SmallFpDoubleImpl is a very low level implementation class
for fast arithmetic in a small, prime finite field. It is not intended
for use by casual CoCoALib users, who should instead see the documentation
QuotientRing (in particular the function
NewZZmod), or possibly
the documentation in
SmallFpImpl the main difference is an implementation
detail: values are represented as
doubles -- on 32-bit computers this
allows a potentially usefully greater range of characteristics at a
probably minor run-time cost.
All operations on values must be effected by calling member functions
SmallFpDoubleImpl class. Here is a brief summary.
SmallFpDoubleImpl::IsGoodCtorArg(p); // true iff ctor SmallFpDoubleImpl(p) will succeed SmallFpDoubleImpl::ourMaxModulus(); // largest permitted modulus SmallFpDoubleImpl ModP(p, convention); // create SmallFpDoubleImpl object long n; BigInt N; BigRat q; SmallFpImpl::value_t a, b, c; ModP.myModulus(); // value of p (as a long) ModP.myReduce(n); // reduce mod p ModP.myReduce(N); // reduce mod p ModP.myReduce(q); // reduce mod p ModP.myExport(a); // returns a preimage (of type long) according to symm/non-neg convention. ModP.myNegate(a); // -a mod p ModP.myAdd(a, b); // (a+b)%p; ModP.mySub(a, b); // (a-b)%p; ModP.myMul(a, b); // (a*b)%p; ModP.myDiv(a, b); // (a*inv(b))%p; where inv(b) is inverse of b ModP.myPower(a, n); // (a^n)%p; where ^ means "to the power of" ModP.myIsZeroAddMul(a,b,c) // a = (a+b*c)%p; result is (a==0)
myExport the choice between least non-negative and symmetric
residues is determined by the convention specified when constructing
SmallFpDoubleImpl object. This convention may be either
Most functions are implemented inline, and no sanity checks are
performed (except when
CoCoA_DEBUG is enabled). The constructor
does do some checking. The basic idea is to use the extra precision
doubles to allow larger prime finite fields than are
permitted when 32-bit integers are used for all arithmetic. If fast
64-bit arithmetic becomes widespread then this class will probably
become obsolete (unless you have a very fast floating point coprocessor?).
SmallFpDoubleImpl::value_t is simply
double. Note that the
values are always non-negative integers with maximum value less than
myModulusValue; i.e. each residue class is represented
(internally) by its least non-negative member.
To avoid problems with overflow the constructor checks that all
integers from 0 to p*p-p can be represented exactly. We need to allow
numbers as big as p*p-p so that
myIsZeroAddMul can be implemented easily.
It is not strictly necessary that
myModulusValue be prime, though division
becomes only a partial map if
myModulusValue is composite. I believe it is
safest to insist that
myModulusValue be prime.
The implementation is simplistic -- I wanted to dash it off quickly before going on holiday :-)