mirror of
https://github.com/cheat/cheat.git
synced 2024-11-22 22:11:35 +01:00
65 lines
1.5 KiB
Go
65 lines
1.5 KiB
Go
|
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)
|
||
|
}
|