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#include "cbd.h"
#include "params.h"
#include <stddef.h>
#include <stdint.h>
/*************************************************
* Name: load32_littleendian
*
* Description: load bytes into a 32-bit integer
* in little-endian order
*
* Arguments: - const uint8_t *x: pointer to input byte array
*
* Returns 32-bit unsigned integer loaded from x
**************************************************/
static uint32_t load32_littleendian(const uint8_t *x) {
uint32_t r;
r = (uint32_t)x[0];
r |= (uint32_t)x[1] << 8;
r |= (uint32_t)x[2] << 16;
r |= (uint32_t)x[3] << 24;
return r;
}
/*************************************************
* Name: cbd
*
* Description: Given an array of uniformly random bytes, compute
* polynomial with coefficients distributed according to
* a centered binomial distribution with parameter KYBER_ETA
* specialized for KYBER_ETA=2
*
* Arguments: - poly *r: pointer to output polynomial
* - const uint8_t *buf: pointer to input byte array
**************************************************/
void PQCLEAN_KYBER512_CLEAN_cbd(poly *r, const uint8_t *buf) {
int16_t a, b;
for (size_t i = 0; i < KYBER_N / 8; i++) {
uint32_t t = load32_littleendian(buf + 4 * i);
uint32_t d = t & 0x55555555;
d += (t >> 1) & 0x55555555;
for (size_t j = 0; j < 8; j++) {
a = (d >> 4 * j) & 0x3;
b = (d >> (4 * j + 2)) & 0x3;
r->coeffs[8 * i + j] = a - b;
}
}
}
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