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