      ring
      Copyright (c)  2005,2010,2012 John Abbott, Anna M. Bigatti
      GNU Free Documentation License, Version 1.2
%!includeconf: ../aux-txt2tags/config.t2t
      TeXTitle{ring}{John Abbott, Anna M. Bigatti}


== User documentation ==
%======================================================================

The primary use for a variable of type ``ring`` is simply to specify
the ring to which [[RingElem]] variables are associated.  CoCoALib
requires that the user specify first the rings in which to compute,
then values in those rings can be created and manipulated.  We
believe that this explicit approach avoids any possible problem of ambiguity.

The file ``ring.H`` introduces several classes used for representing rings
and their elements.  A normal user of the CoCoA library will use
principally the classes ``ring`` and [[RingElem]]: an object of type
``ring`` represents a mathematical ring with unity, and objects of type
[[RingElem]] represent values from some ring.  To make the documentation
more manageable it has been split into two: this file describes operations
directly applicable to rings, whereas a separate file describes the
operations on a [[RingElem]].  Documentation about the creation and use of
homomorphisms between rings can be found in [[RingHom]].

The documentation here is very general in nature: it applies to all rings
which can be created in the CoCoA library.  To find out how to create rings,
and for more specific documentation about the various special types of ring
CoCoALib offers, look at the relevant file: see the subsection below about
**//Types of Ring//**.

While the CoCoA library was conceived primarily for computing with
commutative rings, the possibility of creating and using certain
non-commutative rings exists.  The documentation for these rings is kept
separately in [[RingWeyl]].


=== Examples ===

Here is a list of example programs (to be found in the ``examples/``
subdirectory) illustrating the creation and use of various sorts of ring
and their elements
- [ex-ring1.C ../../examples/index.html#ex-ring1.C]
- [ex-ring2.C ../../examples/index.html#ex-ring2.C]
- [ex-RingElem1.C ../../examples/index.html#ex-RingElem1.C]
- [ex-RingFp1.C   ../../examples/index.html#ex-RingFp1.C]
- [ex-RingFp2.C   ../../examples/index.html#ex-RingFp2.C]
- [ex-RingQ1.C    ../../examples/index.html#ex-RingQ1.C]
- [ex-RingTwinFloat1.C ../../examples/index.html#ex-RingTwinFloat1.C]
- [ex-RingWeyl1.C ../../examples/index.html#ex-RingWeyl1.C]
- [ex-RingZZ1.C ../../examples/index.html#ex-RingZZ1.C]
-

=== Types of ring (inheritance structure) ===

- [[RingZZ]]
- [[RingTwinFloat]]
- [[FractionField]]
 - generic
 - [[RingQQ]]
- [[QuotientRing]]
 - generic
 - [[RingFp]]
 - [[RingFpLog]]
 - [[RingFpDouble]]
 - Simple algebraic extensions (not yet implemented)
- [[PolyRing]]
 - [[DenseUPolyRing]]
  - [[DenseUPolyClean]]
 - [[SparsePolyRing]]
  - [[RingWeyl]]
  - [[DistrMPoly]]
  - DistrMPolyInlPP
  - [[DistrMPolyInlFpPP]]
  -
 -
-

=== Pseudo-constructors ===
%----------------------------------------------------------------------

The default initial value for a ``ring`` is the ring of integers (``RingZZ``).

You can specify explicitly the initial value using one of the various ring pseudo-constructors:

 | ``RingZZ()``             | see [[RingZZ constructors]] |
 | ``RingQQ()``             | see [[RingQQ constructors]] |
 | ``NewZZMod(n)``          | see [[QuotientRing constructors]] |
 | ``NewRingTwinFloat(n)``  | see [[RingTwinFloat constructors]] |
 | ``NewFractionField(R)``  | see [[FractionField constructors]] |
 | ``NewQuotientRing(R,I)`` | see [[QuotientRing constructors]] |


=== Operations on Rings ===[operations]
%----------------------------------------------------------------------

Let ``R`` and ``R2`` be two variable of type ``ring``.

- ``characteristic(R)`` -- the characteristic of ``R`` (as a [[BigInt]])
- ``symbols(R)``        -- a std::vector of the symbols in ``R``
                          (//e.g.// Q(a)[x,y] contains the symbols a, x, and y)
- ``R = R2``           -- assign ``R2`` to ``R`` (so they both refer to the same identical internal impl)
- ``R == R2``          -- test whether ``R`` and ``R2`` are **identical**
       	                  (//i.e.// they refer to the same internal impl)
- ``R != R2``          -- the logical negation of ``R == R2``
-


 | ``zero(R)``     | the zero element of ``R`` |
 | ``one(R)``      | the one element of ``R``  |




==== Queries ====

In some cases the best algorithm to use may depend on whether the ring in
which we are computing has certain properties or not; so CoCoALib offers
some functions to ask a ring ``R`` about its properties:

- ``IsCommutative(R)``     -- a boolean, true iff ``R`` is commutative
- ``IsIntegralDomain(R)``  -- a boolean, true iff ``R`` has no zero divisors
- ``IsIntegralDomain3(R)`` -- a 3-state boolean, like ``IsIntegralDomain`` but fast, gives ``uncertain3`` if cannot determine proper answer quickly
- ``IsTrueGCDDomain(R)``   -- a boolean, true iff ``R`` is a true GCD domain (**note**: fields are //not// true GCD domains)
- ``IsOrderedDomain(R)``   -- a boolean, true iff ``R`` is arithmetically ordered
- ``IsField(R)``           -- a boolean, true iff ``R`` is a field
- ``IsFiniteField(R)``     -- a boolean, true iff ``R`` is a finite field
- ``LogCardinality(R)``    -- the integer k such that card(R) = p^k where p is char(R)
-

NOTE: a pragmatic approach is taken: //e.g.// ``IsOrderedDomain`` is true only
if comparisons between elements can actually be performed using the CoCoA library.

==== Queries and views ====

It may also be important to discover practical structural details of a ``ring``
(//e.g.// some algorithms make sense only for a polynomial ring).  The
following query functions ``Is...`` tell you how the ``ring`` is implemented,
and the //view// functions ``As...`` give access to the specific operations:

- ``IsZZ(R)``             -- see [[RingZZ query]]
- ``IsQQ(R)``             -- see [[RingQQ query]]
- ``IsDenseUPolyRing(R)`` -- see [[DenseUPolyRing query]]
- ``IsFractionField(R)``  -- see [[FractionField query]]
- ``IsPolyRing(R)``       -- see [[PolyRing query]]
- ``IsQuotientRing(R)``   -- see [[QuotientRing query]]
- ``IsSparsePolyRing(R)`` -- see [[SparsePolyRing query]]
-

In general the function "IsXYZ" should be read as "Is internally
implemented as XYZ": for instance ``IsQuotientRing`` is true only if the
internal implementation is as a quotient ring, so if ``ZZ`` denotes the ring
of integers and ``R = ZZ[x]/ideal(x)`` then ``R`` and ``ZZ`` are obviously
isomorphic but ``IsZZ(R)`` gives ``false`` and ``IsZZ(Z)`` gives ``true``,
while conversely ``IsQuotientRing(R)`` gives ``true`` and
``IsQuotientRing(ZZ)`` gives ``false``.


=== ADVANCED USE OF RINGS ===
%----------------------------------------------------------------------

The rest of this section is for more advanced use of ``ring``s (//e.g.// by
CoCoA library contributors).  If you are new to CoCoA, you need not read
this subsection.


==== Writing C++ classes for new types of ring ====
%----------------------------------------------------------------------

An important convention of the CoCoA library is that the class ``RingBase``
is to be used as an abstract base class for all rings.  You are strongly
urged to familiarize yourself with the maintainer documentation if you want
to understand how and why rings are implemented as they are in the CoCoA
library.

The first decision to make when implementating a new ring class for
CoCoALib is where to place it in the ring inheritance structure.  This
inheritance structure is a (currently) tree with all concrete classes at
the leaves, and all abstract classes being internal nodes.  Usually the new
concrete ring class is attached to the structure by making it derive from
one of the existing abstract ring classes.  You may even decide that it is
appropriate to add a new abstract ring class to this structure, and to make
the new concrete class derive from this new abstract class.

**Note:** I have note used multiple inheritance in the structure, largely
  because I do not trust multiple inheritance (not doubt due in part to my
  ignorance of the topic).


Once you have decided where to attach the new concrete class to the
structure, you will have to make sure that all pure virtual functions in the
abstract class are implemented.  Almost all instances of concrete rings are
built through pseudo-constructors (the rings ZZ and QQ are exceptional cases).

An **important detail** of the constructor for a new concrete ring is that
the reference count of the new ring object must be incremented to 1 at the
start of the constructor body (or more precisely, before any self
references are created, //e.g.// when creating the zero and one elements of
the ring); without this "trick" the constructor is not exception safe.

**NOTE** Every concrete ring creates a copy of its zero and one elements
(kept in auto_ptrs ``myZeroPtr`` and ``myOnePtr``).  This common
implementation detail cannot (safely) be moved up into ``RingBase`` because
during destruction by default the data members of ``RingBase`` are
destroyed after the derived concrete ring.  It seems much safer simply to
duplicate the code for each ring implementation class.



== Maintainer documentation ==
%======================================================================
(**NB** consider consulting also [[QuotientRing]], [[FractionField]]
and [[PolyRing]])

The design underlying rings and their elements is more complex than I
would have liked, but it is not as complex as the source code may make
it appear.  The guiding principles are that the implementation should be
flexible and easy/pleasant to use while offering a good degree of
safety; extreme speed of execution was not a goal (as it is usually
contrary to good flexibility) though an interface offering slightly
better run-time efficiency remains.

Regarding flexibility: in CoCoALib we want to handle polynomials whose
coefficients reside in (almost) any commutative ring.  Furthermore, the
actual rings to be used will be decided at run-time, and cannot
restricted to a given finite set.  We have chosen to use C++ inheritance
to achieve the implementation: the abstract class ``RingBase`` defines the
interface that every concrete ring class must offer.

Regarding ease of use: since C++ allows the common arithmetic operators
to be overloaded, it is essential that these work as expected for
elements of arbitrary rings -- with the caveat that ``/`` means **exact division**,
as this is the only reasonable interpretation.  Due to problems of
ambiguity, arithmetic between elements of different rings is forbidden:
//e.g.// let f be in QQ[x,y] and g in ZZ[y,x], where should f+g reside?

The classes in the file ring.H are closely interrelated, and there is no
obvious starting point for describing them -- you may find that you need
to read the following more than once to comprehend it.  Here is a list
of the classes:

 | ``ring``              | value represents a ring; it is a smart pointer |
 | ``RingBase``          | abstract class //defining what a ring is// |
 | ``RingElem``          | value represents an element of a ring |
 | ``ConstRefRingElem``  | const-reference to a RingElem |
 | ``RingElemConstRawPtr`` | raw pointer to a //const// ring value |
 | ``RingElemRawPtr``    | raw pointer to a ring value |

The class ``RingBase`` is of interest primarily to those wanting to
implement new types of ring (see relevant section below); otherwise you
probably don't need to know about it.  Note that ``RingBase`` includes an
intrusive reference counter -- so every concrete ring instance will have
one.  ``RingBase`` also includes a machine integer field containing a
unique numerical ID -- this is so that distinct copies of otherwise
identical rings can be distinguished when output (//e.g.// in OpenMath).

The class ``ring`` is used to represent mathematical rings (//e.g.// possible
values include ZZ, QQ, or QQ[x,y,z]).  An object of type ``ring`` is just a
reference counting smart pointer to a concrete ring implementation
object -- so copying a ring is fairly cheap.  In particular two rings
are considered equal if and only if they refer to the same identical
concrete ring implementation object.  In other files you will find
classes derived from ``ring`` which represent special subclasses of rings,
for instance ``PolyRing`` is used to represent polynomial rings.  The
intrusive reference count, which must be present in every concrete ring
implementation object, is defined as a data member of ``RingBase``.

For the other classes see [[RingElem]].


==== Further comments about implementation aspects of the above classes. ====

Recall that ``ring`` is essentially a smart pointer to a ``RingBase``
object, //i.e.// a concrete implementation of a ring.  Access to the
implementation is given via ``operator->``.  If necessary, the pointer may
also be read using the member function ``myRingPtr``: this is helpful for
defining functions such as ``IsPolyRing`` where access to the pointer is
required but ``operator->`` cannot be used.


The class ``RingBase`` declares a number of pure virtual functions for
computing with ring elements.  Since these functions are pure they must
all be fully defined in any instantiable ring class (//e.g.// RingZZImpl or
RingFpImpl).  These member functions follow certain conventions:

: **RETURN VALUES**:
   most arithmetic functions return no value, instead the
   result is placed in one of the arguments (normally the first argument
   is the one in which the result is placed), but functions which return
   particularly simple values (//e.g.// booleans or machine integers) do
   indeed return the values by the usual function return mechanism.

: **ARG TYPES**:
   ring element values are passed as //raw pointers//
   (//i.e.// a wrapped ``void*`` pointing to the actual value).  A read-only
   arg is of type ``RingElemConstRawPtr``, while a writable arg is of type
   ``RingElemRawPtr``.  When there are writable args they normally appear
   first.  For brevity there are typedefs ``ConstRawPtr`` and ``RawPtr`` in
   the scope of ``RingBase`` or any derived class.

: **ARG CHECKS**:
   sanity checks on the arguments are **not conducted** (//e.g.// the
   division function assumes the divisor is non-zero).  These member
   functions are supposed to be fast rather than safe.  However, if the
   compilation flag ``CoCoA_DEBUG`` was set then some checks may be performed.


In a few cases there are non-pure virtual member functions in
``RingBase``.  They exist either because there is a simple universal
definition or merely to avoid having to define inappropriate member
functions (//e.g.// gcd functions when the ring cannot be a gcd domain).
Here is a list of them:

 | ``IamTrueGCDDomain()``        | defaults to not ``IamField()``  |
 | ``IamOrderedDomain()``        | defaults to false               |


== Bugs, Shortcomings and other ideas ==
%======================================================================

Printing rings is unsatisfactory.  Need a mechanism for specifying a
print name for a ring; and also a mechanism for printing out the full
definition of the ring avoiding all/some print names.  For instance,
given the definitions ``R = QQ(x)`` and ``S = R[a,b]`` we see that
``S`` could be printed as ``S``, ``R[a,b]`` or ``QQ(x)[a,b]``.  We
should allow at least the first and the last of these possibilities.

Should (some of) the query functions return ``bool3`` values?
What about properties which are hard to determine?

The fn ``IsFiniteField`` is not very smart; it recognises only prime
finite fields, and simple algebraic extensions of them.

