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/*
* Implementation of Metacube2 utility functions.
*
* Note: This file is meant to compile as both C and C++, for easier inclusion
* in other projects.
*/
#include "metacube2.h"
#include <arpa/inet.h>
/*
* https://www.ece.cmu.edu/~koopman/pubs/KoopmanCRCWebinar9May2012.pdf
* recommends this for messages as short as ours (see table at page 34).
*/
#define METACUBE2_CRC_POLYNOMIAL 0x8FDB
/* Semi-random starting value to make sure all-zero won't pass. */
#define METACUBE2_CRC_START 0x1234
/* This code is based on code generated by pycrc. */
uint16_t metacube2_compute_crc(const struct metacube2_block_header *hdr)
{
static const int data_len = sizeof(hdr->size) + sizeof(hdr->flags);
const uint8_t *data = (uint8_t *)&hdr->size;
uint16_t crc = METACUBE2_CRC_START;
int i, j;
for (i = 0; i < data_len; ++i) {
uint8_t c = data[i];
for (j = 0; j < 8; j++) {
int bit = crc & 0x8000;
crc = (crc << 1) | ((c >> (7 - j)) & 0x01);
if (bit) {
crc ^= METACUBE2_CRC_POLYNOMIAL;
}
}
}
/* Finalize. */
for (i = 0; i < 16; i++) {
int bit = crc & 0x8000;
crc = crc << 1;
if (bit) {
crc ^= METACUBE2_CRC_POLYNOMIAL;
}
}
/*
* Invert the checksum for metadata packets, so that clients that
* don't understand metadata will ignore it as broken. There will
* probably be logging, but apart from that, it's harmless.
*/
if (ntohs(hdr->flags) & METACUBE_FLAGS_METADATA) {
crc ^= 0xffff;
}
return crc;
}
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