1package mlsbset
 2
 3import "fmt"
 4
 5// Power is a valid exponent produced by the MLSBSet encoding algorithm.
 6type Power struct {
 7	set Encoder // parameters of code.
 8	s   []int32 // set of signs.
 9	b   []int32 // set of digits.
10	c   int     // carry is {0,1}.
11}
12
13// Exp is calculates x^k, where x is a predetermined element of a group G.
14func (p *Power) Exp(G Group) EltG {
15	a, b := G.Identity(), G.NewEltP()
16	for e := int(p.set.p.E - 1); e >= 0; e-- {
17		G.Sqr(a)
18		for v := uint(0); v < p.set.p.V; v++ {
19			sgnElt, idElt := p.Digit(v, uint(e))
20			G.Lookup(b, v, sgnElt, idElt)
21			G.Mul(a, b)
22		}
23	}
24	if p.set.IsExtended() && p.c == 1 {
25		G.Mul(a, G.ExtendedEltP())
26	}
27	return a
28}
29
30// Digit returns the (v,e)-th digit and its sign.
31func (p *Power) Digit(v, e uint) (sgn, dig int32) {
32	sgn = p.bit(0, v, e)
33	dig = 0
34	for i := p.set.p.W - 1; i > 0; i-- {
35		dig = 2*dig + p.bit(i, v, e)
36	}
37	mask := dig >> 31
38	dig = (dig + mask) ^ mask
39	return sgn, dig
40}
41
42// bit returns the (w,v,e)-th bit of the code.
43func (p *Power) bit(w, v, e uint) int32 {
44	if !(w < p.set.p.W &&
45		v < p.set.p.V &&
46		e < p.set.p.E) {
47		panic(fmt.Errorf("indexes outside (%v,%v,%v)", w, v, e))
48	}
49	if w == 0 {
50		return p.s[p.set.p.E*v+e]
51	}
52	return p.b[p.set.p.D*(w-1)+p.set.p.E*v+e]
53}
54
55func (p *Power) String() string {
56	dig := ""
57	for j := uint(0); j < p.set.p.V; j++ {
58		for i := uint(0); i < p.set.p.E; i++ {
59			s, d := p.Digit(j, i)
60			dig += fmt.Sprintf("(%2v,%2v) = %+2v %+2v\n", j, i, s, d)
61		}
62	}
63	return fmt.Sprintf("len: %v\ncarry: %v\ndigits:\n%v", len(p.b)+len(p.s), p.c, dig)
64}