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#include <stdint.h>
#include "kyber512r3_params.h"
#include "kyber512r3_reduce.h"
S2N_ENSURE_PORTABLE_OPTIMIZATIONS
/*************************************************
* 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 montgomery_reduce(int32_t a) {
int32_t t;
int16_t u;
u = a * S2N_KYBER_512_R3_QINV;
t = (int32_t)u * S2N_KYBER_512_R3_Q;
t = a - t;
t >>= 16;
return 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 barrett_reduce(int16_t a) {
int16_t t;
const int16_t v = ((1U << 26) + S2N_KYBER_512_R3_Q / 2) / S2N_KYBER_512_R3_Q;
t = (int32_t)v * a >> 26;
t *= S2N_KYBER_512_R3_Q;
return a - t;
}
/*************************************************
* Name: csubq
*
* Description: Conditionallly subtract q
*
* Arguments: - int16_t x: input integer
*
* Returns: a - q if a >= q, else a
**************************************************/
int16_t csubq(int16_t a) {
a -= S2N_KYBER_512_R3_Q;
a += (a >> 15) & S2N_KYBER_512_R3_Q;
return a;
}
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