vendor.github.com.cloudflare.circl.math.mlsbset.power.go Maven / Gradle / Ivy
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package mlsbset
import "fmt"
// Power is a valid exponent produced by the MLSBSet encoding algorithm.
type Power struct {
set Encoder // parameters of code.
s []int32 // set of signs.
b []int32 // set of digits.
c int // carry is {0,1}.
}
// Exp is calculates x^k, where x is a predetermined element of a group G.
func (p *Power) Exp(G Group) EltG {
a, b := G.Identity(), G.NewEltP()
for e := int(p.set.p.E - 1); e >= 0; e-- {
G.Sqr(a)
for v := uint(0); v < p.set.p.V; v++ {
sgnElt, idElt := p.Digit(v, uint(e))
G.Lookup(b, v, sgnElt, idElt)
G.Mul(a, b)
}
}
if p.set.IsExtended() && p.c == 1 {
G.Mul(a, G.ExtendedEltP())
}
return a
}
// Digit returns the (v,e)-th digit and its sign.
func (p *Power) Digit(v, e uint) (sgn, dig int32) {
sgn = p.bit(0, v, e)
dig = 0
for i := p.set.p.W - 1; i > 0; i-- {
dig = 2*dig + p.bit(i, v, e)
}
mask := dig >> 31
dig = (dig + mask) ^ mask
return sgn, dig
}
// bit returns the (w,v,e)-th bit of the code.
func (p *Power) bit(w, v, e uint) int32 {
if !(w < p.set.p.W &&
v < p.set.p.V &&
e < p.set.p.E) {
panic(fmt.Errorf("indexes outside (%v,%v,%v)", w, v, e))
}
if w == 0 {
return p.s[p.set.p.E*v+e]
}
return p.b[p.set.p.D*(w-1)+p.set.p.E*v+e]
}
func (p *Power) String() string {
dig := ""
for j := uint(0); j < p.set.p.V; j++ {
for i := uint(0); i < p.set.p.E; i++ {
s, d := p.Digit(j, i)
dig += fmt.Sprintf("(%2v,%2v) = %+2v %+2v\n", j, i, s, d)
}
}
return fmt.Sprintf("len: %v\ncarry: %v\ndigits:\n%v", len(p.b)+len(p.s), p.c, dig)
}