CoCoALib-0.9905 date: 23 May 2007


CoCoA::PPMonoid Class Reference

#include <PPMonoid.H>

Collaboration diagram for CoCoA::PPMonoid:

Collaboration graph
[legend]
List of all members.

Public Member Functions

 PPMonoid (const PPMonoidBase *ptr)
const PPMonoidBaseoperator-> () const
 Allow const member fns to be called.
bool operator== (const PPMonoid &PPM) const

Private Attributes

SmartPtrIRC< const PPMonoidBasemySmartPtr

Detailed Description

      Copyright (c)  2005,2006 John Abbott
      Permission is granted to copy, distribute and/or modify this document
      under the terms of the GNU Free Documentation License, Version 1.2;
      with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
      A copy of the licence is included in the file COPYING in this directory.



User documentation for the classes PPMonoid, PPMonoidElem and PPMonoidBase
==========================================================================

The classes [PPMonoid] and [PPMonoidElem] are analogous to [ring] and
[RingElem].  A [PPMonoid] represents a (multiplicative) power product
monoid with grading and compatible total arithmetic ordering; a
[PPMonoidElem] represents an element of a [PPMonoid], i.e. a power product.



Functions for [PPMonoid]s
-------------------------

Recall that every PP monoid is graded, and has a degree-compatible total
arithmetical ordering; the grading and ordering must be specified when the
[PPMonoid] is created.  For convenient input and output, also the names
of the indeterminates generating the monoid must be specified when the
monoid is created.

If you expect to use large exponents then you should use only the special
PP monoid created by [PPMonoidEvZZ].  The other PP monoids should usually
be fine for exponents up to 1000 or more; the true limit depends on the
specific monoid, the number of indeterminates, and the PP ordering.  At the
moment there is no way to find out what the true limit is (see Bugs), and
no warning is given should the limit be exceeded: you just get a wrong
answer.

To create a PPMonoid use the function [NewPPMonoid].  To create a
[PPMonoid] object of a specific type use one of the pseudo-constructors
related to the concrete monoid classes: [PPMonoidEvOv], [PPMonoidEv],
[PPMonoidEvZZ] and [PPMonoidOv].  Here are some examples
   PPOrdering PPO = ...;            // see PPOrdering.txt
   vector<symbol> IndetNames = ...; // see symbol.txt
   PPMonoid PPM = NewPPMonoid(PPO, IndetNames);
   PPMonoid PPM1 = NewPPMonoidEv(PPO, IndetNames);
   PPMonoid PPM2 = NewPPMonoidOv(PPO, IndetNames);
   PPMonoid PPM3 = NewPPMonoidEvOv(PPO, IndetNames);
   PPMonoid PPM4 = NewPPMonoidEvZZ(PPO, IndetNames);

cout << PPM          -- print PPM on cout
NumIndets(PPM)       -- number of indeterminates
ordering(PPM)        -- the PP ordering inherent in PPM
GradingDim(PPM)      -- the dimension of the grading (zero if ungraded)
indets(PPM)          -- std::vector whose n-th entry is n-th indet as a [PPMonoidElem]
symbols(PPM)         -- std::vector of the symbols in PPM (i.e. names of the indets)
PPM1 == PPM2         -- true iff PPM1 and PPM2 are identical (same addr)
PPM1 != PPM2         -- true unless PPM1 and PPM2 are identical

These pseudo-constructors are described in the section about [PPMonoidElem]s
one(PPM)             -- the identity of PPM
indet(PPM, var)      -- the var-th indeterminate as a PP
IndetPower(PPM, var, exp) -- the exp-th power of the var-th indeterminate as a PP


Summary of functions for [PPMonoidElem]s
----------------------------------------

See also some example programs in the CoCoALib/examples/ directory.

When a new object of type [PPMonoidElem] is created the monoid to which it
belongs must be specified either explicitly as a constructor argument, or
implicitly as the monoid associated with some constructor argument.  Once
the [PPMonoidElem] object has been created it is not possible to make it
belong to any another monoid.  Comparison and arithmetic between objects of
type [PPMonoidElem] is permitted only if they belong to the same identical
monoid.

NOTE: when writing a function which has an argument of type [PPMonoidElem],
you should specify the argument type as [ConstRefPPMonoidElem], or
[RefPPMonoidElem] if you want to modify its value.

Let PPM be a PPMonoid; for convenience, in comments we shall use x[i] to
refer to the i-th indeterminate in PPM.  Let pp be a non-const
PPMonoidElem, and pp1 and pp2 const PPMonoidElems (all belonging PPM).
Let expv be a vector<long> of size equal to the number of indeterminates.

PPMonoidElem t(PPM);       -- create new PP in PPM, value is 1
PPMonoidElem t(PPM, expv); -- create new PP in PPM, value is product x[i]^expv[i]
PPMonoidElem t(pp1);       -- create a new copy of pp1, belongs to same PPMonoid as pp1
one(PPM)                   -- the 1 belonging to PPM
indet(PPM, v)              -- create a new copy of x[v] the v-th indeterminate of PPM
IndetPower(PPM, v, n)      -- create x[v]^n, n-th power of v-th indeterminate of PPM

owner(pp1)                 -- returns the PPMonoid to which pp1 belongs

IsOne(pp)                  -- returns true iff pp = 1
IsIndet(i, pp)             -- returns true iff pp is an indet; if true, puts index of indet into i.
cmp(pp1, pp2)              -- compare pp1 with pp2 using inherent ordering;
                              result is <0 if pp1 < pp2, =0 if pp1 = pp2, and >0 if pp1 > pp2.
pp1 == pp2                 -- the six standard comparison operators...
pp1 != pp2                 --   ...
pp1 < pp2                  --   ... (inequalities use the ordering inherent in PPM)
pp1 <= pp2                 --   ...
pp1 > pp2                  --   ...
pp1 >= pp2                 --   ...

pp1 * pp2                  -- product of pp1 and pp2
pp1 / pp2                  -- quotient of pp1 by pp2, quotient MUST be exact
                              (see the function IsDivisible below)
colon(pp1, pp2)            -- "colon" quotient of pp1 by pp2; equal to pp1/gcd(pp1,pp2)
gcd(pp1, pp2)              -- gcd of pp1 and pp2
lcm(pp1, pp2)              -- lcm of pp1 and pp2
power(pp1, n)              -- n-th power of pp1 (NB: cannot use pp1^n, see below)
IsCoprime(pp1, pp2)        -- tests whether pp1 and pp2 are coprime
IsDivisible(pp1, pp2)      -- tests whether pp1 is divisible by pp2
AssignOne(pp)              -- sets pp = 1
swap(pp, pp0)              -- swaps the values of pp and pp0
pp = pp1                   -- assignment (pp and pp1 must belong to same PPMonoid)
pp *= pp1                  -- same as pp = pp * pp1
pp /= pp1                  -- same as pp = pp / pp1
StdDeg(pp1)                -- standard degree of pp1; result is of type [int]
wdeg(pp1)                  -- weighted degree of pp1 (using specified grading); result is of type [degree]
CmpWDeg(pp1, pp2)          -- <0 =0 >0 according as wdeg(pp1) < = > wdeg(pp2); order is lex, see degree.txt.
log(pp1, v)                -- power to which x[v] appears in pp1 (result is a signed long)
exponents(expv, pp)        -- fills array expv so that expv[i] = log(pp, i) for i=0,..,NumIndets(PPM)-1
cout << pp1                -- print out the value of pp1



Library Contributor Documentation
=================================

This section comprises two parts: the first is about creating a new type
of PP monoid; the second comments about calling the member functions of
[PPMonoidBase] directly.


To add a new type of concrete PPMonoid class
--------------------------------------------

My first suggestion is to look at the code implementing [PPMonoidEv].
This is a simple PP monoid implementation: the values are represented as
C arrays of exponents.  Initially you should ignore the class [CmpBase]
and those derived from it; they are simply to permit fast comparison of
PPs in certain special cases.


First, a note about "philosophy".  As far as we can tell the programming
language C++ does not have a built-in type system sufficiently flexible
(and efficient) for our needs, consequently we have to build our own type
system on top of what C++ offers.  The way we have chosen to do this is as
follows (note that the overall scheme used here is similar to that used for
rings and their elements).

To fit into CoCoALib your new class must be derived from [PPMonoidBase].
Remember that any operation on elements of your PP monoid will be effected
by calling a member function of your new monoid class.


The monoid must be a cartesian power of N, the natural numbers, with the
monoid operation (called "multiplication") being vector addition -- the
vector should be thought of as the vector of exponents in a power product.
The monoid must have a total arithmetic ordering; often this will be specified
when the monoid is created.  The class [PPOrdering] represents the possible
orderings.


Here is a summary of the member functions which must be implemented.  All
the functions may be called for a "const" [PPMonoid], for brevity the [const]
qualifier is omitted.  I use two abbreviations:
  RawPP       is short for  PPMonoidElemRawPtr
  ConstRawPP  is short for  PPMonoidElemConstRawPtr

NOTE: all arithmetic function must tolerate argument aliasing (i.e. any
pair of arguments may be identical).

Constructors: these all allocate memory which must eventually be freed (by
calling myDelete); the result is a pointer to the memory allocated.

PPMonoidElemRawPtr PPMonoidBase::myNew()                     // initialize pp to the identity
PPMonoidElemRawPtr PPMonoidBase::myNew(const vector<int>& v) // initialize pp from exponent vector v
PPMonoidElemRawPtr PPMonoidBase::myNew(const RawPP& pp1)     // initialize pp from pp1


Destructor: there is only one of these, its argument must be initialized
void PPMonoidBase::myDelete(PPMonoidElemRawPtr pp)           // destroy pp, frees memory

Assignment etc:
void PPMonoidBase::mySwap(RawPP pp1, RawPP pp2)             // swap the values of pp1 and pp2
void PPMonoidBase::myAssign(RawPP pp, ConstRawPP pp1)       // assign the value of pp1 to pp
void PPMonoidBase::myAssign(RawPP pp, const vector<int>& v) // assign to pp the PP with exponent vector v

Arithmetic: in all cases the first arg is where the answer is placed,
            aliasing is permitted (i.e. arguments need not be distinct);
            div result is undefined if the quotient does not exist!
const PPMonoidElem& myOne()                                 // reference to 1 in the monoid
void myMul(RawPP pp, ConstRawPP pp1, ConstRawPP pp2)        // pp = pp1*pp2
void myMulIndetPower(RawPtr pp, size_t var, unsigned long exp) // pp *= indet(var)^exp
void myDiv(RawPP pp, ConstRawPP pp1, ConstRawPP pp2)        // pp = pp1/pp2 (if it exists)
void myColon(RawPP pp, ConstRawPP pp1, Const RawPP pp2)     // pp = pp1/gcd(pp1,pp2)
void myGcd(RawPP pp, ConstRawPP pp1, ConstRawPP pp2)        // pp = gcd(pp1, pp2)
void myLcm(RawPP pp, ConstRawPP pp1, ConstRawPP pp2)        // pp = lcm(pp1, pp2)
void myPower(RawPP pp, ConstRawPP pp1, int exp)             // pp = pp1^exp

Comparison and testing:
   each PPMonoid has associated with it a "term ordering", a total
   ordering which respects the monoid operation (multiplication)
bool myIsCoprime(ConstRawPP pp1, ConstRawPP pp2)     // true iff gcd(pp1, pp2) is 1
bool myIsDivisible(ConstRawPP t1, ConstRawPP t2)     // true iff t1 is divisible by t2
int myCmp(ConstRawPP t1, ConstRawPP t2)              // result is <0, =0, >0 according as t1 <,=,> t2
NYI int myHomogCmp(ConstRawPP t1, ConstRawPP t2)         // as cmp, but assumes t1 and t2 have the same degree


Sundries:
degree myDeg(ConstRawPP t)                           // total degree
long myLog(ConstRawPP t, unsigned int var)// degree in var (in an unsigned type)
void myExponents(vector<long>& v, ConstRawPP t)      // get exponents
ostream& myOutput(ostream& out, const RawPP& t)      // prints t on out; default defn in PPMonoid.C

Query functions:
size_t myNumIndets()                                 // number of indeterminates generating the monoid
const string& myIndetName(size_t var)                // name of indet with index var



To add a new member function to PPMonoidBase
--------------------------------------------

You will have to edit PPMonoid.H and possibly PPMonoid.C (e.g. if there is
to be a default definition).  Arguments representing PPs should be of type
[RawPP] if they may be modified, or of type [ConstRawPP] if they should
never be modified.  See also the Coding Conventions about names of member
functions.

If you do add a new pure virtual member function, you will have to add
definitions to all the existing concrete PP monoid classes (otherwise they
will become uninstantiable).  Don't forget to update the documentation too!



Calculating directly with raw PPs
---------------------------------

Values of type [PPMonoidElem] are intended to be simple and safe to use
but with some performance penalty.  There is also a "fast, ugly, unsafe"
option which we shall describe here.

The most important fact to heed is that a [PPMonoidElemRawPtr] value is NOT
a C++ object -- it does not generally know enough about itself even to
destroy itself.  This places a considerable responsibility on the
programmer, and probably makes it difficult to write exception clean code.
You really must view the performance issue as paramount if you plan to use
raw PPs!  In any case the gain in speed will likely be only slight.


The model for creation/destruction and use of raw PPs is as follows:
[NB see "bug" section about exception-safety]
 (1) an uninitialized raw PP is acquired from the system;
 (2) the raw PP is "initialized" by calling an "init" function -- this will
     generally acquire further resources;
 (3) now the RawPP may be used for i/o, arithmetic, and so forth;
 (4) finally, when the value is no longer required the extra resources
     acquired during initialization should be released by calling the "kill"
     function -- failure to call "kill" will probably result in a memory leak.

Here is some pseudo C++ code to give an idea
  const PPMonoid& M;     // A PPMonoid from somewhere

  PPMonoidElemRawPtr t;  // A wrapped opaque pointer; initially points into hyperspace.

  t = M->myNew();        // Allocate resources for a new PP belonging to M;
                         // there are two other myNew functions.
  .... operations on t; always via a member function of the monoid M ...

  M->myDelete(t);        // "destroy" the value t held; t points into hyperspace again.

NOTE: the only functions which take a pointer into hyperspace are PPMonoidBase::myNew;
      many functions, e.g. PPMonoidBase::myMul, write their result into the first argument
      and require that that first argument be already allocated/initialized.

NOTE: if an exception is thrown after M->myNew and before M->myDelete then
      there will be a memory leak.  If t is just to hold a temporary local
      value then it is better to create a full PPMonoidElem and then let t
      be its "RawPtr"; this should avoid memory leaks.




Maintainer documentation for PPMonoid, PPMonoidElem, and PPMonoidBase
=====================================================================

The general structure here mirrors that of rings and their elements, so
you may find it helpful to read ring.txt if the following seems too
opaque.  At first sight the design may seem complex (because it
comprises several classes), but there's no need to be afraid.


The class [PPMonoid] is a reference counting smart pointer to an object
derived from [PPMonoidBase].  This means that making copies of a
[PPMonoid] is very cheap, and that it is easy to tell if two [PPMonoid]s
are identical.  Assignment of [PPMonoid]s is disabled because I am not
sure whether it is useful/meaningful.  [operator->] allows member
functions of [PPMonoidBase] to be called using a simple syntax.


The class [PPMonoidBase] is what specifies the class interface for each
concrete PP monoid implementation, i.e. the operations that it must offer.
It includes an intrusive reference count for compatibility with
[PPMonoid].  Since it is inconceivable to have a PP monoid without an
ordering, there is a data member for memorizing the inherent [PPOrdering].
This data member is [protected] so that it is accessible only to friends
and derived classes.

The function [PPMonoidBase::myOutput] for printing PPs has a reasonable
default definition.



The situation for elements of a PP monoid could easily appear horrendously
complicated.  The basic idea is that a PP monoid element comprises two
components: one indicating the [PPMonoid] to which the value belongs, and
the other indicating the actual value.  This allows the user to employ a
notationally convenient syntax for many operations -- the emphasis is on
notational convenience rather than ultimate run-time efficiency.


For an element of a PP monoid, the owning [PPMonoid] is specified during
creation and remains fixed throughout the life of the object; in contrast
the value may be varied (if C++ const rules permit).  The value is
indicated by an opaque pointer (essentially a wrapped [void*]): only the
owning [PPMonoid] knows how to interpret the data pointed to, and so all
operations on the value are effected by member functions of the owning
[PPMonoid].

I do not like the idea of having naked [void*] values in programs: it is
too easy to get confused about what is pointing to what.  Since the
value part of a [PPMonoidElem] is an opaque pointer (morally a [void*]),
I chose to wrap it in a lightweight class; actually there are two classes
depending on whether the pointed to value is [const] or not.  These
classes are [PPMonoidElemRawPtr] and [PPMonoidElemConstRawPtr]; they
are opaque pointers pointing to a value belonging to some concrete PP
monoid (someone else must keep track of precisely which PP monoid is the
owner).

The constructors for [PPMonoidElemRawPtr] and [PPMonoidElemConstRawPtr]
are [explicit] to avoid potentially risky automatic conversion of any
old pointer into one of these types.  The naked pointer may be accessed
via the member functions [myRawPtr].  Only implementors of new PP
monoid classes are likely to find these two opaque pointer classes useful.


I now return to the classes for representing fully qualified PPs.
There are three very similar yet distinct classes for elements of PP
monoids; the distinction is to keep track of constness and ownership.
I have used inheritance to allow natural automatic conversion among
these three classes (analogously to [RingElem], [RefRingElem] etc.)

* A [PPMonoidElem] is the owner of its value; the value will be deleted
  when the object ceases to exist.
* A [RefPPMonoidElem] is not the owner of its value, but the value may be
  changed (and the owner of the value will see the change too).
* A [ConstRefPPMonoidElem] is not the owner of its value, and its value
  may not be changed (through this reference).


The data layout is determined in [ConstRefPPMonoidElem], and the more
permissive classes inherit the data members.  I have deliberately used a
non-constant [PPMonoidElemRawPtr] for the value pointer as it is easier for
the class [ConstRefPPMonoidElem] to add in constness appropriately than it
is for the other two classes to remove it.  The four assignment operators
must all be defined since C++ does not allow polymorphism in the destination
object (e.g. because of potential problems with slicing).  Ideally it would
be enough to define assignment just from a [ConstRefPPMonoidElem], but I
have to define also the "homogeneous" assignment operator since the default
definition would not work properly.  It is a bit tedious to have four copies
of the relevant code (but it is only a handful of lines each time).

By convention the member functions of [PPMonoidBase] which operate on
raw PP values assume that the values are valid (e.g. belong to the same
PP monoid, division is exact in [myDiv]).  The validity of the arguments
is checked by the syntactically nice equivalent operations (see the code
in PPMonoid.C).  This permits a programmer to choose between safe clean
code (with nice syntax) or faster unsafe code (albeit with uglier syntax).



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

The section on "Advanced Use" is a bit out of date and too long.

(1) Should more operations on [PPMonoidElem]s be inlined?
    With the current design, since speed is not so important for [PPMonoidElem]s.

(2) We would like a way of performing divisibility tests faster when
    there are few indeterminates and relatively high degrees.  In this
    case the DivMask is useless.  The "gonnet" example is slow because
    it entails many divisibility tests.  One suggestion would be to
    maintain a "randomly weighted" degree and use that as a simple
    heuristic for deciding quickly some cases.

(3) I've fixed the various arithmetic functions for [PPMonoidElem]s so
    that they are obviously exception safe, BUT they now make an extra
    copy of the computed value (as it is returned from a local variable
    to the caller).  Here is an idea for avoiding that extra copy.
    Create a new type (say PPMonoidElem_local) which offers just raw(..)
    and a function export(..) which allows the return mechanism to
    create a full [PPMonoidElem] (just by copying pointers) and empty
    out the PPMonoidElem_local.  If the PPMonoidElem_local is not
    empty then it can destroy the value held within it.  By not
    attempting to make PPMonoidElem_locals behave like full
    PPMonoidElems I save a lot of "useless" function definitions.
    Indeed the "export" function need not exist: an implicit ctor for
    a PPMonoidElem from a PPMonoidElem_local could do all the work.
    I'll wait to see profiling information before considering implementing.

(4) Is assignment for [PPMonoid]s likely to be useful to anyone?
    I prefer to forbid it, as I suspect a program needing to use it
    is really suffering from poor design...

(5) I have chosen not to use [operator^] for computing powers because of a
    significant risk of misunderstanding between programmer and compiler.
    The syntax/grammar of C++ cannot be changed, and [operator^] binds less
    tightly than (binary) [operator*], so any expression of the form [a*b^c]
    will be parsed as [(a*b)^c]; this is almost certainly not what the
    programmer intended.  To avoid such problems of misunderstanding I
    have preferred not to define [operator^]; it seems too dangerous.

(6) The absence of a [deg] function for [PPMonoidElem]s is deliberate;
    you should choose either [StdDeg] or [wdeg] according to the type
    of degree you want to compute.  This is unnatural; is is a bug?

(7) I have deliberately not made the destructors for [ConstRefPPMonoidElem]
    and its descendants virtual.  This is marginally risky: it might be
    possible to leak memory if you convert a raw pointer to [PPMonoidElem]
    into a raw pointer to [ConstRefPPMonoidElem]; of course, if you do this
    you're asking for trouble anyway.

(8) Should [exponents] give an error if the values exceed the limits for [long]?

(9) Offer the user some means of checking for and handling exponent overflow.
    Probably a PPMonoid with a compact internal representation and checking
    would combine the slowness of PPMonoidEvZZ with the limitations imposed
    by a compact representation --  i.e. it would not be very useful!

Definition at line 48 of file PPMonoid.H.


Constructor & Destructor Documentation

CoCoA::PPMonoid::PPMonoid const PPMonoidBase ptr  )  [inline, explicit]
 

Definition at line 51 of file PPMonoid.H.


Member Function Documentation

const PPMonoidBase* CoCoA::PPMonoid::operator->  )  const [inline]
 

Allow const member fns to be called.

Definition at line 53 of file PPMonoid.H.

References mySmartPtr.

bool CoCoA::PPMonoid::operator== const PPMonoid PPM  )  const [inline]
 

Definition at line 54 of file PPMonoid.H.

References mySmartPtr, and CoCoA::PPM().


Member Data Documentation

SmartPtrIRC<const PPMonoidBase> CoCoA::PPMonoid::mySmartPtr [private]
 

Definition at line 56 of file PPMonoid.H.

Referenced by operator->(), operator==(), and CoCoA::module::operator==().


The documentation for this class was generated from the following file:
Generated on Wed May 23 13:44:37 2007 for CoCoALib by  doxygen 1.4.6