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tlss.cc
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tlss.cc
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// tlss.cc: implementation of class TLSS for sieving E(Q)/pE(Q) at one prime q
//////////////////////////////////////////////////////////////////////////
//
// Copyright 1990-2012 John Cremona
//
// This file is part of the eclib package.
//
// eclib is free software; you can redistribute it and/or modify it
// under the terms of the GNU General Public License as published by the
// Free Software Foundation; either version 2 of the License, or (at your
// option) any later version.
//
// eclib is distributed in the hope that it will be useful, but WITHOUT
// ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
// for more details.
//
// You should have received a copy of the GNU General Public License
// along with eclib; if not, write to the Free Software Foundation,
// Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301, USA
//
//////////////////////////////////////////////////////////////////////////
// NB: TLSS = Tate--Lichtenbaum--Samir-Siksek: we use a simple
// discrete log a la Siksek when the p-torsion in E(F_q) is cyclic,
// else use the Tate-Lichtenbaum pairing
#include <eclib/matrix.h>
#include <eclib/subspace.h>
#include <eclib/points.h>
#include <eclib/polys.h>
#include <eclib/curvemod.h>
#include <eclib/pointsmod.h>
#include <eclib/ffmod.h>
#include <eclib/tlss.h>
void TLSS::init(int pp, int verb)
{
verbose=verb;
p=pp;
Pi=Emodq.get_pbasis(p);
rank=Pi.size();
if((verbose>1)&&(rank>0))
{
cout<<"Generators of "<<p<<"-torsion mod "<<q<<": \n";
cout<<"P1 = "<<Pi[0]<<endl;
if(rank>1) cout<<"P2 = "<<Pi[1]<<endl;
}
if(rank==2) init_tlpolys();
}
void TLSS::init(int pp, const vector<bigint>& pdivpol, int verb)
{
verbose=verb;
p=pp;
Pi=Emodq.get_pbasis_via_divpol(p, pdivpol);
rank=Pi.size();
if((verbose>1)&&(rank>0))
{
cout<<"Generators of "<<p<<"-torsion mod "<<q<<": \n";
cout<<"P1 = "<<Pi[0]<<endl;
if(rank>1) cout<<"P2 = "<<Pi[1]<<endl;
}
if(rank==2) init_tlpolys();
}
void TLSS::init_tlpolys()
{
if (rank<2) return;
// case rank=2: prepare to use TL map
// first find p'th root of 1 mod q
// initialize the array of all p'th roots mod q
q1p=(q-1)/p;
mu_p = roots_of_unity(Fq,p);
if(verbose>1)
{
cout<<"q="<<q<<endl;
cout<<"p'th roots of unity mod q = "<<mu_p<<endl;
cout<<"Rank of p-torsion mod q = "<<rank<<endl;
}
// initialize the ffmodq class
ffmodq dummy((const curvemodq)Emodq);
// initialize the TL-functions
TLpolys.resize(0);
int i;
for(i=0; i<rank; i++) TLpolys.push_back(weil_pol(Pi[i],p));
if(verbose>1) for(i=0; i<rank; i++) cout<<"TL poly: "<<TLpolys[i]<<endl;
}
//#define debugTL
// apply map to P, result in (rank*)[0..p-1]:
vector<int> TLSS::map1point(const Point& P) const
{
#ifdef debugTL
cout<<"Applying TLSS::map1point (q="<<q<<") to P="<<P<<endl;
#endif
pointmodq Pmodq = reduce_point(P,(const curvemodq&)Emodq);
#ifdef debugTL
cout<<"P mod q ="<<Pmodq<<endl;
#endif
int i, sw;
vector<int> ans;
ans.resize(rank);
for(i=0; i<rank; i++) ans[i]=0;
if(Pmodq.is_zero()) return ans;
if(rank==1)
{
pointmodq P1=Pi[0];
bigint n1 = Emodq.n1; // order of relevant cyclic factor if known
if(n1==0) n1 = Emodq.n; // else full group order if not
Pmodq = I2long(n1/p) * Pmodq;
if(Pmodq.is_zero()) return ans;
#ifdef debugTL
cout<<"after multiplying by "<<I2long(n1/p)<<", get "<<Pmodq<<endl;
cout<<"finding discrete log of "<<Pmodq<<" w.r.t. "<<P1<<endl;
#endif
pointmodq Q(Emodq);
pointmodq minusPmodq=-Pmodq;
for(i=0; i<p; i++)
{
if(Q==Pmodq) {ans[0]=i; return ans;}
if(i) if(Q==minusPmodq) {ans[0]=p-i; return ans;}
Q+=P1;
}
// Optional check:
#ifdef debugTL
int check = ans[0]*P1 == Pmodq;
if(!check)
{
cout<<"Error: discrete log of "<<Pmodq<<" w.r.t. "<<P1<<" returns "<<ans[0]<<endl;
abort();
}
#endif
return ans;
}
// else apply TL maps
gf_element xP=Pmodq.get_x();
gf_element yP=Pmodq.get_y();
gf_element lambda, mu, t;
gf_element a1,a2,a3,a4,a6;
Emodq.get_ai(a1,a2,a3,a4,a6);
gf_element b2 = a1*a1 + 4*a2;
gf_element b4 = 2*a4 + a1*a3;
for(i=0; i<rank; i++)
{
const pointmodq& T = Pi[i];
gf_element xT=T.get_x(), yT=T.get_y();
#ifdef debugTL
cout<<"(xT,yT)=("<<xT<<","<<yT<<")"<<endl;
#endif
switch(p) {
case 2:{
t=xP-xT;
if(t==0)
{
lambda=4*xT+b2;
mu=xT*lambda+b4+b4;
t=4*xP*xP+lambda*xP+mu;
}
#ifdef debugTL
cout<<"calling power_mod with (t,q1p,q) = ("<<mu<<","<<t<<","<<q1p<<","<<q<<")"<<endl;
#endif
power(mu,t,q1p);
#ifdef debugTL
cout<<"mu = "<<mu<<endl;
#endif
ans[i] = ((mu==1)? 0 : 1);
break;
}
default:{
#ifdef debugTL
cout<<"applying TL poly = "<<TLpolys[i]<<" to "<<Pmodq<<endl;
#endif
t=TLpolys[i](Pmodq);
sw=0;
if(t==0)
{
sw=1;
t=TLpolys[i](-Pmodq);
}
if(t==0)
{
cout<<"Error: both P and -P map to 0"<<endl;
abort();
}
power(mu,t,q1p);
#ifdef debugTL
cout<<"t="<<t<<endl;
cout<<"mu = "<<mu<<endl;
gf_element t2, mu2;
if(!(p*Pmodq).is_zero()) t2 = evaluate_weil_pol(T,p,Pmodq);
else
{
cout<<"(new method having to use shifting trick)"<<endl;
pointmodq R=Pmodq.get_curve().random_point();
while((p*R).is_zero())
R=Pmodq.get_curve().random_point();
t2 = evaluate_weil_pol(T,p,R+Pmodq)/evaluate_weil_pol(T,p,R);
}
power(mu2,t2,q1p);
if(mu==mu2) cout<<"New method agrees"<<endl;
else cout<<"New method gives mu value "<<mu2<<" instead!"<<endl;
#endif
if(mu==1) ans[i]=0; else // discrete log: mu is some power of mu_p
{
int dl=find(mu_p.begin(),mu_p.end(),mu)-mu_p.begin();
#ifdef debugTL
cout<<"discrete log of mu = "<<dl<<endl;
#endif
if(dl==p)
{
cout<<"Error: mu="<<mu<<" (mod "<<q<<") is not a "<<p<<"'th root of unity!"<<endl;
abort();
}
ans[i] = (sw? p-dl: dl);
}
break;
} // end of default case
} // switch(p)
}
#ifdef debugTL
cout<<"ans = "<<ans<<endl;
#endif
return ans;
}
// apply map to all P in Plist, result is a (rank*#Plist) matrix:
mat TLSS::map_points(const vector<Point>& Plist) const
{
int npts = Plist.size();
mat TLim(rank,npts);
int i,j;
for(i=0; i<npts; i++)
{
Point P=Plist[i];
vector<int> tlP = map1point(P);
if(verbose>1) cout<<"P="<<P<<" -> "<<tlP<<endl;
for(j=0; j<rank; j++)
{
TLim(j+1,i+1)=tlP[j];
}
}
return TLim;
}