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demod_2400.c
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demod_2400.c
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// Part of dump1090, a Mode S message decoder for RTLSDR devices.
//
// demod_2400.c: 2.4MHz Mode S demodulator.
//
// Copyright (c) 2014,2015 Oliver Jowett <[email protected]>
//
// 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.
//
// 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"
#include <assert.h>
#ifdef MODEAC_DEBUG
#include <gd.h>
#endif
// 2.4MHz sampling rate version
//
// When sampling at 2.4MHz we have exactly 6 samples per 5 symbols.
// Each symbol is 500ns wide, each sample is 416.7ns wide
//
// We maintain a phase offset that is expressed in units of 1/5 of a sample i.e. 1/6 of a symbol, 83.333ns
// Each symbol we process advances the phase offset by 6 i.e. 6/5 of a sample, 500ns
//
// The correlation functions below correlate a 1-0 pair of symbols (i.e. manchester encoded 1 bit)
// starting at the given sample, and assuming that the symbol starts at a fixed 0-5 phase offset within
// m[0]. They return a correlation value, generally interpreted as >0 = 1 bit, <0 = 0 bit
// TODO check if there are better (or more balanced) correlation functions to use here
// nb: the correlation functions sum to zero, so we do not need to adjust for the DC offset in the input signal
// (adding any constant value to all of m[0..3] does not change the result)
static inline int slice_phase0(uint16_t *m) {
return 5 * m[0] - 3 * m[1] - 2 * m[2];
}
static inline int slice_phase1(uint16_t *m) {
return 4 * m[0] - m[1] - 3 * m[2];
}
static inline int slice_phase2(uint16_t *m) {
return 3 * m[0] + m[1] - 4 * m[2];
}
static inline int slice_phase3(uint16_t *m) {
return 2 * m[0] + 3 * m[1] - 5 * m[2];
}
static inline int slice_phase4(uint16_t *m) {
return m[0] + 5 * m[1] - 5 * m[2] - m[3];
}
//
// Given 'mlen' magnitude samples in 'm', sampled at 2.4MHz,
// try to demodulate some Mode S messages.
//
void demodulate2400(struct mag_buf *mag)
{
static struct modesMessage zeroMessage;
struct modesMessage mm;
unsigned char msg1[MODES_LONG_MSG_BYTES], msg2[MODES_LONG_MSG_BYTES], *msg;
uint32_t j;
unsigned char *bestmsg;
int bestscore, bestphase;
uint16_t *m = mag->data;
uint32_t mlen = mag->length;
uint64_t sum_scaled_signal_power = 0;
msg = msg1;
for (j = 0; j < mlen; j++) {
uint16_t *preamble = &m[j];
int high;
uint32_t base_signal, base_noise;
int try_phase;
int msglen;
// Look for a message starting at around sample 0 with phase offset 3..7
// Ideal sample values for preambles with different phase
// Xn is the first data symbol with phase offset N
//
// sample#: 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0
// phase 3: 2/4\0/5\1 0 0 0 0/5\1/3 3\0 0 0 0 0 0 X4
// phase 4: 1/5\0/4\2 0 0 0 0/4\2 2/4\0 0 0 0 0 0 0 X0
// phase 5: 0/5\1/3 3\0 0 0 0/3 3\1/5\0 0 0 0 0 0 0 X1
// phase 6: 0/4\2 2/4\0 0 0 0 2/4\0/5\1 0 0 0 0 0 0 X2
// phase 7: 0/3 3\1/5\0 0 0 0 1/5\0/4\2 0 0 0 0 0 0 X3
//
// quick check: we must have a rising edge 0->1 and a falling edge 12->13
if (! (preamble[0] < preamble[1] && preamble[12] > preamble[13]) )
continue;
if (preamble[1] > preamble[2] && // 1
preamble[2] < preamble[3] && preamble[3] > preamble[4] && // 3
preamble[8] < preamble[9] && preamble[9] > preamble[10] && // 9
preamble[10] < preamble[11]) { // 11-12
// peaks at 1,3,9,11-12: phase 3
high = (preamble[1] + preamble[3] + preamble[9] + preamble[11] + preamble[12]) / 4;
base_signal = preamble[1] + preamble[3] + preamble[9];
base_noise = preamble[5] + preamble[6] + preamble[7];
} else if (preamble[1] > preamble[2] && // 1
preamble[2] < preamble[3] && preamble[3] > preamble[4] && // 3
preamble[8] < preamble[9] && preamble[9] > preamble[10] && // 9
preamble[11] < preamble[12]) { // 12
// peaks at 1,3,9,12: phase 4
high = (preamble[1] + preamble[3] + preamble[9] + preamble[12]) / 4;
base_signal = preamble[1] + preamble[3] + preamble[9] + preamble[12];
base_noise = preamble[5] + preamble[6] + preamble[7] + preamble[8];
} else if (preamble[1] > preamble[2] && // 1
preamble[2] < preamble[3] && preamble[4] > preamble[5] && // 3-4
preamble[8] < preamble[9] && preamble[10] > preamble[11] && // 9-10
preamble[11] < preamble[12]) { // 12
// peaks at 1,3-4,9-10,12: phase 5
high = (preamble[1] + preamble[3] + preamble[4] + preamble[9] + preamble[10] + preamble[12]) / 4;
base_signal = preamble[1] + preamble[12];
base_noise = preamble[6] + preamble[7];
} else if (preamble[1] > preamble[2] && // 1
preamble[3] < preamble[4] && preamble[4] > preamble[5] && // 4
preamble[9] < preamble[10] && preamble[10] > preamble[11] && // 10
preamble[11] < preamble[12]) { // 12
// peaks at 1,4,10,12: phase 6
high = (preamble[1] + preamble[4] + preamble[10] + preamble[12]) / 4;
base_signal = preamble[1] + preamble[4] + preamble[10] + preamble[12];
base_noise = preamble[5] + preamble[6] + preamble[7] + preamble[8];
} else if (preamble[2] > preamble[3] && // 1-2
preamble[3] < preamble[4] && preamble[4] > preamble[5] && // 4
preamble[9] < preamble[10] && preamble[10] > preamble[11] && // 10
preamble[11] < preamble[12]) { // 12
// peaks at 1-2,4,10,12: phase 7
high = (preamble[1] + preamble[2] + preamble[4] + preamble[10] + preamble[12]) / 4;
base_signal = preamble[4] + preamble[10] + preamble[12];
base_noise = preamble[6] + preamble[7] + preamble[8];
} else {
// no suitable peaks
continue;
}
// Check for enough signal
if (base_signal * 2 < 3 * base_noise) // about 3.5dB SNR
continue;
// Check that the "quiet" bits 6,7,15,16,17 are actually quiet
if (preamble[5] >= high ||
preamble[6] >= high ||
preamble[7] >= high ||
preamble[8] >= high ||
preamble[14] >= high ||
preamble[15] >= high ||
preamble[16] >= high ||
preamble[17] >= high ||
preamble[18] >= high) {
continue;
}
// try all phases
Modes.stats_current.demod_preambles++;
bestmsg = NULL; bestscore = -2; bestphase = -1;
for (try_phase = 4; try_phase <= 8; ++try_phase) {
uint16_t *pPtr;
int phase, i, score, bytelen;
// Decode all the next 112 bits, regardless of the actual message
// size. We'll check the actual message type later
pPtr = &m[j+19] + (try_phase/5);
phase = try_phase % 5;
bytelen = MODES_LONG_MSG_BYTES;
for (i = 0; i < bytelen; ++i) {
uint8_t theByte = 0;
switch (phase) {
case 0:
theByte =
(slice_phase0(pPtr) > 0 ? 0x80 : 0) |
(slice_phase2(pPtr+2) > 0 ? 0x40 : 0) |
(slice_phase4(pPtr+4) > 0 ? 0x20 : 0) |
(slice_phase1(pPtr+7) > 0 ? 0x10 : 0) |
(slice_phase3(pPtr+9) > 0 ? 0x08 : 0) |
(slice_phase0(pPtr+12) > 0 ? 0x04 : 0) |
(slice_phase2(pPtr+14) > 0 ? 0x02 : 0) |
(slice_phase4(pPtr+16) > 0 ? 0x01 : 0);
phase = 1;
pPtr += 19;
break;
case 1:
theByte =
(slice_phase1(pPtr) > 0 ? 0x80 : 0) |
(slice_phase3(pPtr+2) > 0 ? 0x40 : 0) |
(slice_phase0(pPtr+5) > 0 ? 0x20 : 0) |
(slice_phase2(pPtr+7) > 0 ? 0x10 : 0) |
(slice_phase4(pPtr+9) > 0 ? 0x08 : 0) |
(slice_phase1(pPtr+12) > 0 ? 0x04 : 0) |
(slice_phase3(pPtr+14) > 0 ? 0x02 : 0) |
(slice_phase0(pPtr+17) > 0 ? 0x01 : 0);
phase = 2;
pPtr += 19;
break;
case 2:
theByte =
(slice_phase2(pPtr) > 0 ? 0x80 : 0) |
(slice_phase4(pPtr+2) > 0 ? 0x40 : 0) |
(slice_phase1(pPtr+5) > 0 ? 0x20 : 0) |
(slice_phase3(pPtr+7) > 0 ? 0x10 : 0) |
(slice_phase0(pPtr+10) > 0 ? 0x08 : 0) |
(slice_phase2(pPtr+12) > 0 ? 0x04 : 0) |
(slice_phase4(pPtr+14) > 0 ? 0x02 : 0) |
(slice_phase1(pPtr+17) > 0 ? 0x01 : 0);
phase = 3;
pPtr += 19;
break;
case 3:
theByte =
(slice_phase3(pPtr) > 0 ? 0x80 : 0) |
(slice_phase0(pPtr+3) > 0 ? 0x40 : 0) |
(slice_phase2(pPtr+5) > 0 ? 0x20 : 0) |
(slice_phase4(pPtr+7) > 0 ? 0x10 : 0) |
(slice_phase1(pPtr+10) > 0 ? 0x08 : 0) |
(slice_phase3(pPtr+12) > 0 ? 0x04 : 0) |
(slice_phase0(pPtr+15) > 0 ? 0x02 : 0) |
(slice_phase2(pPtr+17) > 0 ? 0x01 : 0);
phase = 4;
pPtr += 19;
break;
case 4:
theByte =
(slice_phase4(pPtr) > 0 ? 0x80 : 0) |
(slice_phase1(pPtr+3) > 0 ? 0x40 : 0) |
(slice_phase3(pPtr+5) > 0 ? 0x20 : 0) |
(slice_phase0(pPtr+8) > 0 ? 0x10 : 0) |
(slice_phase2(pPtr+10) > 0 ? 0x08 : 0) |
(slice_phase4(pPtr+12) > 0 ? 0x04 : 0) |
(slice_phase1(pPtr+15) > 0 ? 0x02 : 0) |
(slice_phase3(pPtr+17) > 0 ? 0x01 : 0);
phase = 0;
pPtr += 20;
break;
}
msg[i] = theByte;
if (i == 0) {
switch (msg[0] >> 3) {
case 0: case 4: case 5: case 11:
bytelen = MODES_SHORT_MSG_BYTES; break;
case 16: case 17: case 18: case 20: case 21: case 24:
break;
default:
bytelen = 1; // unknown DF, give up immediately
break;
}
}
}
// Score the mode S message and see if it's any good.
score = scoreModesMessage(msg, i*8);
if (score > bestscore) {
// new high score!
bestmsg = msg;
bestscore = score;
bestphase = try_phase;
// swap to using the other buffer so we don't clobber our demodulated data
// (if we find a better result then we'll swap back, but that's OK because
// we no longer need this copy if we found a better one)
msg = (msg == msg1) ? msg2 : msg1;
}
}
// Do we have a candidate?
if (bestscore < 0) {
if (bestscore == -1)
Modes.stats_current.demod_rejected_unknown_icao++;
else
Modes.stats_current.demod_rejected_bad++;
continue; // nope.
}
msglen = modesMessageLenByType(bestmsg[0] >> 3);
// Set initial mm structure details
mm = zeroMessage;
// For consistency with how the Beast / Radarcape does it,
// we report the timestamp at the end of bit 56 (even if
// the frame is a 112-bit frame)
mm.timestampMsg = mag->sampleTimestamp + j*5 + (8 + 56) * 12 + bestphase;
// compute message receive time as block-start-time + difference in the 12MHz clock
mm.sysTimestampMsg = mag->sysTimestamp + receiveclock_ms_elapsed(mag->sampleTimestamp, mm.timestampMsg);
mm.score = bestscore;
// Decode the received message
{
int result = decodeModesMessage(&mm, bestmsg);
if (result < 0) {
if (result == -1)
Modes.stats_current.demod_rejected_unknown_icao++;
else
Modes.stats_current.demod_rejected_bad++;
continue;
} else {
Modes.stats_current.demod_accepted[mm.correctedbits]++;
}
}
// measure signal power
{
double signal_power;
uint64_t scaled_signal_power = 0;
int signal_len = msglen*12/5;
int k;
for (k = 0; k < signal_len; ++k) {
uint32_t mag = m[j+19+k];
scaled_signal_power += mag * mag;
}
signal_power = scaled_signal_power / 65535.0 / 65535.0;
mm.signalLevel = signal_power / signal_len;
Modes.stats_current.signal_power_sum += signal_power;
Modes.stats_current.signal_power_count += signal_len;
sum_scaled_signal_power += scaled_signal_power;
if (mm.signalLevel > Modes.stats_current.peak_signal_power)
Modes.stats_current.peak_signal_power = mm.signalLevel;
if (mm.signalLevel > 0.50119)
Modes.stats_current.strong_signal_count++; // signal power above -3dBFS
}
// Skip over the message:
// (we actually skip to 8 bits before the end of the message,
// because we can often decode two messages that *almost* collide,
// where the preamble of the second message clobbered the last
// few bits of the first message, but the message bits didn't
// overlap)
j += msglen*12/5;
// Pass data to the next layer
useModesMessage(&mm);
}
/* update noise power */
{
double sum_signal_power = sum_scaled_signal_power / 65535.0 / 65535.0;
Modes.stats_current.noise_power_sum += (mag->mean_power * mag->length - sum_signal_power);
Modes.stats_current.noise_power_count += mag->length;
}
}
#ifdef MODEAC_DEBUG
static int yscale(unsigned signal)
{
return (int) (299 - 299.0 * signal / 65536.0);
}
static void draw_modeac(uint16_t *m, unsigned modeac, unsigned f1_clock, unsigned noise_threshold, unsigned signal_threshold, unsigned bits, unsigned noisy_bits, unsigned uncertain_bits)
{
// 25 bits at 87*60MHz
// use 1 pixel = 30MHz = 1087 pixels
gdImagePtr im = gdImageCreate(1088, 300);
int red = gdImageColorAllocate(im, 255, 0, 0);
int brightgreen = gdImageColorAllocate(im, 0, 255, 0);
int darkgreen = gdImageColorAllocate(im, 0, 180, 0);
int blue = gdImageColorAllocate(im, 0, 0, 255);
int grey = gdImageColorAllocate(im, 200, 200, 200);
int white = gdImageColorAllocate(im, 255, 255, 255);
int black = gdImageColorAllocate(im, 0, 0, 0);
gdImageFilledRectangle(im, 0, 0, 1087, 299, white);
// draw samples
for (unsigned pixel = 0; pixel < 1088; ++pixel) {
int clock_offset = (pixel - 150) * 2;
int bit = clock_offset / 87;
int sample = (f1_clock + clock_offset) / 25;
int bitoffset = clock_offset % 87;
int color;
if (sample < 0)
continue;
if (clock_offset < 0 || bit >= 20) {
color = grey;
} else if (bitoffset < 27 && (uncertain_bits & (1 << (19-bit)))) {
color = red;
} else if (bitoffset >= 27 && (noisy_bits & (1 << (19-bit)))) {
color = red;
} else if (bitoffset >= 27) {
color = grey;
} else if (bits & (1 << (19-bit))) {
color = brightgreen;
} else {
color = darkgreen;
}
gdImageLine(im, pixel, 299, pixel, yscale(m[sample]), color);
}
// draw bit boundaries
for (unsigned bit = 0; bit < 20; ++bit) {
unsigned clock = 87 * bit;
unsigned pixel0 = clock / 2 + 150;
unsigned pixel1 = (clock + 27) / 2 + 150;
gdImageLine(im, pixel0, 0, pixel0, 299, (bit == 0 || bit == 14) ? black : grey);
gdImageLine(im, pixel1, 0, pixel1, 299, (bit == 0 || bit == 14) ? black : grey);
}
// draw thresholds
gdImageLine(im, 0, yscale(noise_threshold), 1087, yscale(noise_threshold), blue);
gdImageLine(im, 0, yscale(signal_threshold), 1087, yscale(signal_threshold), blue);
// save it
static int file_counter;
char filename[PATH_MAX];
sprintf(filename, "modeac_%04X_%04d.png", modeac, ++file_counter);
fprintf(stderr, "writing %s\n", filename);
FILE *pngout = fopen(filename, "wb");
gdImagePng(im, pngout);
fclose(pngout);
gdImageDestroy(im);
}
#endif
//////////
////////// MODE A/C
//////////
// Mode A/C bits are 1.45us wide, consisting of 0.45us on and 1.0us off
// We track this in terms of a (virtual) 60MHz clock, which is the lowest common multiple
// of the bit frequency and the 2.4MHz sampling frequency
//
// 0.45us = 27 cycles }
// 1.00us = 60 cycles } one bit period = 1.45us = 87 cycles
//
// one 2.4MHz sample = 25 cycles
void demodulate2400AC(struct mag_buf *mag)
{
struct modesMessage mm;
uint16_t *m = mag->data;
uint32_t mlen = mag->length;
unsigned f1_sample;
memset(&mm, 0, sizeof(mm));
double noise_stddev = sqrt(mag->mean_power - mag->mean_level * mag->mean_level); // Var(X) = E[(X-E[X])^2] = E[X^2] - (E[X])^2
unsigned noise_level = (unsigned) ((mag->mean_power + noise_stddev) * 65535 + 0.5);
for (f1_sample = 1; f1_sample < mlen; ++f1_sample) {
// Mode A/C messages should match this bit sequence:
// bit # value
// -1 0 quiet zone
// 0 1 framing pulse (F1)
// 1 C1
// 2 A1
// 3 C2
// 4 A2
// 5 C4
// 6 A4
// 7 0 quiet zone (X1)
// 8 B1
// 9 D1
// 10 B2
// 11 D2
// 12 B4
// 13 D4
// 14 1 framing pulse (F2)
// 15 0 quiet zone (X2)
// 16 0 quiet zone (X3)
// 17 SPI
// 18 0 quiet zone (X4)
// 19 0 quiet zone (X5)
// Look for a F1 and F2 pair,
// with F1 starting at offset f1_sample.
// the first framing pulse covers 3.5 samples:
//
// |----| |----|
// | F1 |________| C1 |_
//
// | 0 | 1 | 2 | 3 | 4 |
//
// and there is some unknown phase offset of the
// leading edge e.g.:
//
// |----| |----|
// __| F1 |________| C1 |_
//
// | 0 | 1 | 2 | 3 | 4 |
//
// in theory the "on" period can straddle 3 samples
// but it's not a big deal as at most 4% of the power
// is in the third sample.
if (!(m[f1_sample-1] < m[f1_sample+0]))
continue; // not a rising edge
if (m[f1_sample+2] > m[f1_sample+0] || m[f1_sample+2] > m[f1_sample+1])
continue; // quiet part of bit wasn't sufficiently quiet
unsigned f1_level = (m[f1_sample+0] + m[f1_sample+1]) / 2;
if (noise_level * 2 > f1_level) {
// require 6dB above noise
continue;
}
// estimate initial clock phase based on the amount of power
// that ended up in the second sample
float f1a_power = (float)m[f1_sample] * m[f1_sample];
float f1b_power = (float)m[f1_sample+1] * m[f1_sample+1];
float fraction = f1b_power / (f1a_power + f1b_power);
unsigned f1_clock = (unsigned) (25 * (f1_sample + fraction * fraction) + 0.5);
// same again for F2
// F2 is 20.3us / 14 bit periods after F1
unsigned f2_clock = f1_clock + (87 * 14);
unsigned f2_sample = f2_clock / 25;
assert(f2_sample < mlen + Modes.trailing_samples);
if (!(m[f2_sample-1] < m[f2_sample+0]))
continue;
if (m[f2_sample+2] > m[f2_sample+0] || m[f2_sample+2] > m[f2_sample+1])
continue; // quiet part of bit wasn't sufficiently quiet
unsigned f2_level = (m[f2_sample+0] + m[f2_sample+1]) / 2;
if (noise_level * 2 > f2_level) {
// require 6dB above noise
continue;
}
unsigned f1f2_level = (f1_level > f2_level ? f1_level : f2_level);
float midpoint = sqrtf(noise_level * f1f2_level); // geometric mean of the two levels
unsigned signal_threshold = (unsigned) (midpoint * M_SQRT2 + 0.5); // +3dB
unsigned noise_threshold = (unsigned) (midpoint / M_SQRT2 + 0.5); // -3dB
// Looks like a real signal. Demodulate all the bits.
unsigned uncertain_bits = 0;
unsigned noisy_bits = 0;
unsigned bits = 0;
unsigned bit;
unsigned clock;
for (bit = 0, clock = f1_clock; bit < 20; ++bit, clock += 87) {
unsigned sample = clock / 25;
bits <<= 1;
noisy_bits <<= 1;
uncertain_bits <<= 1;
// check for excessive noise in the quiet period
if (m[sample+2] >= signal_threshold) {
noisy_bits |= 1;
}
// decide if this bit is on or off
if (m[sample+0] >= signal_threshold || m[sample+1] >= signal_threshold) {
bits |= 1;
} else if (m[sample+0] > noise_threshold && m[sample+1] > noise_threshold) {
/* not certain about this bit */
uncertain_bits |= 1;
} else {
/* this bit is off */
}
}
// framing bits must be on
if ((bits & 0x80020) != 0x80020) {
continue;
}
// quiet bits must be off
if ((bits & 0x0101B) != 0) {
continue;
}
if (noisy_bits || uncertain_bits) {
continue;
}
// Convert to the form that we use elsewhere:
// 00 A4 A2 A1 00 B4 B2 B1 SPI C4 C2 C1 00 D4 D2 D1
unsigned modeac =
((bits & 0x40000) ? 0x0010 : 0) | // C1
((bits & 0x20000) ? 0x1000 : 0) | // A1
((bits & 0x10000) ? 0x0020 : 0) | // C2
((bits & 0x08000) ? 0x2000 : 0) | // A2
((bits & 0x04000) ? 0x0040 : 0) | // C4
((bits & 0x02000) ? 0x4000 : 0) | // A4
((bits & 0x00800) ? 0x0100 : 0) | // B1
((bits & 0x00400) ? 0x0001 : 0) | // D1
((bits & 0x00200) ? 0x0200 : 0) | // B2
((bits & 0x00100) ? 0x0002 : 0) | // D2
((bits & 0x00080) ? 0x0400 : 0) | // B4
((bits & 0x00040) ? 0x0004 : 0) | // D4
((bits & 0x00004) ? 0x0080 : 0); // SPI
#ifdef MODEAC_DEBUG
draw_modeac(m, modeac, f1_clock, noise_threshold, signal_threshold, bits, noisy_bits, uncertain_bits);
#endif
// This message looks good, submit it
// For consistency with how the Beast / Radarcape does it,
// we report the timestamp at the second framing pulse (F2)
mm.timestampMsg = mag->sampleTimestamp + f2_clock / 5; // 60MHz -> 12MHz
// compute message receive time as block-start-time + difference in the 12MHz clock
mm.sysTimestampMsg = mag->sysTimestamp + receiveclock_ms_elapsed(mag->sampleTimestamp, mm.timestampMsg);
decodeModeAMessage(&mm, modeac);
// Pass data to the next layer
useModesMessage(&mm);
f1_sample += (20*87 / 25);
Modes.stats_current.demod_modeac++;
}
}