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#include "reduce.h"
#include "params.h"
#include <stdint.h>
/*************************************************
* Name: montgomery_reduce
*
* Description: Montgomery reduction; given a 32-bit integer a, computes
* 16-bit integer congruent to a * R^-1 mod q,
* where R=2^16
*
* Arguments: - int32_t a: input integer to be reduced; has to be in {-q2^15,...,q2^15-1}
*
* Returns: integer in {-q+1,...,q-1} congruent to a * R^-1 modulo q.
**************************************************/
int16_t PQCLEAN_KYBER512_CLEAN_montgomery_reduce(int32_t a) {
int32_t t;
int16_t u;
u = (int16_t)(a * (int64_t)QINV);
t = (int32_t)u * KYBER_Q;
t = a - t;
t >>= 16;
return (int16_t)t;
}
/*************************************************
* Name: barrett_reduce
*
* Description: Barrett reduction; given a 16-bit integer a, computes
* 16-bit integer congruent to a mod q in {0,...,q}
*
* Arguments: - int16_t a: input integer to be reduced
*
* Returns: integer in {0,...,q} congruent to a modulo q.
**************************************************/
int16_t PQCLEAN_KYBER512_CLEAN_barrett_reduce(int16_t a) {
int32_t t;
const int32_t v = (1U << 26) / KYBER_Q + 1;
t = v * a;
t >>= 26;
t *= KYBER_Q;
return a - (int16_t)t;
}
/*************************************************
* Name: csubq
*
* Description: Conditionallly subtract q
*
* Arguments: - int16_t a: input integer
*
* Returns: a - q if a >= q, else a
**************************************************/
int16_t PQCLEAN_KYBER512_CLEAN_csubq(int16_t a) {
a -= KYBER_Q;
a += (a >> 15) & KYBER_Q;
return a;
}
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