (i < usedsize-1 && table[i+1].syndrome == table[i].syndrome) {
// skip over this entry and all collisions
while (i < usedsize && table[i+1].syndrome == table[i].syndrome)
++i;
// now table[i] is the last duplicate
continue;
}
if (i != j)
table[j] = table[i];
++j;
}
if (j < usedsize) {
#ifdef CRCDEBUG
fprintf(stderr, "Discarded %d collisions.\n", usedsize - j);
#endif
usedsize = j;
}
// Flag collisions we want to detect but not correct
if (max_detect > max_correct) {
int flagged;
#ifdef CRCDEBUG
fprintf(stderr, "Flagging collisions between %d - %d bits..\n", max_correct+1, max_detect);
#endif
flagged = flagCollisions(table, usedsize, 112 - bits, 5, bits, 0, 1, max_correct+1, max_detect);
#ifdef CRCDEBUG
fprintf(stderr, "Flagged %d collisions for removal.\n", flagged);
#else
#endif
if (flagged > 0) {
for (i = 0, j = 0; i < usedsize; ++i) {
if (table[i].errors != -1) {
if (i != j)
table[j] = table[i];
++j;
}
}
#ifdef CRCDEBUG
fprintf(stderr, "Discarded %d flagged collisions.\n", usedsize - j);
#endif
usedsize = j;
}
}
if (usedsize < maxsize) {
#ifdef CRCDEBUG
fprintf(stderr, "Shrinking table from %d to %d..\n", maxsize, usedsize);
table = realloc(table, usedsize * sizeof(struct errorinfo));
#endif
}
*size_out = usedsize;
#ifdef CRCDEBUG
{
// Check the table.
unsigned char *msg = malloc(bits/8);
for (i = 0; i < usedsize; ++i) {
int j;
struct errorinfo *ei;
uint32_t result;
memset(msg, 0, bits/8);
ei = &table[i];
for (j = 0; j < ei->errors; ++j) {
msg[ei->bit[j] >> 3] ^= 1 << (7 - (ei->bit[j]&7));
}
result = modesChecksum(msg, bits);
if (result != ei->syndrome) {
fprintf(stderr, "PROBLEM: entry %6d/%6d syndrome %06x errors %d bits ", i, usedsize, ei->syndrome, ei->errors);
for (j = 0; j < ei->errors; ++j)
fprintf(stderr, "%3d ", ei->bit[j]);
fprintf(stderr, " checksum %06x\n", result);
}
}
free(msg);
// Show the table stats
fprintf(stderr, "Syndrome table summary:\n");
for (i = 1; i <= max_correct; ++i) {
int j, count, possible;
count = 0;
for (j = 0; j < usedsize; ++j)
if (table[j].errors == i)
++count;
possible = combinations(bits-5, i);
fprintf(stderr, " %d entries for %d-bit errors (%d possible, %d%% coverage)\n", count, i, possible, 100 * count / possible);
}
fprintf(stderr, " %d entries total\n", usedsize);
}
#endif
return table;
}
// Precompute syndrome tables for 56- and 112-bit messages.
void modesChecksumInit(int fixBits)
{
initLookupTables();
switch (fixBits) {
case 0:
bitErrorTable_short = bitErrorTable_long = NULL;
bitErrorTableSize_short = bitErrorTableSize_long = 0;
break;
case 1:
// For 1 bit correction, we have 100% coverage up to 4 bit detection, so don't bother
// with flagging collisions there.
bitErrorTable_short = prepareErrorTable(MODES_SHORT_MSG_BITS, 1, 1, &bitErrorTableSize_short);
bitErrorTable_long = prepareErrorTable(MODES_LONG_MSG_BITS, 1, 1, &bitErrorTableSize_long);
break;
default:
// Detect out to 4 bit errors; this reduces our 2-bit coverage to about 65%.
// This can take a little while - tell the user.
fprintf(stderr, "Preparing error correction tables.. ");
bitErrorTable_short = prepareErrorTable(MODES_SHORT_MSG_BITS, 2, 4, &bitErrorTableSize_short);
bitErrorTable_long = prepareErrorTable(MODES_LONG_MSG_BITS, 2, 4, &bitErrorTableSize_long);
fprintf(stderr, "done.\n");
break;
}
bitErrorTable[n].syndrome = crc; // two bit error case
bitErrorTable[n].bits = 2;
bitErrorTable[n].pos[0] = i;
bitErrorTable[n].pos[1] = j;
n += 1;
msg[bytepos1] ^= mask1; // revert error1
}
}
msg[bytepos0] ^= mask0; // revert error0
}
qsort(bitErrorTable, NERRORINFO, sizeof(struct errorinfo), cmpErrorInfo);
// Test code: report if any syndrome appears at least twice. In this
// case the correction cannot be done without ambiguity.
// Tried it, does not happen for 1- and 2-bit errors.
/*
for (i = 1; i < NERRORINFO; i++) {
if (bitErrorTable[i-1].syndrome == bitErrorTable[i].syndrome) {
fprintf(stderr, "modesInitErrorInfo: Collision for syndrome %06x\n",
(int)bitErrorTable[i].syndrome);
}
}
for (i = 0; i < NERRORINFO; i++) {
printf("syndrome %06x bit0 %3d bit1 %3d\n",
bitErrorTable[i].syndrome,
bitErrorTable[i].pos0, bitErrorTable[i].pos1);
}
*/
}
//
//=========================================================================
//
// Search for syndrome in table and if an entry is found, flip the necessary
// bits. Make sure the indices fit into the array
// Additional parameter: fix only less than maxcorrected bits, and record
// fixed bit positions in corrected[]. This array can be NULL, otherwise
// must be of length at least maxcorrected.
// Return number of fixed bits.
//
int fixBitErrors(unsigned char *msg, int bits, int maxfix, char *fixedbits) {
struct errorinfo *pei;
// Given an error syndrome and message length, return
// an error-correction descriptor, or NULL if the
// syndrome is uncorrectable
struct errorinfo *modesChecksumDiagnose(uint32_t syndrome, int bitlen)
{
struct errorinfo *table;
int tablesize;
struct errorinfo ei;
int bitpos, offset, res, i;
memset(&ei, 0, sizeof(struct errorinfo));
ei.syndrome = modesChecksum(msg, bits);
pei = bsearch(&ei, bitErrorTable, NERRORINFO,
sizeof(struct errorinfo), cmpErrorInfo);
if (pei == NULL) {
return 0; // No syndrome found
}
// Check if the syndrome fixes more bits than we allow
if (maxfix < pei->bits) {
return 0;
}
if (syndrome == 0)
return NULL; // no errors
// Check that all bit positions lie inside the message length
offset = MODES_LONG_MSG_BITS-bits;
for (i = 0; i < pei->bits; i++) {
bitpos = pei->pos[i] - offset;
if ((bitpos < 0) || (bitpos >= bits)) {
return 0;
}
}
assert (bitlen == 56 || bitlen == 112);
if (bitlen == 56) { table = bitErrorTable_short; tablesize = bitErrorTableSize_short; }
else { table = bitErrorTable_long; tablesize = bitErrorTableSize_long; }
// Fix the bits
for (i = res = 0; i < pei->bits; i++) {
bitpos = pei->pos[i] - offset;
msg[bitpos >> 3] ^= (1 << (7 - (bitpos & 7)));
if (fixedbits) {
fixedbits[res++] = bitpos;
}
}
return res;
if (!table)
return NULL;
ei.syndrome = syndrome;
return bsearch(&ei, table, tablesize, sizeof(struct errorinfo), syndrome_compare);
}
// Given a message and an error-correction descriptor,
// apply the error correction to the given message.
void modesChecksumFix(uint8_t *msg, struct errorinfo *info)
{
int i;
if (!info)
return;
for (i = 0; i < info->errors; ++i)
msg[info->bit[i] >> 3] ^= 1 << (7 - (info->bit[i] & 7));
}
#ifdef CRCDEBUG
int main(int argc, char **argv)
{
int len;
if (argc < 3) {
fprintf(stderr, "syntax: crctests <ncorrect> <ndetect>\n");
return 1;
}
initLookupTables();
prepareErrorTable(MODES_SHORT_MSG_BITS, atoi(argv[1]), atoi(argv[2]), &len);
prepareErrorTable(MODES_LONG_MSG_BITS, atoi(argv[1]), atoi(argv[2]), &len);
return 0;
}
#endif

16
crc.h
View file

@ -22,8 +22,18 @@
#include <stdint.h>
void modesInitErrorInfo();
uint32_t modesChecksum(unsigned char *msg, int bits);
int fixBitErrors(unsigned char *msg, int bits, int maxfix, char *fixedbits);
// Global max for fixable bit erros
#define MODES_MAX_BITERRORS 2
struct errorinfo {
uint32_t syndrome; // CRC syndrome
int errors; // number of errors
int8_t bit[MODES_MAX_BITERRORS]; // bit positions to fix (-1 = no bit)
};
void modesChecksumInit(int fixBits);
uint32_t modesChecksum(uint8_t *msg, int bitlen);
struct errorinfo *modesChecksumDiagnose(uint32_t syndrome, int bitlen);
void modesChecksumFix(uint8_t *msg, struct errorinfo *info);
#endif

23
demod_2000.c
View file

@ -109,7 +109,7 @@ static void dumpMagnitudeVector(uint16_t *m, uint32_t offset) {
// loadable by debug.html.
//
static void dumpRawMessageJS(char *descr, unsigned char *msg,
uint16_t *m, uint32_t offset, int fixable, char *bitpos)
uint16_t *m, uint32_t offset, struct errorinfo *ei)
{
int padding = 5; // Show a few samples before the actual start.
int start = offset - padding;
@ -117,7 +117,6 @@ static void dumpRawMessageJS(char *descr, unsigned char *msg,
FILE *fp;
int j;
MODES_NOTUSED(fixable);
if ((fp = fopen("frames.js","a")) == NULL) {
fprintf(stderr, "Error opening frames.js: %s\n", strerror(errno));
exit(1);
@ -128,8 +127,10 @@ static void dumpRawMessageJS(char *descr, unsigned char *msg,
fprintf(fp,"%d", j < 0 ? 0 : m[j]);
if (j != end) fprintf(fp,",");
}
fprintf(fp,"], \"fix1\": %d, \"fix2\": %d, \"bits\": %d, \"hex\": \"",
bitpos[0], bitpos[1] , modesMessageLenByType(msg[0]>>3));
fprintf(fp, "], ");
for (j = 0; j < MODES_MAX_BITERRORS; ++j)
fprintf(fp,"\"fix%d\": %d, ", j, ei->bit[j]);
fprintf(fp, "\"bits\": %d, \"hex\": \"", modesMessageLenByType(msg[0]>>3));
for (j = 0; j < MODES_LONG_MSG_BYTES; j++)
fprintf(fp,"\\x%02x",msg[j]);
fprintf(fp,"\"});\n");
@ -153,18 +154,16 @@ static void dumpRawMessageJS(char *descr, unsigned char *msg,
static void dumpRawMessage(char *descr, unsigned char *msg, uint16_t *m, uint32_t offset) {
int j;
int msgtype = msg[0] >> 3;
int fixable = 0;
char bitpos[MODES_MAX_BITERRORS];
struct errorinfo *ei = NULL;
for (j = 0; j < MODES_MAX_BITERRORS; j++) {
bitpos[j] = -1;
}
if (msgtype == 17) {
fixable = fixBitErrors(msg, MODES_LONG_MSG_BITS, MODES_MAX_BITERRORS, bitpos);
int len = modesMessageLenByType(msgtype);
uint32_t csum = modesChecksum(msg, len);
ei = modesChecksumDiagnose(csum, len);
}
if (Modes.debug & MODES_DEBUG_JS) {
dumpRawMessageJS(descr, msg, m, offset, fixable, bitpos);
dumpRawMessageJS(descr, msg, m, offset, ei);
return;
}
@ -173,7 +172,7 @@ static void dumpRawMessage(char *descr, unsigned char *msg, uint16_t *m, uint32_
printf("%02x",msg[j]);
if (j == MODES_SHORT_MSG_BYTES-1) printf(" ... ");
}
printf(" (DF %d, Fixable: %d)\n", msgtype, fixable);
printf(" (DF %d, Fixable: %d)\n", msgtype, ei ? ei->errors : 0);
dumpMagnitudeVector(m,offset);
printf("---\n\n");
}

2
dump1090.c
View file

@ -217,7 +217,7 @@ void modesInit(void) {
}
// Prepare error correction tables
modesInitErrorInfo();
modesChecksumInit(Modes.nfix_crc);
}
//
// =============================== RTLSDR handling ==========================

4
dump1090.h
View file

@ -118,9 +118,6 @@
#define MODES_MSG_SQUELCH_DB 4.0 // Minimum SNR, in dB
#define MODES_MSG_ENCODER_ERRS 3 // Maximum number of encoding errors
// When changing, change also fixBitErrors() and modesInitErrorTable() !!
#define MODES_MAX_BITERRORS 2 // Global max for fixable bit erros
#define MODES_MAX_PHASE_STATS 10
#define MODEAC_MSG_SAMPLES (25 * 2) // include up to the SPI bit
@ -471,7 +468,6 @@ void decodeModesMessage (struct modesMessage *mm, unsigned char *msg);
void displayModesMessage(struct modesMessage *mm);
void useModesMessage (struct modesMessage *mm);
void computeMagnitudeVector(uint16_t *pData);
void modesInitErrorInfo ();
//
// Functions exported from interactive.c
//

6
mode_s.c
View file

@ -333,7 +333,11 @@ void decodeModesMessage(struct modesMessage *mm, unsigned char *msg) {
// using the results. Perhaps check the ICAO against known aircraft, and check
// IID against known good IID's. That's a TODO.
//
mm->correctedbits = fixBitErrors(msg, mm->msgbits, Modes.nfix_crc, mm->corrected);
struct errorinfo *ei = modesChecksumDiagnose(mm->crc, mm->msgbits);
if (ei != NULL && ei->errors <= Modes.nfix_crc) {
modesChecksumFix(msg, ei);
mm->correctedbits = ei->errors;
}
// If we correct, validate ICAO addr to help filter birthday paradox solutions.
if (mm->correctedbits) {

2
view1090.c
View file

@ -126,7 +126,7 @@ void view1090Init(void) {
}
// Prepare error correction tables
modesInitErrorInfo();
modesChecksumInit(Modes.nfix_crc);
}
// Set up data connection