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dsd-fme_18_05_2023/src/nxdn_deperm.c

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//NXDN descramble/deperm/depuncture and crc/utility functions
//Reworked portions from Osmocom OP25
/* -*- c++ -*- */
/*
* NXDN Encoder/Decoder (C) Copyright 2019 Max H. Parke KA1RBI
*
*
* This is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation; either version 3, or (at your option)
* any later version.
*
* This software 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 software; see the file COPYING. If not, write to
* the Free Software Foundation, Inc., 51 Franklin Street,
* Boston, MA 02110-1301, USA.
*/
#include "dsd.h"
#include "nxdn_const.h"
static const uint8_t scramble_t[] = { //values are the position values we need to invert in the descramble
2, 5, 6, 7, 10, 12, 14, 16, 17, 22, 23, 25, 26, 27, 28, 30, 33, 34, 36, 37, 38, 41, 45, 47,
52, 54, 56, 57, 59, 62, 63, 64, 65, 66, 67, 69, 70, 73, 76, 79, 81, 82, 84, 85, 86, 87, 88,
89, 92, 95, 96, 98, 100, 103, 104, 107, 108, 116, 117, 121, 122, 125, 127, 131, 132, 134,
137, 139, 140, 141, 142, 143, 144, 145, 147, 151, 153, 154, 158, 159, 160, 162, 164, 165,
168, 170, 171, 174, 175, 176, 177, 181
};
static const int PARITY[] = {
0, 1, 1, 0, 1, 0, 0, 1, 1, 0, 0,
1, 0, 1, 1, 0, 1, 0, 0, 1, 0, 1,
1, 0, 0, 1, 1, 0, 1, 0, 0, 1
};
//decoding functions here
void nxdn_descramble(uint8_t dibits[], int len)
{
for (int i=0; i<len; i++)
{
if (scramble_t[i] >= len)
break;
dibits[scramble_t[i]] ^= 0x2; // invert sign of scrambled dibits
}
}
void nxdn_deperm_facch(dsd_opts * opts, dsd_state * state, uint8_t bits[144])
{
uint8_t deperm[144];
uint8_t depunc[192];
uint8_t trellis_buf[96];
uint16_t crc = 0; //crc calculated by function
uint16_t check = 0; //crc from payload for comparison
int out;
char buf[128];
for (int i=0; i<144; i++)
deperm[PERM_16_9[i]] = bits[i];
out = 0;
for (int i=0; i<144; i+=3) {
depunc[out++] = deperm[i+0];
depunc[out++] = 0;
depunc[out++] = deperm[i+1];
depunc[out++] = deperm[i+2];
}
//switch to the convolutional decoder
uint8_t temp[210];
uint8_t s0;
uint8_t s1;
uint8_t m_data[13];
memset (temp, 0, sizeof(temp));
memset (m_data, 0, sizeof(m_data));
memset (trellis_buf, 0, sizeof(trellis_buf));
for (int i = 0; i < 192; i++)
{
temp[i] = depunc[i] << 1;
}
for (int i = 0; i < 8; i++)
{
temp[i+192] = 0;
}
CNXDNConvolution_start();
for (int i = 0U; i < 100U; i++)
{
s0 = temp[(2*i)];
s1 = temp[(2*i)+1];
CNXDNConvolution_decode(s0, s1);
}
CNXDNConvolution_chainback(m_data, 96U);
for(int i = 0; i < 12; i++)
{
trellis_buf[(i*8)+0] = (m_data[i] >> 7) & 1;
trellis_buf[(i*8)+1] = (m_data[i] >> 6) & 1;
trellis_buf[(i*8)+2] = (m_data[i] >> 5) & 1;
trellis_buf[(i*8)+3] = (m_data[i] >> 4) & 1;
trellis_buf[(i*8)+4] = (m_data[i] >> 3) & 1;
trellis_buf[(i*8)+5] = (m_data[i] >> 2) & 1;
trellis_buf[(i*8)+6] = (m_data[i] >> 1) & 1;
trellis_buf[(i*8)+7] = (m_data[i] >> 0) & 1;
}
crc = crc12f (trellis_buf, 84); //80
for (int i = 0; i < 12; i++)
{
check = check << 1;
check = check | trellis_buf[84+i]; //80
}
if (crc == check) NXDN_Elements_Content_decode(opts, state, 1, trellis_buf);
else if (opts->aggressive_framesync == 0) NXDN_Elements_Content_decode(opts, state, 0, trellis_buf);
if (opts->payload == 1)
{
fprintf (stderr, "\n");
fprintf (stderr, " FACCH Payload ");
for (int i = 0; i < 12; i++)
{
fprintf (stderr, "[%02X]", m_data[i]);
}
fprintf (stderr, " - %03X %03X", check, crc);
}
}
//new sacch
void nxdn_deperm_sacch(dsd_opts * opts, dsd_state * state, uint8_t bits[60])
{
//see about initializing these variables
uint8_t deperm[60];
uint8_t depunc[72];
uint8_t trellis_buf[32];
uint8_t answer[26];
memset (deperm, 0, sizeof(deperm));
memset (depunc, 0, sizeof(depunc));
memset (trellis_buf, 0, sizeof(trellis_buf));
memset (answer, 0, sizeof(answer));
int o = 0;
uint8_t crc = 0; //value computed by crc6 on payload
uint8_t check = 0; //value pulled from last 6 bits
int sf = 0;
int ran = 0;
int part_of_frame = 0;
for (int i=0; i<60; i++)
deperm[PERM_12_5[i]] = bits[i];
for (int p=0; p<60; p+= 10) {
depunc[o++] = deperm[p+0];
depunc[o++] = deperm[p+1];
depunc[o++] = deperm[p+2];
depunc[o++] = deperm[p+3];
depunc[o++] = deperm[p+4];
depunc[o++] = 0;
depunc[o++] = deperm[p+5];
depunc[o++] = deperm[p+6];
depunc[o++] = deperm[p+7];
depunc[o++] = deperm[p+8];
depunc[o++] = deperm[p+9];
depunc[o++] = 0;
}
//switch to the convolutional decoder
uint8_t temp[90];
uint8_t s0;
uint8_t s1;
uint8_t m_data[5];
memset (temp, 0, sizeof (temp));
memset (m_data, 0, sizeof (m_data));
memset (trellis_buf, 0, sizeof(trellis_buf));
for (int i = 0; i < 72; i++)
{
temp[i] = depunc[i] << 1;
}
for (int i = 0; i < 8; i++)
{
temp[i+72] = 0;
}
CNXDNConvolution_start();
for (int i = 0U; i < 36U; i++) //40
{
s0 = temp[(2*i)];
s1 = temp[(2*i)+1];
CNXDNConvolution_decode(s0, s1);
}
//stored as 5 bytes, will need to convert to trellis_buf after running
CNXDNConvolution_chainback(m_data, 32U); //36
for(int i = 0; i < 4; i++)
{
trellis_buf[(i*8)+0] = (m_data[i] >> 7) & 1;
trellis_buf[(i*8)+1] = (m_data[i] >> 6) & 1;
trellis_buf[(i*8)+2] = (m_data[i] >> 5) & 1;
trellis_buf[(i*8)+3] = (m_data[i] >> 4) & 1;
trellis_buf[(i*8)+4] = (m_data[i] >> 3) & 1;
trellis_buf[(i*8)+5] = (m_data[i] >> 2) & 1;
trellis_buf[(i*8)+6] = (m_data[i] >> 1) & 1;
trellis_buf[(i*8)+7] = (m_data[i] >> 0) & 1;
}
crc = crc6(trellis_buf, 26); //32
for (int i = 0; i < 6; i++)
{
check = check << 1;
check = check | trellis_buf[i+26];
}
//FIRST! If part of a non_superframe, and CRC is good, send directly to NXDN_Elements_Content_decode
if (state->nxdn_sacch_non_superframe == TRUE)
{
if (state->nxdn_last_ran != -1) fprintf (stderr, " RAN %02d ", state->nxdn_last_ran);
else fprintf (stderr, " ");
uint8_t nsf_sacch[26];
memset (nsf_sacch, 0, sizeof(nsf_sacch));
for (int i = 0; i < 26; i++)
{
nsf_sacch[i] = trellis_buf[i+8];
}
if (crc == check) NXDN_Elements_Content_decode(opts, state, 1, nsf_sacch);
else if (opts->aggressive_framesync == 0) NXDN_Elements_Content_decode(opts, state, 0, nsf_sacch);
if (opts->payload == 1)
{
fprintf (stderr, "\n SACCH NSF ");
for (int i = 0; i < 4; i++)
{
fprintf (stderr, "[%02X]", m_data[i]);
}
if (crc != check) fprintf (stderr, " CRC ERR - %02X %02X", check, crc);
}
}
//If part of superframe, collect the fragments and send to NXDN_SACCH_Full_decode instead
else if (state->nxdn_sacch_non_superframe == FALSE)
{
//sf and ran together are denoted as SR in the manual (more confusing acronyms)
//sf (structure field) and RAN will always exist in first 8 bits of each SACCH, then the next 18 bits are the fragment of the superframe
sf = (trellis_buf[0] << 1) | trellis_buf[1];
ran = (trellis_buf[2] << 5) | (trellis_buf[3] << 4) | (trellis_buf[4] << 3) | (trellis_buf[5] << 2) | (trellis_buf[6] << 1) | trellis_buf[7];
if (sf == 3) part_of_frame = 0;
else if (sf == 2) part_of_frame = 1;
else if (sf == 1) part_of_frame = 2;
else if (sf == 0) part_of_frame = 3;
else part_of_frame = 0;
fprintf (stderr, "%s", KCYN);
if (state->nxdn_last_ran != -1) fprintf (stderr, " RAN %02d ", state->nxdn_last_ran);
else fprintf (stderr, " ");
fprintf (stderr, "%s", KNRM);
//reset scrambler seed to key value on new superframe
if (part_of_frame == 0 && state->nxdn_cipher_type == 0x1) state->payload_miN = 0;
if (crc == check)
{
state->nxdn_ran = state->nxdn_last_ran = ran;
state->nxdn_sf = sf;
state->nxdn_part_of_frame = part_of_frame;
state->nxdn_sacch_frame_segcrc[part_of_frame] = 0; //zero indicates good check
}
else state->nxdn_sacch_frame_segcrc[part_of_frame] = 1; //1 indicates bad check
int sacch_segment = 0;
for (int i = 0; i < 18; i++)
{
sacch_segment = sacch_segment << 1;
sacch_segment = sacch_segment + trellis_buf[i+8];
state->nxdn_sacch_frame_segment[part_of_frame][i] = trellis_buf[i+8];
}
//Hand off to LEH NXDN_SACCH_Full_decode
if (part_of_frame == 3)
{
NXDN_SACCH_Full_decode (opts, state);
}
if (opts->payload == 1)
{
fprintf (stderr, "\n");
fprintf (stderr, " SACCH SF Segment #%d ", part_of_frame+1);
for (int i = 0; i < 4; i++)
{
fprintf (stderr, "[%02X]", m_data[i]);
}
if (crc != check) fprintf (stderr, " CRC ERR - %02X %02X", crc, check);
}
}
}
void nxdn_deperm_facch2_udch(dsd_opts * opts, dsd_state * state, uint8_t bits[348])
{
uint8_t deperm[348];
uint8_t depunc[406];
uint8_t trellis_buf[199];
int id = 0;
uint16_t crc = 0;
uint16_t check = 0;
for (int i=0; i<348; i++) {
deperm[PERM_12_29[i]] = bits[i];
}
for (int i=0; i<29; i++) {
depunc[id++] = deperm[i*12];
depunc[id++] = deperm[i*12+1];
depunc[id++] = deperm[i*12+2];
depunc[id++] = 0;
depunc[id++] = deperm[i*12+3];
depunc[id++] = deperm[i*12+4];
depunc[id++] = deperm[i*12+5];
depunc[id++] = deperm[i*12+6];
depunc[id++] = deperm[i*12+7];
depunc[id++] = deperm[i*12+8];
depunc[id++] = deperm[i*12+9];
depunc[id++] = 0;
depunc[id++] = deperm[i*12+10];
depunc[id++] = deperm[i*12+11];
}
//switch to the convolutional decoder
uint8_t temp[220];
uint8_t s0;
uint8_t s1;
uint8_t m_data[26];
memset (trellis_buf, 0, sizeof(trellis_buf));
memset (temp, 0, sizeof (temp));
memset (m_data, 0, sizeof (m_data));
for (int i = 0; i < 406; i++)
{
temp[i] = depunc[i] << 1;
}
for (int i = 0; i < 8; i++)
{
temp[i+203] = 0;
}
CNXDNConvolution_start();
for (int i = 0U; i < 207U; i++)
{
s0 = temp[(2*i)];
s1 = temp[(2*i)+1];
CNXDNConvolution_decode(s0, s1);
}
//numerals seem okay now
CNXDNConvolution_chainback(m_data, 203U);
for(int i = 0; i < 26; i++)
{
trellis_buf[(i*8)+0] = (m_data[i] >> 7) & 1;
trellis_buf[(i*8)+1] = (m_data[i] >> 6) & 1;
trellis_buf[(i*8)+2] = (m_data[i] >> 5) & 1;
trellis_buf[(i*8)+3] = (m_data[i] >> 4) & 1;
trellis_buf[(i*8)+4] = (m_data[i] >> 3) & 1;
trellis_buf[(i*8)+5] = (m_data[i] >> 2) & 1;
trellis_buf[(i*8)+6] = (m_data[i] >> 1) & 1;
trellis_buf[(i*8)+7] = (m_data[i] >> 0) & 1;
}
crc = crc15(trellis_buf, 199);
for (int i = 0; i < 15; i++)
{
check = check << 1;
check = check | trellis_buf[i+184];
}
uint8_t f2u_message_buffer[199];
memset (f2u_message_buffer, 0, sizeof(f2u_message_buffer));
//just going to leave this all here in case its needed, like in cac,
//don't have any samples with f2 or udch data in it
for (int i = 0; i < 199-8; i++)
{
f2u_message_buffer[i] = trellis_buf[i+8];
}
if (crc == check)
{
fprintf (stderr, " F2/U ");
//NXDN_Elements_Content_decode(opts, state, 1, trellis_buf);
NXDN_Elements_Content_decode(opts, state, 1, f2u_message_buffer);
}
else if (opts->aggressive_framesync == 0) NXDN_Elements_Content_decode(opts, state, 0, f2u_message_buffer);
if (opts->payload == 1)
{
fprintf (stderr, "\n");
fprintf (stderr, " F2/U Payload\n ");
for (int i = 0; i < 26; i++)
{
fprintf (stderr, "[%02X]", m_data[i]);
if (i == 12) fprintf (stderr, "\n ");
}
if (crc != check) fprintf (stderr, " CRC ERR ");
}
}
void nxdn_deperm_cac(dsd_opts * opts, dsd_state * state, uint8_t bits[300])
{
uint8_t deperm[300];
uint8_t depunc[350];
uint8_t trellis_buf[171];
int id = 0;
uint16_t crc = 0;
for (int i=0; i<300; i++) {
deperm[PERM_12_25[i]] = bits[i];
}
for (int i=0; i<25; i++) {
depunc[id++] = deperm[i*12];
depunc[id++] = deperm[i*12+1];
depunc[id++] = deperm[i*12+2];
depunc[id++] = 0;
depunc[id++] = deperm[i*12+3];
depunc[id++] = deperm[i*12+4];
depunc[id++] = deperm[i*12+5];
depunc[id++] = deperm[i*12+6];
depunc[id++] = deperm[i*12+7];
depunc[id++] = deperm[i*12+8];
depunc[id++] = deperm[i*12+9];
depunc[id++] = 0;
depunc[id++] = deperm[i*12+10];
depunc[id++] = deperm[i*12+11];
}
//switch to the convolutional decoder
uint8_t temp[360];
uint8_t s0;
uint8_t s1;
uint8_t m_data[26];
memset (trellis_buf, 0, sizeof(trellis_buf));
memset (temp, 0, sizeof (temp));
memset (m_data, 0, sizeof (m_data));
for (int i = 0; i < 350; i++)
{
temp[i] = depunc[i] << 1;
}
CNXDNConvolution_start();
for (int i = 0U; i < 179U; i++)
{
s0 = temp[(2*i)];
s1 = temp[(2*i)+1];
CNXDNConvolution_decode(s0, s1);
}
CNXDNConvolution_chainback(m_data, 175U);
for(int i = 0; i < 22; i++)
{
trellis_buf[(i*8)+0] = (m_data[i] >> 7) & 1;
trellis_buf[(i*8)+1] = (m_data[i] >> 6) & 1;
trellis_buf[(i*8)+2] = (m_data[i] >> 5) & 1;
trellis_buf[(i*8)+3] = (m_data[i] >> 4) & 1;
trellis_buf[(i*8)+4] = (m_data[i] >> 3) & 1;
trellis_buf[(i*8)+5] = (m_data[i] >> 2) & 1;
trellis_buf[(i*8)+6] = (m_data[i] >> 1) & 1;
trellis_buf[(i*8)+7] = (m_data[i] >> 0) & 1;
}
crc = crc16cac(trellis_buf, 171);
uint8_t cac_message_buffer[171];
memset (cac_message_buffer, 0, sizeof(cac_message_buffer));
//shift the cac_message into the appropriate byte arrangement for element_decoder -- skip SR field
for (int i = 0; i < 160; i++)
{
cac_message_buffer[i] = trellis_buf[i+8];
}
if (crc == 0)
{
fprintf (stderr, " CAC ");
NXDN_Elements_Content_decode(opts, state, 1, cac_message_buffer);
}
else if (opts->aggressive_framesync == 0) NXDN_Elements_Content_decode(opts, state, 0, cac_message_buffer);
if (opts->payload == 1)
{
fprintf (stderr, "\n");
fprintf (stderr, " CAC Payload\n ");
for (int i = 0; i < 22; i++)
{
fprintf (stderr, "[%02X]", m_data[i]);
if (i == 10) fprintf (stderr, "\n ");
}
if (crc != 0) fprintf (stderr, " CRC ERR ");
}
}
void nxdn_message_type (dsd_opts * opts, dsd_state * state, uint8_t MessageType)
{
fprintf (stderr, "%s", KYEL);
if (MessageType == 0x10) fprintf(stderr, " IDLE");
else if (MessageType == 0x11) fprintf(stderr, " DISC"); //disconnect
else if (MessageType == 0x01) fprintf(stderr, " VCALL");
else if (MessageType == 0x03) fprintf(stderr, " VCALL_IV");
else if (MessageType == 0x07) fprintf(stderr, " TX_REL_EX");
else if (MessageType == 0x08) fprintf(stderr, " TX_REL");
else if (MessageType == 0x04) fprintf(stderr, " VCALL_ASSGN");
else if (MessageType == 0x05) fprintf(stderr, " VCALL_ASSGN_DUP");
else if (MessageType == 0x0E) fprintf(stderr, " DCALL_ASSGN");
else if (MessageType == 0x18) fprintf(stderr, " SITE_INFO");
else if (MessageType == 0x19) fprintf(stderr, " SRV_INFO");
else if (MessageType == 0x1C) fprintf(stderr, " FAIL_STAT_INFO");
else if (MessageType == 0x1A) fprintf(stderr, " CCH_INFO");
else if (MessageType == 0x1B) fprintf(stderr, " ADJ_SITE_INFO");
else if (MessageType == 0x20) fprintf(stderr, " REG_RESP");
else if (MessageType == 0x22) fprintf(stderr, " REG_C_RESP");
else if (MessageType == 0x24) fprintf(stderr, " GRP_REG_RESP");
else if (MessageType == 0x32) fprintf(stderr, " STAT_REQ");
else if (MessageType == 0x33) fprintf(stderr, " STAT_RESP");
else if (MessageType == 0x38) fprintf(stderr, " SDCALL_REQ_HEADER");
else if (MessageType == 0x39) fprintf(stderr, " SDCALL_REQ_USERDATA");
else if (MessageType == 0x3B) fprintf(stderr, " SDCALL_RESP");
else if (MessageType == 0x3F) fprintf(stderr, " ALIAS");
else if (MessageType == 0x0C) fprintf(stderr, " DCALL_ACK");
else fprintf(stderr, " Unknown M-%02X", MessageType);
fprintf (stderr, "%s", KNRM);
//on nxdn48 trunking -- zero out stale values so they won't persist after a transmit release, idle, or disconnect
if (opts->frame_nxdn48 == 1)
{
if (MessageType == 0x08 || MessageType == 0x10 || MessageType == 0x11) //tx_rel, idle, or disc
{
memset (state->nxdn_alias_block_segment, 0, sizeof(state->nxdn_alias_block_segment));
state->nxdn_last_rid = 0;
state->nxdn_last_tg = 0;
state->nxdn_cipher_type = 0;
memset (state->nxdn_sacch_frame_segcrc, 1, sizeof(state->nxdn_sacch_frame_segcrc));
memset (state->nxdn_sacch_frame_segment, 1, sizeof(state->nxdn_sacch_frame_segment));
sprintf (state->nxdn_call_type, "%s", "");
}
}
//if nxdn96, only zero out on disc or tx_rel, some systems have data frames have idle that wipe out the tg/rid and cipher
else
{
if (MessageType == 0x08 || MessageType == 0x11) //tx_rel, or disc
{
memset (state->nxdn_alias_block_segment, 0, sizeof(state->nxdn_alias_block_segment));
state->nxdn_last_rid = 0;
state->nxdn_last_tg = 0;
state->nxdn_cipher_type = 0;
// memset (state->nxdn_sacch_frame_segcrc, 1, sizeof(state->nxdn_sacch_frame_segcrc));
// memset (state->nxdn_sacch_frame_segment, 1, sizeof(state->nxdn_sacch_frame_segment));
sprintf (state->nxdn_call_type, "%s", "");
}
}
}
//voice descrambler
void LFSRN(char * BufferIn, char * BufferOut, dsd_state * state)
{
int i;
int lfsr;
int pN[49] = {0};
int bit = 0;
lfsr = state->payload_miN & 0x7FFF;
for (i = 0; i < 49; i++)
{
pN[i] = lfsr & 0x1;
bit = ( (lfsr >> 1) ^ (lfsr >> 0) ) & 1;
lfsr = ( (lfsr >> 1 ) | (bit << 14) );
BufferOut[i] = BufferIn[i] ^ pN[i];
}
state->payload_miN = lfsr & 0x7FFF;
}
static inline int load_i(const uint8_t val[], int len) {
int acc = 0;
for (int i=0; i<len; i++){
acc = (acc << 1) + (val[i] & 1);
}
return acc;
}
static uint8_t crc6(const uint8_t buf[], int len)
{
uint8_t s[6];
uint8_t a;
for (int i=0;i<6;i++)
s[i] = 1;
for (int i=0;i<len;i++) {
a = buf[i] ^ s[0];
s[0] = a ^ s[1];
s[1] = s[2];
s[2] = s[3];
s[3] = a ^ s[4];
s[4] = a ^ s[5];
s[5] = a;
}
return load_i(s, 6);
}
static uint16_t crc12f(const uint8_t buf[], int len)
{
uint8_t s[12];
uint8_t a;
for (int i=0;i<12;i++)
s[i] = 1;
for (int i=0;i<len;i++) {
a = buf[i] ^ s[0];
s[0] = a ^ s[1];
s[1] = s[2];
s[2] = s[3];
s[3] = s[4];
s[4] = s[5];
s[5] = s[6];
s[6] = s[7];
s[7] = s[8];
s[8] = a ^ s[9];
s[9] = a ^ s[10];
s[10] = a ^ s[11];
s[11] = a;
}
return load_i(s, 12);
}
static uint16_t crc15(const uint8_t buf[], int len)
{
uint8_t s[15];
uint8_t a;
for (int i=0;i<15;i++)
s[i] = 1;
for (int i=0;i<len;i++) {
a = buf[i] ^ s[0];
s[0] = a ^ s[1];
s[1] = s[2];
s[2] = s[3];
s[3] = a ^ s[4];
s[4] = a ^ s[5];
s[5] = s[6];
s[6] = s[7];
s[7] = a ^ s[8];
s[8] = a ^ s[9];
s[9] = s[10];
s[10] = s[11];
s[11] = s[12];
s[12] = a ^ s[13];
s[13] = s[14];
s[14] = a;
}
return load_i(s, 15);
}
static uint16_t crc16cac(const uint8_t buf[], int len)
{
uint32_t crc = 0xc3ee; //not sure why this though
uint32_t poly = (1<<12) + (1<<5) + 1; //poly is fine
for (int i=0;i<len;i++)
{
crc = ((crc << 1) | buf[i]) & 0x1ffff;
if(crc & 0x10000) crc = (crc & 0xffff) ^ poly;
}
crc = crc ^ 0xffff;
return crc & 0xffff;
}
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