xs/vendor/blitter.com/go/kyber/reduce.go

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// reduce.go - Montgomery, Barret, and Full reduction.
//
// To the extent possible under law, Yawning Angel has waived all copyright
// and related or neighboring rights to the software, using the Creative
// Commons "CC0" public domain dedication. See LICENSE or
// <http://creativecommons.org/publicdomain/zero/1.0/> for full details.
package kyber
const (
qinv = 7679 // -inverse_mod(q,2^18)
rlog = 18
)
// Montgomery reduction; given a 32-bit integer a, computes 16-bit integer
// congruent to a * R^-1 mod q, where R=2^18 (see value of rlog).
func montgomeryReduce(a uint32) uint16 {
u := a * qinv
u &= (1 << rlog) - 1
u *= kyberQ
a += u
return uint16(a >> rlog)
}
// Barrett reduction; given a 16-bit integer a, computes 16-bit integer
// congruent to a mod q in {0,...,11768}.
func barrettReduce(a uint16) uint16 {
u := uint32(a >> 13) // ((uint32_t) a * sinv) >> 16
u *= kyberQ
a -= uint16(u)
return a
}
// Full reduction; given a 16-bit integer a, computes unsigned integer a mod q.
func freeze(x uint16) uint16 {
r := barrettReduce(x)
m := r - kyberQ
c := int16(m)
c >>= 15
r = m ^ ((r ^ m) & uint16(c))
return r
}