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#include <stdint.h>
// CRC code from http://www.hackersdelight.org/hdcodetxt/crc.c.txt
// Reverses (reflects) bits in a 32-bit word.
unsigned reverse(unsigned x) {
x = ((x & 0x55555555) << 1) | ((x >> 1) & 0x55555555);
x = ((x & 0x33333333) << 2) | ((x >> 2) & 0x33333333);
x = ((x & 0x0F0F0F0F) << 4) | ((x >> 4) & 0x0F0F0F0F);
x = (x << 24) | ((x & 0xFF00) << 8) |
((x >> 8) & 0xFF00) | (x >> 24);
return x;
}
// ----------------------------- crc32a --------------------------------
/* This is the basic CRC algorithm with no optimizations. It follows the
logic circuit as closely as possible. */
unsigned int crc32a(uint8_t *message, unsigned int size) {
int i, j;
unsigned int byte, crc;
i = 0;
crc = 0xFFFFFFFF;
while (i < size) {
byte = message[i]; // Get next byte.
byte = reverse(byte); // 32-bit reversal.
for (j = 0; j <= 7; j++) { // Do eight times.
if ((int)(crc ^ byte) < 0)
crc = (crc << 1) ^ 0x04C11DB7;
else crc = crc << 1;
byte = byte << 1; // Ready next msg bit.
}
i = i + 1;
}
return reverse(~crc);
}
extern uint8_t *data;
extern uint32_t length;
uint32_t main()
{
/* Compute a simple checksum. */
return crc32a(data, length);
}
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