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// reduce.cc: implementation of quartic reduction functions
//////////////////////////////////////////////////////////////////////////
//
// 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
//
//////////////////////////////////////////////////////////////////////////
#include <eclib/marith.h>
#include <eclib/unimod.h>
#include <eclib/points.h>
#include <eclib/mquartic.h>
#include <eclib/transform.h>
#include <eclib/msoluble.h>
#include <eclib/minim.h>
#include <eclib/reduce.h>
#define REDUCE_B
//#define DEBUG_REDUCE
//#define DEBUG_ONESTEP
//#define DEBUG_FIRSTSTEP
//#define USE_OLD_BSD // to use bsd-reduction for type 3 (worse!)
void reduce_b(bigint& a, bigint& b, bigint& c, bigint& d, bigint& e,
unimod& m)
{
bigint a4=a<<2;
bigint bmod4a = mod(b,a4);
bigint alpha;
divide_exact((bmod4a-b),a4,alpha);
if(is_zero(alpha)) return;
xshift(alpha,a,b,c,d,e,m);
return;
}
bigfloat show(bigfloat x) {cout<<x<<endl; return x;}
bigfloat* types12_covar(const bigint& a, const bigint& b, const bigint& c, const bigint& d,
const bigfloat& xH, const bigfloat& phi)
{
// The following covariant quadratic is the unique real quadratic factor
// of g6 which is positive definite
bigfloat xa=I2bigfloat(a), xb=I2bigfloat(b), xc=I2bigfloat(c);
bigfloat* ans = new bigfloat[3];
ans[0] = 3*(4*xa*phi-xH);
ans[1] = 6*(xb*phi+I2bigfloat(b*c-6*a*d));
ans[2] = 2*phi*(xc-phi)+I2bigfloat(4*c*c-9*b*d);
#ifdef DEBUG_REDUCE
cout<<"Before scaling, quadratic has coefficients: "
<<ans[0]<<", "<<ans[1]<<", "<<ans[2]<<endl;
#endif
ans[1]/=ans[0];
ans[2]/=ans[0];
ans[0]=1;
return ans;
}
bigfloat* type3_covar(const bigfloat& xa, const bigfloat& xb, const bigfloat& xH,
const bigfloat& rphi, const bigcomplex& cphi, int Risneg)
{
bigfloat* ans = new bigfloat[3];
bigfloat a4=4*xa;
static bigfloat three(to_bigfloat(3));
bigcomplex r1 = sqrt((a4*cphi-xH)/three);
bigfloat r3 = sqrt(abs(a4*rphi-xH)/three);
if(Risneg) r3 = -r3;
#ifdef DEBUG_REDUCE
cout<<"r1, r3 = "<<r1<<", "<<r3<<endl;
#endif
bigfloat rr1 = abs(real(r1));
bigfloat ir1 = abs(imag(r1));
bigfloat rbeta = ( r3 - xb ) / a4;
bigfloat ibeta = ( 2*ir1 ) / a4;
bigfloat alpha_1 = ( 2*rr1 - r3 - xb) / a4;
bigfloat alpha_2 = (-2*rr1 - r3 - xb) / a4;
// roots are alpha_1, alpha_2 (real), rbeta+/-i*ibeta (non-real)
bigfloat p = -2*rbeta;
bigfloat lambda = 2*ibeta;
bigfloat q = (p*p+lambda*lambda)/4;
#ifdef DEBUG_REDUCE
cout<<"phi = "<<rphi<<endl;
cout<<"Complex phi = "<<cphi<<endl;
cout<<"Roots are "<<alpha_1<<", "<<alpha_2<<", "<<rbeta<<" +/-i* "<<ibeta<<endl;
cout<<"p = "<<p<<", q = "<<q<<endl;
cout << "lambda = " << lambda << endl;
#endif
#ifdef USE_OLD_BSD
// B&SD's covariant:
ans[0]=1; ans[1] = p; ans[2] = q;
#else
//
// Julia's covariant:
bigfloat ar = abs(r1+r3);
bigfloat br = abs(r1-r3);
bigfloat t1sq = ir1*ar*ar;
bigfloat t2sq = ir1*br*br;
bigfloat usq = rr1*ar*br;
ans[0] = t1sq + t2sq + 2*usq;
ans[1] = -2*alpha_1*t1sq -2*alpha_2*t2sq + 2*p*usq;
ans[2] = alpha_1*alpha_1*t1sq + alpha_2*alpha_2*t2sq + 2*q*usq;
#ifdef DEBUG_REDUCE
cout << "quadratic has coefficients "<<ans[0]<<", "<<ans[1]<<", "<<ans[2]<<endl;
cout << "-disc = 4*q0*q2-q1^2 = " << (4*ans[0]*ans[2]-ans[1]*ans[1])
<< " (should be positive!)\n";
#endif
#endif
ans[1] /= ans[0];
ans[2] /= ans[0];
ans[0] = 1;
return ans;
}
// Compute the quadratic covariant of a real quartic:
bigfloat* quadratic_covariant(bigint& a, bigint& b, bigint& c, bigint& d, bigint& e)
{
bigint ii = II(a,b,c,d,e);
bigint jj = JJ(a,b,c,d,e);
bigint disc = 4*pow(ii,3)-sqr(jj);
bigint H = H_invariant(a,b,c), R = R_invariant(a,b,c,d);
bigfloat xH = I2bigfloat(H);
bigfloat xii = I2bigfloat(ii), xjj=I2bigfloat(jj);
bigcomplex c1(to_bigfloat(0)), c2(-3*xii), c3(xjj);
vector<bigcomplex> cphi = solvecubic( c1, c2, c3);
#ifdef DEBUG_REDUCE
cout<<"Three roots phi are initially "<<cphi<<"\n";
#endif
bigfloat * hcoeffs; // will hold coeffs of covariant quadratic
bigfloat realphi, phi; // will hold specific real roots
if(is_positive(disc))
{
// all the phi are real; order them so that a*phi[i] decreases
bigfloat phi1 = real(cphi[0]);
bigfloat phi2 = real(cphi[1]);
bigfloat phi3 = real(cphi[2]);
if(is_positive(a)) orderreal(phi1,phi2,phi3);
else orderreal(phi3,phi2,phi1);
#ifdef DEBUG_REDUCE
cout<<"Three real phi are "<<phi1<<", "<<phi2<<", "<<phi3<<"\n";
#endif
// So now a*phi1>a*phi2>a*phi3
if((is_negative(H))&&(H*H>16*a*a*ii))
{
realphi = phi2; phi = phi2;
#ifdef DEBUG_REDUCE
cout<<"Type = 2, phi = "<<phi<<"\n";
#endif
}
else
{
realphi = phi1; phi = phi3;
#ifdef DEBUG_REDUCE
cout<<"Type = 1, phi = "<<phi<<"\n";
#endif
}
hcoeffs = types12_covar(a,b,c,d,xH,phi);
}
else // disc < 0
{
#ifdef DEBUG_REDUCE
cout<<"Type = 3, ";
#endif
if (is_real(cphi[1]))
{
realphi=real(cphi[1]);
cphi[1]=cphi[2];
cphi[2]=realphi;
}
else
if (is_real(cphi[2]))
{
realphi=real(cphi[2]);
}
else
{
realphi=real(cphi[0]);
cphi[0]=cphi[2];
cphi[2]=realphi;
}
#ifdef DEBUG_REDUCE
cout<<"real phi = "<<realphi<<"\n";
cout<<"complex phi = "<<cphi[0]<<"\n";
#endif
bigfloat xa=I2bigfloat(a),xb=I2bigfloat(b);
hcoeffs = type3_covar(xa,xb,xH,realphi,cphi[0],(sign(R)<0));
}
return hcoeffs;
}
void reduce(bigint& a, bigint& b, bigint& c, bigint& d, bigint& e,
unimod& m)
// Construct a covariant quadratic, and reduce this
{
bigfloat* hcoeffs = quadratic_covariant(a,b,c,d,e);
unimod m1 = reduce_quad(hcoeffs[1],hcoeffs[2]);
delete [] hcoeffs;
// m1 contains the transform to reduce the quadratic;
// now update the input transform and quartic:
m*=m1;
apply_transform(a,b,c,d,e,m1);
#ifdef DEBUG_REDUCE
quartic newg(a,b,c,d,e);
cout << "reduced quartic has coefficients " << newg << endl;
#endif
#ifdef REDUCE_B
// Now reduce b so -2a < b <= 2a:
bigint newa4=a<<2;
bigint bmod4a = mod(b,newa4);
bigint alpha;
divide_exact((bmod4a-b),newa4,alpha);
if(!is_zero(alpha))
{
xshift(alpha,a,b,c,d,e,m);
#ifdef DEBUG_REDUCE
newg.assign(a,b,c,d,e);
cout << "after reducing b (shifting by "<<alpha<<"), reduced quartic has coefficients "
<< newg << endl;
#endif
}
#endif
}
// Finds a good unimodular matrix (a,b;c,d)
// which raises z=x0+i*y0 when y0 is small
int first_step(const bigfloat& x0, const bigfloat& y0,
bigint& a, bigint& b, bigint& c, bigint& d);
// Finds a unimodular matrix (a,b;c,d)
// which raises z=x0+i*y0 so im(z)>h
int one_step(const bigfloat& x0, const bigfloat& y0, const bigfloat& h,
bigint& a, bigint& b, bigint& c, bigint& d);
unimod reduce_quad_1(const bigfloat& b, const bigfloat& c);
unimod reduce_quad_2(const bigfloat& b, const bigfloat& c);
unimod reduce_quad(const bigfloat& b, const bigfloat& c)
{
return reduce_quad_1(b,c);
}
// Given a pos. def. quadratic x^2+b*x+c, returns a unimod which
// reduces it (whose inverse takes its root into the fundamental
// region).
unimod reduce_quad_1(const bigfloat& bb, const bigfloat& cc)
{
bigfloat b=bb, c=cc;
bigfloat dz = 4*c-b*b; // should be positive!
bigfloat xz = -b/2;
bigfloat yz = sqrt(abs(dz))/2;
#ifdef DEBUG_REDUCE_QUAD
bigfloat az = c;
cout << "Before any reduction:\n";
cout << "Covariant quadratic has coefficients 1, "<<b<<", "<<c<<endl;
cout << "-disc = " << dz << " (should be positive!)\n";
cout << "Root has real part = " << xz << endl;
cout << "imaginary part = " << yz << endl;
cout << "modulus^2 = " << az << endl;
#endif
// Now do reduction.
// Special first step: should raise so Im(z)>height
bigfloat h(to_bigfloat(0.1));
bigint ma, mb, mc, md;
ma=1; mb=0; mc=0; md=1;
one_step(xz,yz,h,ma,mb,mc,md);
#ifdef DEBUG_REDUCE_QUAD
cout<<"one_step returns [a,b;c,d] = ["<<ma<<","<<mb<<";"<<mc<<","<<md<<"]\n";
#endif
bigfloat xd=I2bigfloat(ma), xb=-I2bigfloat(mb),
xc=-I2bigfloat(mc), xa=I2bigfloat(md);
bigfloat a1=(xa*xa+(b*xa+c*xc)*xc);
bigfloat b1=2*(xa*xb+c*xc*xd)+b*(xa*xd+xb*xc);
bigfloat c1=(xb*xb+(b*xb+c*xd)*xd);
b=b1/a1;
c=c1/a1;
unimod m1(md,-mb,-mc,ma);
#ifdef DEBUG_REDUCE_QUAD
dz = 4*a1*c1-b1*b1; // should be positive!
xz = -b1/(2*a1);
yz = sqrt(abs(dz))/(2*a1);
az = c1/a1;
cout << "After first step of reduction:\n";
cout << "Covariant quadratic has coefficients "<<a1<<", "<<b1<<", "<<c1<<endl;
cout << "-disc = " << dz << " (should be positive!)\n";
cout << "Root has real part = " << xz << endl;
cout << "imaginary part = " << yz << endl;
cout << "modulus^2 = " << az << endl;
#endif
bigint s = Iround(xz);
bigfloat xk=I2bigfloat(s);
m1.x_shift(s);
c+=xk*(xk+b); b+=2*xk; xz-=xk;
int reduced = (c>0.99999);
#ifdef DEBUG_REDUCE_QUAD
dz = 4*c-b*b; // should be positive!
cout << "After preliminary shift by "<<s<<":\n";
cout << "Covariant quadratic has coefficients 1, "<<b<<", "<<c<<endl;
cout << "-disc = = " << dz << " (should be positive!)\n";
cout << "Root has real part = " << xz << endl;
cout << "imaginary part = " << yz << endl;
cout << "modulus^2 = " << c << endl;
#endif
while(!reduced)
{
// invert:
c=1/c; b*=(-c);
m1.invert();
// shift:
xz = -b/2;
s = Iround(xz);
xk=I2bigfloat(s);
c+=xk*(xk+b); b+=2*xk; xz -= xk;
m1.x_shift(s);
reduced = (c>0.99999);
#ifdef DEBUG_REDUCE_QUAD
//recompute root:
dz = 4*c-b*b; // should be positive!
yz = sqrt(abs(dz))/2;
cout << "After inversion and shift by "<<s<<":\n";
cout << "Covariant quadratic has coefficients "<<a<<", "<<b<<", "<<c<<endl;
cout << "-disc = = " << dz << " (should be positive!)\n";
cout << "Root has real part = " << xz << endl;
cout << "imaginary part = " << yz << endl;
cout << "modulus^2 = " << c << endl;
#endif
}
return m1;
}
unimod reduce_quad_2(const bigfloat& b, const bigfloat& c)
{
bigfloat xz = -b/2;
bigfloat dz = 4*c-b*b; // should be positive!
bigfloat az = c;
bigfloat yz = sqrt(abs(dz))/2;
bigcomplex z(xz,yz);
#ifdef DEBUG_REDUCE_QUAD
bigcomplex z0=z;
cout << "Before any reduction:\n";
cout << "Covariant quadratic has coefficients 1, "<<b<<", "<<c<<endl;
cout << "-disc = " << dz << " (should be positive!)\n";
cout << "Root = " << z << " with modulus^2 = " << az << endl;
#endif
// Now do reduction.
// Preliminary step shifts real part of root to [-1/2,1/2]
// gives better stability in low precision
bigint s = Iround(xz);
bigfloat xk=I2bigfloat(s);
unimod m1; // default constructor initializes to identity
m1.x_shift(s);
xz-=xk;
// yz is unchanged literally but we recompute it for better precision
bigfloat b1=b+2*xk;
bigfloat c1=xk*(xk+b)+c;
dz = 4*c1-b1*b1; // should be positive!
az = c1;
yz = sqrt(abs(dz))/2;
z = bigcomplex(xz,yz);
int reduced = (az>0.999);
#ifdef DEBUG_REDUCE_QUAD
cout << "After preliminary shift:\n";
cout << "Covariant quadratic has coefficients 1, "<<b1<<", "<<c1<<endl;
cout << "-disc = = " << dz << " (should be positive!)\n";
cout << "Root = " << z << " with modulus^2 = " << az << endl;
#endif
if(reduced) return m1;
// First stage: crude but guaranteed to double im(z):
bigfloat a11, a12, a21, a22;
bigint m11, m12, m21, m22;
static bigfloat one(to_bigfloat(1));
int changed=1;
first_step(xz, yz, m11, m12, m21, m22);
while(changed)
{
a11=I2bigfloat(m11); a12=I2bigfloat(m12);
a21=I2bigfloat(m21); a22=I2bigfloat(m22);
z = (a11*z+a12) / (a21*z+a22);
xz=real(z); yz=imag(z);
az = xz*xz + yz*yz;
unimod n(Iround(a22),Iround(-a12),
Iround(-a21),Iround(a11)); // rounding the inverse matrix
m1 *= n;
#ifdef DEBUG_REDUCE_QUAD
cout << "z = "<<z<<endl;
cout << "one matrix = "<<n<<"\n";
cout << "cumulative transform matrix = "<<m1<<"\n";
#endif
changed=first_step(xz, yz, m11, m12, m21, m22);
}
// Second stage: standard algorithm
#ifdef DEBUG_REDUCE_QUAD
cout << "After first stage , "<<endl;
cout << "z = "<<z<<endl;
#endif
reduced=0;
while(!reduced)
{
s = Iround(xz);
xk=I2bigfloat(s);
m1.x_shift(s);
xz-=xk;
z -=xk;
az = xz*xz + yz*yz;
#ifdef DEBUG_REDUCE_QUAD
cout << "After shift by "<<s<<", "<<endl;
cout << "z = "<<z<<endl;
cout << "|z|^2 = "<<az<<endl;
#endif
reduced = (az>0.999);
if(!reduced)
{
z=-one/z;
xz=real(z); yz=imag(z);
#ifdef DEBUG_REDUCE_QUAD
cout << "After inverting, "<<endl;
cout << "z = "<<z<<endl;
cout << "|z|^2 = "<<(1/az)<<endl;
#endif
m1.invert();
}
}
// final shift of x-coord
s = Iround(xz);
m1.x_shift(s);
#ifdef DEBUG_REDUCE_QUAD
bigfloat xs = I2bigfloat(s);
xz -= xs;
az = xz*xz + yz*yz;
z = bigcomplex(xz,yz);
cout << "After reduction, root = " << z << " with modulus^2 = "
<< az << endl;
cout<<"Transform matrix = "<<m1<<endl;
cout<<"This (inverted) applied to original root z0 = "<<z0<<endl;
a11=I2bigfloat(m1(1,1)); a12=I2bigfloat(m1(1,2));
a21=I2bigfloat(m1(2,1)); a22=I2bigfloat(m1(2,2));
bigcomplex z1=(a22*z0-a12) / (-a21*z0+a11);
cout<<" gives z = "<<z1<<endl;
#endif
return m1;
}
int first_step(const bigfloat& x0, const bigfloat& y0,
bigint& a, bigint& b, bigint& c, bigint& d)
// Finds a good unimodular matrix (a,b;c,d)
// which raises z=x0+i*y0 when y0 is small
{
#ifdef DEBUG_FIRSTSTEP
cout<<"In first_step with x0="<<x0<<", y0="<<y0<<endl;
#endif
a=1; b=0; c=0; d=1;
bigfloat xc = to_bigfloat(1)/(2*y0);
c=Ifloor(xc);
if(c<10) // return without doing anything
{
c=0;
#ifdef DEBUG_FIRSTSTEP
cout<<"first_step returns with no change"<<endl;
#endif
return 0;
}
d=-Iround(xc*x0);
bigint g = bezout(-c,d,b,a); // a*d-b*c=g, may be >1
if(g>1) {c/=g; d/=g;}
#ifdef DEBUG_FIRSTSTEP
cout<<"first_step returns (a,b;c,d) = ("<<a<<","<<b<<";"<<c<<","<<d<<")"<<endl;
#endif
return 1;
}
int one_step(const bigfloat& x0, const bigfloat& y0, const bigfloat& h,
bigint& a, bigint& b, bigint& c, bigint& d)
// Finds a good unimodular matrix (a,b;c,d)
// which raises z=x0+i*y0
{
#ifdef DEBUG_ONESTEP
cout<<"In one_step with x0="<<x0<<", y0="<<y0<<endl;
#endif
bigint k, newc, newd;
bigfloat x(x0), s0(y0/h);
bigfloat xk, x2, s1, s2, s=to_bigfloat(1), news;
int i, ans=0;
a=1; b=0; c=0; d=1;
for(i=0; ; i++)
{
k = Iround(x); xk=I2bigfloat(k); x2=xk-x;
#ifdef DEBUG_ONESTEP
cout<<i<<":\tk = "<<k<<", xk-x = "<<x2<<endl;
#endif
newc=k*c-a; newd=k*d-b;
#ifdef DEBUG_ONESTEP
cout<<"c'= "<<newc<<", d' = "<<newd<<endl;
#endif
s1 = x0*I2bigfloat(newc)+I2bigfloat(newd);
s2 = y0*I2bigfloat(newc);
news = s1*s1+s2*s2; // =Im(z)/Im(z')
#ifdef DEBUG_ONESTEP
cout<<"s = "<<news<<endl;
cout<<"Im(z) = "<<y0/news<<endl;
#endif
if(news>s) return ans; // new z lower than previous
s=news;
a= c; c= newc; b= d; d= newd;
if(s0>s) return ans; // Im(new z)>0.1
if(is_approx_zero(x2)) return ans;
x = 1/x2;
#ifdef DEBUG_ONESTEP
cout<<"New x = "<<x<<endl;
#endif
ans=1;
}
return ans;
}
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