source: git/libpolys/polys/ext_fields/algext.cc @ 7e9f12

/****************************************
*  Computer Algebra System SINGULAR     *
****************************************/
/**
  * ABSTRACT: numbers in an algebraic extension field K[a] / < f(a) >
  *           Assuming that we have a coeffs object cf, then these numbers
  *           are polynomials in the polynomial ring K[a] represented by
  *           cf->extRing.
  *           IMPORTANT ASSUMPTIONS:
  *           1.) So far we assume that cf->extRing is a valid polynomial
  *               ring in exactly one variable, i.e., K[a], where K is allowed
  *               to be any field (representable in SINGULAR and which may
  *               itself be some extension field, thus allowing for extension
  *               towers).
  *           2.) Moreover, this implementation assumes that
  *               cf->extRing->qideal is not NULL but an ideal with at
  *               least one non-zero generator which may be accessed by
  *               cf->extRing->qideal->m[0] and which represents the minimal
  *               polynomial f(a) of the extension variable 'a' in K[a].
  *           3.) As soon as an std method for polynomial rings becomes
  *               availabe, all reduction steps modulo f(a) should be replaced
  *               by a call to std. Moreover, in this situation one can finally
  *               move from K[a] / < f(a) > to
  *                  K[a_1, ..., a_s] / I, with I some zero-dimensional ideal
  *                                        in K[a_1, ..., a_s] given by a lex
  *                                        Gröbner basis.
  *               The code in algext.h and algext.cc is then capable of
  *               computing in K[a_1, ..., a_s] / I.
  **/

#include "config.h"
#include <misc/auxiliary.h>

#include <omalloc/omalloc.h>

#include <reporter/reporter.h>

#include <coeffs/coeffs.h>
#include <coeffs/numbers.h>
#include <coeffs/longrat.h>

#include <polys/monomials/ring.h>
#include <polys/monomials/p_polys.h>
#include <polys/simpleideals.h>

#ifdef HAVE_FACTORY
#include <polys/clapconv.h>
#include <factory/factory.h>
#endif

#include "ext_fields/algext.h"
#define TRANSEXT_PRIVATES 1
#include "ext_fields/transext.h"

#ifdef LDEBUG
#define naTest(a) naDBTest(a,__FILE__,__LINE__,cf)
BOOLEAN  naDBTest(number a, const char *f, const int l, const coeffs r);
#else
#define naTest(a) (TRUE)
#endif

/// Our own type!
static const n_coeffType ID = n_algExt;

/* polynomial ring in which our numbers live */
#define naRing cf->extRing

/* coeffs object in which the coefficients of our numbers live;
 * methods attached to naCoeffs may be used to compute with the
 * coefficients of our numbers, e.g., use naCoeffs->nAdd to add
 * coefficients of our numbers */
#define naCoeffs cf->extRing->cf

/* minimal polynomial */
#define naMinpoly naRing->qideal->m[0]

/// forward declarations
BOOLEAN  naGreaterZero(number a, const coeffs cf);
BOOLEAN  naGreater(number a, number b, const coeffs cf);
BOOLEAN  naEqual(number a, number b, const coeffs cf);
BOOLEAN  naIsOne(number a, const coeffs cf);
BOOLEAN  naIsMOne(number a, const coeffs cf);
BOOLEAN  naIsZero(number a, const coeffs cf);
number   naInit(long i, const coeffs cf);
int      naInt(number &a, const coeffs cf);
number   naNeg(number a, const coeffs cf);
number   naInvers(number a, const coeffs cf);
number   naAdd(number a, number b, const coeffs cf);
number   naSub(number a, number b, const coeffs cf);
number   naMult(number a, number b, const coeffs cf);
number   naDiv(number a, number b, const coeffs cf);
void     naPower(number a, int exp, number *b, const coeffs cf);
number   naCopy(number a, const coeffs cf);
void     naWriteLong(number &a, const coeffs cf);
void     naWriteShort(number &a, const coeffs cf);
number   naRePart(number a, const coeffs cf);
number   naImPart(number a, const coeffs cf);
number   naGetDenom(number &a, const coeffs cf);
number   naGetNumerator(number &a, const coeffs cf);
number   naGcd(number a, number b, const coeffs cf);
//number   naLcm(number a, number b, const coeffs cf);
int      naSize(number a, const coeffs cf);
void     naDelete(number *a, const coeffs cf);
void     naCoeffWrite(const coeffs cf, BOOLEAN details);
number   naIntDiv(number a, number b, const coeffs cf);
const char * naRead(const char *s, number *a, const coeffs cf);

static BOOLEAN naCoeffIsEqual(const coeffs cf, n_coeffType n, void * param);

#ifdef LDEBUG
BOOLEAN naDBTest(number a, const char *f, const int l, const coeffs cf)
{
  assume(getCoeffType(cf) == ID);
  if (a == NULL) return TRUE;
  p_Test((poly)a, naRing);
  if((((poly)a)!=naMinpoly)
  && p_Totaldegree((poly)a, naRing) >= p_Totaldegree(naMinpoly, naRing))
  {
    Print("deg >= deg(minpoly) in %s:%d\n",f,l);
    return FALSE;
  }
  return TRUE;
}
#endif

void heuristicReduce(poly &p, poly reducer, const coeffs cf);
void definiteReduce(poly &p, poly reducer, const coeffs cf);

/* returns the bottom field in this field extension tower; if the tower
   is flat, i.e., if there is no extension, then r itself is returned;
   as a side-effect, the counter 'height' is filled with the height of
   the extension tower (in case the tower is flat, 'height' is zero) */
static coeffs nCoeff_bottom(const coeffs r, int &height)
{
  assume(r != NULL);
  coeffs cf = r;
  height = 0;
  while (nCoeff_is_Extension(cf))
  {
    assume(cf->extRing != NULL); assume(cf->extRing->cf != NULL);
    cf = cf->extRing->cf;
    height++;
  }
  return cf;
}

BOOLEAN naIsZero(number a, const coeffs cf)
{
  naTest(a);
  return (a == NULL);
}

void naDelete(number * a, const coeffs cf)
{
  if (*a == NULL) return;
  if (((poly)*a)==naMinpoly) { *a=NULL;return;}
  poly aAsPoly = (poly)(*a);
  p_Delete(&aAsPoly, naRing);
  *a = NULL;
}

BOOLEAN naEqual(number a, number b, const coeffs cf)
{
  naTest(a); naTest(b);

  /// simple tests
  if (a == b) return TRUE;
  if ((a == NULL) && (b != NULL)) return FALSE;
  if ((b == NULL) && (a != NULL)) return FALSE;

  /// deg test
  int aDeg = 0;
  if (a != NULL) aDeg = p_Totaldegree((poly)a, naRing);
  int bDeg = 0;
  if (b != NULL) bDeg = p_Totaldegree((poly)b, naRing);
  if (aDeg != bDeg) return FALSE;

  /// subtraction test
  number c = naSub(a, b, cf);
  BOOLEAN result = naIsZero(c, cf);
  naDelete(&c, cf);
  return result;
}

number naCopy(number a, const coeffs cf)
{
  naTest(a);
  if (a == NULL) return NULL;
  if (((poly)a)==naMinpoly) return a;
  return (number)p_Copy((poly)a, naRing);
}

number naGetNumerator(number &a, const coeffs cf)
{
  return naCopy(a, cf);
}

number naGetDenom(number &a, const coeffs cf)
{
  naTest(a);
  return naInit(1, cf);
}

BOOLEAN naIsOne(number a, const coeffs cf)
{
  naTest(a);
  poly aAsPoly = (poly)a;
  if (!p_IsConstant(aAsPoly, naRing)) return FALSE;
  return n_IsOne(p_GetCoeff(aAsPoly, naRing), naCoeffs);
}

BOOLEAN naIsMOne(number a, const coeffs cf)
{
  naTest(a);
  poly aAsPoly = (poly)a;
  if (!p_IsConstant(aAsPoly, naRing)) return FALSE;
  return n_IsMOne(p_GetCoeff(aAsPoly, naRing), naCoeffs);
}

/// this is in-place, modifies a
number naNeg(number a, const coeffs cf)
{
  naTest(a);
  if (a != NULL) a = (number)p_Neg((poly)a, naRing);
  return a;
}

number naImPart(number a, const coeffs cf)
{
  naTest(a);
  return NULL;
}

number naInit_bigint(number longratBigIntNumber, const coeffs src, const coeffs cf)
{
  assume( cf != NULL );

  const ring A = cf->extRing;

  assume( A != NULL );

  const coeffs C = A->cf;

  assume( C != NULL );

  number n = n_Init_bigint(longratBigIntNumber, src, C);

  if ( n_IsZero(n, C) )
  {
    n_Delete(&n, C);
    return NULL;
  }

  return (number)p_NSet(n, A);
}



number naInit(long i, const coeffs cf)
{
  if (i == 0) return NULL;
  else        return (number)p_ISet(i, naRing);
}

int naInt(number &a, const coeffs cf)
{
  naTest(a);
  poly aAsPoly = (poly)a;
  if(aAsPoly == NULL)
    return 0;
  if (!p_IsConstant(aAsPoly, naRing))
    return 0;
  assume( aAsPoly != NULL );
  return n_Int(p_GetCoeff(aAsPoly, naRing), naCoeffs);
}

/* TRUE iff (a != 0 and (b == 0 or deg(a) > deg(b))) */
BOOLEAN naGreater(number a, number b, const coeffs cf)
{
  naTest(a); naTest(b);
  if (naIsZero(a, cf)) return FALSE;
  if (naIsZero(b, cf)) return TRUE;
  int aDeg = 0;
  if (a != NULL) aDeg = p_Totaldegree((poly)a, naRing);
  int bDeg = 0;
  if (b != NULL) bDeg = p_Totaldegree((poly)b, naRing);
  return (aDeg > bDeg);
}

/* TRUE iff a != 0 and (LC(a) > 0 or deg(a) > 0) */
BOOLEAN naGreaterZero(number a, const coeffs cf)
{
  naTest(a);
  if (a == NULL)                                            return FALSE;
  if (n_GreaterZero(p_GetCoeff((poly)a, naRing), naCoeffs)) return TRUE;
  if (p_Totaldegree((poly)a, naRing) > 0)                   return TRUE;
  return FALSE;
}

void naCoeffWrite(const coeffs cf, BOOLEAN details)
{
  assume( cf != NULL );

  const ring A = cf->extRing;

  assume( A != NULL );
  assume( A->cf != NULL );

  n_CoeffWrite(A->cf, details);

//  rWrite(A);

  const int P = rVar(A);
  assume( P > 0 );

  Print("//   %d parameter    : ", P);

  for (int nop=0; nop < P; nop ++)
    Print("%s ", rRingVar(nop, A));

  PrintLn();

  const ideal I = A->qideal;

  assume( I != NULL );
  assume( IDELEMS(I) == 1 );


  if ( details )
  {
    PrintS("//   minpoly        : (");
    p_Write0( I->m[0], A);
    PrintS(")");
  }
  else
    PrintS("//   minpoly        : ...");

  PrintLn();

/*
  char *x = rRingVar(0, A);

  Print("//   Coefficients live in the extension field K[%s]/<f(%s)>\n", x, x);
  Print("//   with the minimal polynomial f(%s) = %s\n", x,
        p_String(A->qideal->m[0], A));
  PrintS("//   and K: ");
*/
}

number naAdd(number a, number b, const coeffs cf)
{
  naTest(a); naTest(b);
  if (a == NULL) return naCopy(b, cf);
  if (b == NULL) return naCopy(a, cf);
  poly aPlusB = p_Add_q(p_Copy((poly)a, naRing),
                        p_Copy((poly)b, naRing), naRing);
  definiteReduce(aPlusB, naMinpoly, cf);
  return (number)aPlusB;
}

number naSub(number a, number b, const coeffs cf)
{
  naTest(a); naTest(b);
  if (b == NULL) return naCopy(a, cf);
  poly minusB = p_Neg(p_Copy((poly)b, naRing), naRing);
  if (a == NULL) return (number)minusB;
  poly aMinusB = p_Add_q(p_Copy((poly)a, naRing), minusB, naRing);
  definiteReduce(aMinusB, naMinpoly, cf);
  return (number)aMinusB;
}

number naMult(number a, number b, const coeffs cf)
{
  naTest(a); naTest(b);
  if (a == NULL) return NULL;
  if (b == NULL) return NULL;
  poly aTimesB = p_Mult_q(p_Copy((poly)a, naRing),
                          p_Copy((poly)b, naRing), naRing);
  definiteReduce(aTimesB, naMinpoly, cf);
  return (number)aTimesB;
}

number naDiv(number a, number b, const coeffs cf)
{
  naTest(a); naTest(b);
  if (b == NULL) WerrorS(nDivBy0);
  if (a == NULL) return NULL;
  poly bInverse = (poly)naInvers(b, cf);
  poly aDivB = p_Mult_q(p_Copy((poly)a, naRing), bInverse, naRing);
  definiteReduce(aDivB, naMinpoly, cf);
  return (number)aDivB;
}

/* 0^0 = 0;
   for |exp| <= 7 compute power by a simple multiplication loop;
   for |exp| >= 8 compute power along binary presentation of |exp|, e.g.
      p^13 = p^1 * p^4 * p^8, where we utilise that
      p^(2^(k+1)) = p^(2^k) * p^(2^k);
   intermediate reduction modulo the minimal polynomial is controlled by
   the in-place method heuristicReduce(poly, poly, coeffs); see there.
*/
void naPower(number a, int exp, number *b, const coeffs cf)
{
  naTest(a);

  /* special cases first */
  if (a == NULL)
  {
    if (exp >= 0) *b = NULL;
    else          WerrorS(nDivBy0);
  }
  else if (exp ==  0) { *b = naInit(1, cf); return; }
  else if (exp ==  1) { *b = naCopy(a, cf); return; }
  else if (exp == -1) { *b = naInvers(a, cf); return; }

  int expAbs = exp; if (expAbs < 0) expAbs = -expAbs;

  /* now compute a^expAbs */
  poly pow; poly aAsPoly = (poly)a;
  if (expAbs <= 7)
  {
    pow = p_Copy(aAsPoly, naRing);
    for (int i = 2; i <= expAbs; i++)
    {
      pow = p_Mult_q(pow, p_Copy(aAsPoly, naRing), naRing);
      heuristicReduce(pow, naMinpoly, cf);
    }
    definiteReduce(pow, naMinpoly, cf);
  }
  else
  {
    pow = p_ISet(1, naRing);
    poly factor = p_Copy(aAsPoly, naRing);
    while (expAbs != 0)
    {
      if (expAbs & 1)
      {
        pow = p_Mult_q(pow, p_Copy(factor, naRing), naRing);
        heuristicReduce(pow, naMinpoly, cf);
      }
      expAbs = expAbs / 2;
      if (expAbs != 0)
      {
        factor = p_Mult_q(factor, p_Copy(factor, naRing), naRing);
        heuristicReduce(factor, naMinpoly, cf);
      }
    }
    p_Delete(&factor, naRing);
    definiteReduce(pow, naMinpoly, cf);
  }

  /* invert if original exponent was negative */
  number n = (number)pow;
  if (exp < 0)
  {
    number m = naInvers(n, cf);
    naDelete(&n, cf);
    n = m;
  }
  *b = n;
}

/* may reduce p modulo the reducer by calling definiteReduce;
   the decision is made based on the following heuristic
   (which should also only be changed here in this method):
      if (deg(p) > 10*deg(reducer) then perform reduction;
   modifies p */
void heuristicReduce(poly &p, poly reducer, const coeffs cf)
{
  #ifdef LDEBUG
  p_Test((poly)p, naRing);
  p_Test((poly)reducer, naRing);
  #endif
  if (p_Totaldegree(p, naRing) > 10 * p_Totaldegree(reducer, naRing))
    definiteReduce(p, reducer, cf);
}

void naWriteLong(number &a, const coeffs cf)
{
  naTest(a);
  if (a == NULL)
    StringAppendS("0");
  else
  {
    poly aAsPoly = (poly)a;
    /* basically, just write aAsPoly using p_Write,
       but use brackets around the output, if a is not
       a constant living in naCoeffs = cf->extRing->cf */
    BOOLEAN useBrackets = !(p_IsConstant(aAsPoly, naRing));
    if (useBrackets) StringAppendS("(");
    p_String0Long(aAsPoly, naRing, naRing);
    if (useBrackets) StringAppendS(")");
  }
}

void naWriteShort(number &a, const coeffs cf)
{
  naTest(a);
  if (a == NULL)
    StringAppendS("0");
  else
  {
    poly aAsPoly = (poly)a;
    /* basically, just write aAsPoly using p_Write,
       but use brackets around the output, if a is not
       a constant living in naCoeffs = cf->extRing->cf */
    BOOLEAN useBrackets = !(p_IsConstant(aAsPoly, naRing));
    if (useBrackets) StringAppendS("(");
    p_String0Short(aAsPoly, naRing, naRing);
    if (useBrackets) StringAppendS(")");
  }
}

const char * naRead(const char *s, number *a, const coeffs cf)
{
  poly aAsPoly;
  const char * result = p_Read(s, aAsPoly, naRing);
  definiteReduce(aAsPoly, naMinpoly, cf);
  *a = (number)aAsPoly;
  return result;
}

#if 0
/* implemented by the rule lcm(a, b) = a * b / gcd(a, b) */
number naLcm(number a, number b, const coeffs cf)
{
  naTest(a); naTest(b);
  if (a == NULL) return NULL;
  if (b == NULL) return NULL;
  number theProduct = (number)p_Mult_q(p_Copy((poly)a, naRing),
                                       p_Copy((poly)b, naRing), naRing);
  /* note that theProduct needs not be reduced w.r.t. naMinpoly;
     but the final division will take care of the necessary reduction */
  number theGcd = naGcd(a, b, cf);
  return naDiv(theProduct, theGcd, cf);
}
#endif

/* expects *param to be castable to AlgExtInfo */
static BOOLEAN naCoeffIsEqual(const coeffs cf, n_coeffType n, void * param)
{
  if (ID != n) return FALSE;
  AlgExtInfo *e = (AlgExtInfo *)param;
  /* for extension coefficient fields we expect the underlying
     polynomial rings to be IDENTICAL, i.e. the SAME OBJECT;
     this expectation is based on the assumption that we have properly
     registered cf and perform reference counting rather than creating
     multiple copies of the same coefficient field/domain/ring */
  if (naRing == e->r)
    return TRUE;
  /* (Note that then also the minimal ideals will necessarily be
     the same, as they are attached to the ring.) */

  // NOTE: Q(a)[x] && Q(a)[y] should better share the _same_ Q(a)...
  if( rEqual(naRing, e->r, TRUE) ) // also checks the equality of qideals
  {
    const ideal mi = naRing->qideal; 
    assume( IDELEMS(mi) == 1 );
    const ideal ii = e->r->qideal;
    assume( IDELEMS(ii) == 1 );

    // TODO: the following should be extended for 2 *equal* rings...
    assume( p_EqualPolys(mi->m[0], ii->m[0], naRing, e->r) );
    
    rDelete(e->r);
    
    return TRUE;
  }

  return FALSE;

}

int naSize(number a, const coeffs cf)
{
  if (a == NULL) return -1;
  /* this has been taken from the old implementation of field extensions,
     where we computed the sum of the degree and the number of terms in
     (poly)a; so we leave it at that, for the time being;
     maybe, the number of terms alone is a better measure? */
  poly aAsPoly = (poly)a;
  int theDegree = 0; int noOfTerms = 0;
  while (aAsPoly != NULL)
  {
    noOfTerms++;
    int d = p_GetExp(aAsPoly, 1, naRing);
    if (d > theDegree) theDegree = d;
    pIter(aAsPoly);
  }
  return theDegree + noOfTerms;
}

/* performs polynomial division and overrides p by the remainder
   of division of p by the reducer;
   modifies p */
void definiteReduce(poly &p, poly reducer, const coeffs cf)
{
  #ifdef LDEBUG
  p_Test((poly)p, naRing);
  p_Test((poly)reducer, naRing);
  #endif
  p_PolyDiv(p, reducer, FALSE, naRing);
}

void  naNormalize(number &a, const coeffs cf)
{
  poly aa=(poly)a;
  if (aa!=naMinpoly)
    definiteReduce(aa,naMinpoly,cf);
  a=(number)aa;
}

#ifdef HAVE_FACTORY
number naConvFactoryNSingN( const CanonicalForm n, const coeffs cf)
{
  if (n.isZero()) return NULL;
  poly p=convFactoryPSingP(n,naRing);
  return (number)p;
}
CanonicalForm naConvSingNFactoryN( number n, BOOLEAN /*setChar*/, const coeffs cf )
{
  naTest(n);
  if (n==NULL) return CanonicalForm(0);

  return convSingPFactoryP((poly)n,naRing);
}
#endif


/* IMPORTANT NOTE: Since an algebraic field extension is again a field,
                   the gcd of two elements is not very interesting. (It
                   is actually any unit in the field, i.e., any non-
                   zero element.) Note that the below method does not operate
                   in this strong sense but rather computes the gcd of
                   two given elements in the underlying polynomial ring. */
number naGcd(number a, number b, const coeffs cf)
{
  naTest(a); naTest(b);
  if ((a == NULL) && (b == NULL)) WerrorS(nDivBy0);
  return (number)p_Gcd((poly)a, (poly)b, naRing);
}

number naInvers(number a, const coeffs cf)
{
  naTest(a);
  if (a == NULL) WerrorS(nDivBy0);
  poly aFactor = NULL; poly mFactor = NULL;
  poly theGcd = p_ExtGcd((poly)a, aFactor, naMinpoly, mFactor, naRing);
  naTest((number)theGcd); naTest((number)aFactor); naTest((number)mFactor);
  /* the gcd must be 1 since naMinpoly is irreducible and a != NULL: */
  assume(naIsOne((number)theGcd, cf));
  p_Delete(&theGcd, naRing);
  p_Delete(&mFactor, naRing);
  return (number)(aFactor);
}

/* assumes that src = Q, dst = Q(a) */
number naMap00(number a, const coeffs src, const coeffs dst)
{
  if (n_IsZero(a, src)) return NULL;
  assume(src == dst->extRing->cf);
  poly result = p_One(dst->extRing);
  p_SetCoeff(result, n_Copy(a, src), dst->extRing);
  return (number)result;
}

/* assumes that src = Z/p, dst = Q(a) */
number naMapP0(number a, const coeffs src, const coeffs dst)
{
  if (n_IsZero(a, src)) return NULL;
  /* mapping via intermediate int: */
  int n = n_Int(a, src);
  number q = n_Init(n, dst->extRing->cf);
  poly result = p_One(dst->extRing);
  p_SetCoeff(result, q, dst->extRing);
  return (number)result;
}

#if 0
/* assumes that either src = Q(a), dst = Q(a), or
                       src = Z/p(a), dst = Z/p(a)     */
number naCopyMap(number a, const coeffs src, const coeffs dst)
{
  return naCopy(a, dst);
}
#endif

number naCopyExt(number a, const coeffs src, const coeffs)
{
  fraction fa=(fraction)a;
  return (number)p_Copy(NUM(fa),src->extRing);
}

/* assumes that src = Q, dst = Z/p(a) */
number naMap0P(number a, const coeffs src, const coeffs dst)
{
  if (n_IsZero(a, src)) return NULL;
  int p = rChar(dst->extRing);

  number q = nlModP(a, src, dst->extRing->cf);

  poly result = p_NSet(q, dst->extRing);

  return (number)result;
}

/* assumes that src = Z/p, dst = Z/p(a) */
number naMapPP(number a, const coeffs src, const coeffs dst)
{
  if (n_IsZero(a, src)) return NULL;
  assume(src == dst->extRing->cf);
  poly result = p_One(dst->extRing);
  p_SetCoeff(result, n_Copy(a, src), dst->extRing);
  return (number)result;
}

/* assumes that src = Z/u, dst = Z/p(a), where u != p */
number naMapUP(number a, const coeffs src, const coeffs dst)
{
  if (n_IsZero(a, src)) return NULL;
  /* mapping via intermediate int: */
  int n = n_Int(a, src);
  number q = n_Init(n, dst->extRing->cf);
  poly result = p_One(dst->extRing);
  p_SetCoeff(result, q, dst->extRing);
  return (number)result;
}

nMapFunc naSetMap(const coeffs src, const coeffs dst)
{
  /* dst is expected to be an algebraic field extension */
  assume(getCoeffType(dst) == ID);

  if( src == dst ) return ndCopyMap;

  int h = 0; /* the height of the extension tower given by dst */
  coeffs bDst = nCoeff_bottom(dst, h); /* the bottom field in the tower dst */
  coeffs bSrc = nCoeff_bottom(src, h); /* the bottom field in the tower src */

  /* for the time being, we only provide maps if h = 1 and if b is Q or
     some field Z/pZ: */
  if (h==0)
  {
    if (nCoeff_is_Q(src) && nCoeff_is_Q(bDst))
      return naMap00;                            /// Q     -->  Q(a)
    if (nCoeff_is_Zp(src) && nCoeff_is_Q(bDst))
      return naMapP0;                            /// Z/p   -->  Q(a)
    if (nCoeff_is_Q(src) && nCoeff_is_Zp(bDst))
      return naMap0P;                            /// Q      --> Z/p(a)
    if (nCoeff_is_Zp(src) && nCoeff_is_Zp(bDst))
    {
      if (src->ch == dst->ch) return naMapPP;    /// Z/p    --> Z/p(a)
      else return naMapUP;                       /// Z/u    --> Z/p(a)
    }
  }
  if (h != 1) return NULL;
  if ((!nCoeff_is_Zp(bDst)) && (!nCoeff_is_Q(bDst))) return NULL;
  if ((!nCoeff_is_Zp(bSrc)) && (!nCoeff_is_Q(bSrc))) return NULL;

  if (nCoeff_is_Q(bSrc) && nCoeff_is_Q(bDst))
  {
    if (strcmp(rRingVar(0, src->extRing),
               rRingVar(0, dst->extRing)) == 0)
    {
      if (src->type==n_algExt)
         return ndCopyMap; // naCopyMap;         /// Q(a)   --> Q(a)
      else
         return naCopyExt;
    }
    else
      return NULL;                               /// Q(b)   --> Q(a)
  }

  if (nCoeff_is_Zp(bSrc) && nCoeff_is_Zp(bDst))
  {
    if (strcmp(rRingVar(0,src->extRing),rRingVar(0,dst->extRing))==0)
    {
      if (src->type==n_algExt)
        return ndCopyMap; // naCopyMap;          /// Z/p(a) --> Z/p(a)
      else
         return naCopyExt;
    }
    else
      return NULL;                               /// Z/p(b) --> Z/p(a)
  }

  return NULL;                                           /// default
}

static int naParDeg(number a, const coeffs cf)
{
  if (a == NULL) return -1;
  poly aa=(poly)a;
  return cf->extRing->pFDeg(aa,cf->extRing);
}

/// return the specified parameter as a number in the given alg. field
static number naParameter(const int iParameter, const coeffs cf)
{
  assume(getCoeffType(cf) == ID);

  const ring R = cf->extRing;
  assume( R != NULL );
  assume( 0 < iParameter && iParameter <= rVar(R) );

  poly p = p_One(R); p_SetExp(p, iParameter, 1, R); p_Setm(p, R);

  return (number) p;
}


/// if m == var(i)/1 => return i,
int naIsParam(number m, const coeffs cf)
{
  assume(getCoeffType(cf) == ID);

  const ring R = cf->extRing;
  assume( R != NULL );

  return p_Var( (poly)m, R );
}

BOOLEAN naInitChar(coeffs cf, void * infoStruct)
{
  assume( infoStruct != NULL );

  AlgExtInfo *e = (AlgExtInfo *)infoStruct;
  /// first check whether cf->extRing != NULL and delete old ring???

  assume(e->r                     != NULL);      // extRing;
  assume(e->r->cf                 != NULL);      // extRing->cf;

  assume((e->r->qideal            != NULL) &&    // minideal has one
         (IDELEMS(e->r->qideal)   == 1)    &&    // non-zero generator
         (e->r->qideal->m[0]      != NULL)    ); // at m[0];

  assume( cf != NULL );
  assume(getCoeffType(cf) == ID);                     // coeff type;

  e->r->ref ++; // increase the ref.counter for the ground poly. ring!
  const ring R = e->r; // no copy!
  assume( R->qideal == e->r->qideal );
  cf->extRing  = R;

  /* propagate characteristic up so that it becomes
     directly accessible in cf: */
  cf->ch = R->cf->ch;

  #ifdef LDEBUG
  p_Test((poly)naMinpoly, naRing);
  #endif

  cf->cfGreaterZero  = naGreaterZero;
  cf->cfGreater      = naGreater;
  cf->cfEqual        = naEqual;
  cf->cfIsZero       = naIsZero;
  cf->cfIsOne        = naIsOne;
  cf->cfIsMOne       = naIsMOne;
  cf->cfInit         = naInit;
  cf->cfInit_bigint  = naInit_bigint;
  cf->cfInt          = naInt;
  cf->cfNeg          = naNeg;
  cf->cfAdd          = naAdd;
  cf->cfSub          = naSub;
  cf->cfMult         = naMult;
  cf->cfDiv          = naDiv;
  cf->cfExactDiv     = naDiv;
  cf->cfPower        = naPower;
  cf->cfCopy         = naCopy;

  cf->cfWriteLong        = naWriteLong;

  if( rCanShortOut(naRing) )
    cf->cfWriteShort = naWriteShort;
  else
    cf->cfWriteShort = naWriteLong;

  cf->cfRead         = naRead;
  cf->cfDelete       = naDelete;
  cf->cfSetMap       = naSetMap;
  cf->cfGetDenom     = naGetDenom;
  cf->cfGetNumerator = naGetNumerator;
  cf->cfRePart       = naCopy;
  cf->cfImPart       = naImPart;
  cf->cfCoeffWrite   = naCoeffWrite;
  cf->cfNormalize    = naNormalize;
#ifdef LDEBUG
  cf->cfDBTest       = naDBTest;
#endif
  cf->cfGcd          = naGcd;
  //cf->cfLcm          = naLcm;
  cf->cfSize         = naSize;
  cf->nCoeffIsEqual  = naCoeffIsEqual;
  cf->cfInvers       = naInvers;
  cf->cfIntDiv       = naDiv;
#ifdef HAVE_FACTORY
  cf->convFactoryNSingN=naConvFactoryNSingN;
  cf->convSingNFactoryN=naConvSingNFactoryN;
#endif
  cf->cfParDeg = naParDeg;

  cf->iNumberOfParameters = rVar(R);
  cf->pParameterNames = R->names;
  cf->cfParameter = naParameter;

  return FALSE;
}