SmallFpImpl 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 in
QuotientRing (in particular the function
NewZZmod), or possibly the
SmallFpImpl offers the possibility of efficient arithmetic in
small, prime finite fields. This efficiency comes at a cost: the interface
is rather unnatural. The emphasis is on speed rather than convenience;
this speed depends on many functions being inlined.
The overall structure is modelled on that of
namely, operations on values are via member functions of
SmallFpImpl records the modulus, while the actual values are
SmallFpImpl::value, and record only the residue class. Also
see below for the special type
The ctor for a
SmallFpImpl object takes 1 or 2 args:
SmallFpImpl(p)- create a
p; error if
pis not prime, or too large.
SmallFpImpl(p,conv)- specify export convention
The default export convention is
SymmResidues (unless changed in the
This convention may be either
the default convention is determined by the
ModP be a
pis a valid
SmallFpImplctor arg; otherwise
SmallFpImpl::ourMaxModulus()-- returns largest ctor arg allowed by the implementation
ModP.myModulus()-- returns the prime
ModP.myMaxIters()-- see section on unnormalized computation
All operations (except for
must be effected by calling member functions of the
The member function
myReduce is effectively a ctor. Here is a brief summary.
long n; BigInt N; BigRat q; SmallFpImpl::value a, b, c; a = zero(SmallFp); // equiv to a = ModP.myReduce(0); b = one(SmallFp); // equiv to b = ModP.myReduce(1); IsZero(a); // equiv to (a == ModP.myReduce(0)) IsOne(b); // equiv to (b == ModP.myReduce(1)) a == b; // test for equality a != b; // logical negation of (a == b) ModP.myReduce(n); // reduce mod p ModP.myReduce(N); // reduce mod p ModP.myReduce(q); // reduce mod p ModP.myExportNonNeg(a); // returns the least non negative preimage (of type long), between 0 and p-1. ModP.myExportSymm(a); // returns a symmetric preimage (of type long), between -p/2 and p/2. ModP.myExport(a); // returns a preimage (of type long) between -p/2 and p-1; see note below! ModP.myNegate(a); // -a mod p, additive inverse ModP.myRecip(a); // inv(a), multiplicative inverse 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) ModP.myAddMul(a,b,c) // (a+b*c)%p
We suggest using the function
myExport principally for values to be printed;
in other contexts we recommend using
myExportNonNeg if possible.
myExport should assume only that the value returned is
p-1; the actual range of return values is determined
by the convention specified when the
SmallFpImpl object was constructed.
The normal mod p arithmetic operations listed above always produce a normalized result, but this normalization incurs a run-time cost. In some loops (e.g. for an inner product) it may be possible to compute several iterations before having to normalize the result.
SmallFpImpl supports this by offering the type
for unnormalized values; this type is effectively an unsigned integer,
and such values may be added and multiplied without normalization
(but also without overflow checks!) using the usual
SmallFpImpl offers the following three functions to help implement
a delayed normalization strategy.
SmallFpImpl::NonRedValue a; ModP.myNormalize(a); -- FULL normalization of a, result is a SmallFpImpl::value ModP.myHalfNormalize(a); -- *fast*, PARTIAL normalization of a, result is a NonRedValue ModP.myMaxIters(); -- see comment below
The value of
myMaxIters() is the largest number of unnormalized
products (of normalized values) which may safely be added to a "half
normalized" value without risking overflow. The half normalization
operation is quick (at most a comparison and a subtraction).
Naturally, the final result must be fully normalized. See example
ex-SmallFp1.C for a working implementation.
Most functions are implemented inline, and no sanity checks are
performed (except when
CoCoA_DEBUG is enabled). The constructor
does do some checking.
SmallFpImpl::value_t must be an unsigned integral type; it is a
typedef to a type specified in
CoCoA/config.H -- this should allow
fairly easy platform-specific customization.
This code is valid only if the square of
myModulus can be represented
SmallFpImpl::value_t; the constructor checks this condition.
Most functions do not require
myModulus to be prime, though division
becomes only a partial map if it is composite; and the function
myIsDivisible is correct only if
myModulus is prime. Currently the
constructor rejects non-prime moduli.
The code assumes that each value modulo p is represented as the least
non-negative residue (i.e. the values are represented as integers in
the range 0 to p-1 inclusive). This decision is linked to the fact
SmallFpImpl::value_t is an unsigned type.
myIterLimit are to allow efficient
exploitation of non-reduced multiplication (e.g. when trying to
compute an inner product modulo p). See example program
The return type of
int even though the result is
always non-negative -- I do not like
Should there be a