rational.qbk

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[section:rational Polynomial and Rational Function Evaluation]

[h4 synopsis]

``
#include <boost/math/tools/rational.hpp>
``

   // Polynomials:
   template <std::size_t N, class T, class V>
   V evaluate_polynomial(const T(&poly)[N], const V& val);
   
   template <std::size_t N, class T, class V>
   V evaluate_polynomial(const boost::array<T,N>& poly, const V& val);
   
   template <class T, class U>
   U evaluate_polynomial(const T* poly, U z, std::size_t count);
   
   // Even polynomials:
   template <std::size_t N, class T, class V>
   V evaluate_even_polynomial(const T(&poly)[N], const V& z);
   
   template <std::size_t N, class T, class V>
   V evaluate_even_polynomial(const boost::array<T,N>& poly, const V& z);
   
   template <class T, class U>
   U evaluate_even_polynomial(const T* poly, U z, std::size_t count);
   
   // Odd polynomials   
   template <std::size_t N, class T, class V>
   V evaluate_odd_polynomial(const T(&a)[N], const V& z);
   
   template <std::size_t N, class T, class V>
   V evaluate_odd_polynomial(const boost::array<T,N>& a, const V& z);
   
   template <class T, class U>
   U evaluate_odd_polynomial(const T* poly, U z, std::size_t count);
   
   // Rational Functions:
   template <std::size_t N, class T, class V>
   V evaluate_rational(const T(&a)[N], const T(&b)[N], const V& z);
   
   template <std::size_t N, class T, class V>
   V evaluate_rational(const boost::array<T,N>& a, const boost::array<T,N>& b, const V& z);
   
   template <class T, class U, class V>
   V evaluate_rational(const T* num, const U* denom, V z, unsigned count);

[h4 Description]

Each of the functions come in three variants: a pair of overloaded functions
where the order of the polynomial or rational function is evaluated at
compile time, and an overload that accepts a runtime variable for the size
of the coefficient array.  Generally speaking, compile time evaluation of the
array size results in better type safety, is less prone to programmer errors,
and may result in better optimised code.  The polynomial evaluation functions
in particular, are specialised for various array sizes, allowing for
loop unrolling, and one hopes, optimal inline expansion.

   template <std::size_t N, class T, class V>
   V evaluate_polynomial(const T(&poly)[N], const V& val);
   
   template <std::size_t N, class T, class V>
   V evaluate_polynomial(const boost::array<T,N>& poly, const V& val);
   
   template <class T, class U>
   U evaluate_polynomial(const T* poly, U z, std::size_t count);

Evaluates the [@http://en.wikipedia.org/wiki/Polynomial polynomial] described by
the coefficients stored in /poly/.

If the size of the array is specified at runtime, then the polynomial 
most have order /count-1/ with /count/ coefficients.  Otherwise it has
order /N-1/ with /N/ coefficients.

Coefficients should be stored such that the coefficients for the x[super i ] terms
are in poly[i].

The types of the coefficients and of variable
/z/ may differ as long as /*poly/ is convertible to type /U/.
This allows, for example, for the coefficient table
to be a table of integers if this is appropriate.

   template <std::size_t N, class T, class V>
   V evaluate_even_polynomial(const T(&poly)[N], const V& z);
   
   template <std::size_t N, class T, class V>
   V evaluate_even_polynomial(const boost::array<T,N>& poly, const V& z);
   
   template <class T, class U>
   U evaluate_even_polynomial(const T* poly, U z, std::size_t count);

As above, but evaluates an even polynomial: one where all the powers
of /z/ are even numbers.  Equivalent to calling 
`evaluate_polynomial(poly, z*z, count)`.

   template <std::size_t N, class T, class V>
   V evaluate_odd_polynomial(const T(&a)[N], const V& z);
   
   template <std::size_t N, class T, class V>
   V evaluate_odd_polynomial(const boost::array<T,N>& a, const V& z);
   
   template <class T, class U>
   U evaluate_odd_polynomial(const T* poly, U z, std::size_t count);

As above but evaluates a polynomial where all the powers are odd numbers.
Equivalent to `evaluate_polynomial(poly+1, z*z, count-1) * z + poly[0]`.

   template <std::size_t N, class T, class U, class V>
   V evaluate_rational(const T(&num)[N], const U(&denom)[N], const V& z);
   
   template <std::size_t N, class T, class U, class V>
   V evaluate_rational(const boost::array<T,N>& num, const boost::array<U,N>& denom, const V& z);
   
   template <class T, class U, class V>
   V evaluate_rational(const T* num, const U* denom, V z, unsigned count);
   
Evaluates the rational function (the ratio of two polynomials) described by
the coefficients stored in /num/ and /demom/.

If the size of the array is specified at runtime then both 
polynomials most have order /count-1/ with /count/ coefficients.
Otherwise both polynomials have order /N-1/ with /N/ coefficients.

Array /num/ describes the numerator, and /demon/ the denominator.

Coefficients should be stored such that the coefficients for the x[super i ] terms
are in num[i] and denom[i].

The types of the coefficients and of variable
/v/ may differ as long as /*num/ and /*denom/ are convertible to type /V/.
This allows, for example, for one or both of the coefficient tables
to be a table of integers if this is appropriate.

These functions are designed to safely evaluate the result, even when the value
/z/ is very large.  As such they do not take advantage of compile time array
sizes to make any optimisations.  These functions are best reserved for situations
where /z/ may be large: if you can be sure that numerical overflow will not occur
then polynomial evaluation with compile-time array sizes may offer slightly
better performance.

[h4 Implementation]

Polynomials are evaluated by 
[@http://en.wikipedia.org/wiki/Horner_algorithm Horners method].  
If the array size is known at
compile time then the functions dispatch to size-specific implementations
that unroll the evaluation loop.

Rational evaluation is by 
[@http://en.wikipedia.org/wiki/Horner_algorithm Horners method]:
with the two polynomials being evaluated
in parallel to make the most of the processors floating-point pipeline.
If /v/ is greater than one, then the polynomials are evaluated in reverse
order as polynomials in ['1\/v]: this avoids unnecessary numerical overflow when the
coefficients are large.

Both the polynomial and rational function evaluation algorithms can be
tuned using various configuration macros to provide optimal performance
for a particular combination of compiler and platform.  This includes
support for second-order Horner's methods.  The various options are
[link math_toolkit.perf.tuning documented here].  However, the performance
benefits to be gained from these are marginal on most current hardware,
consequently it's best to run the 
[link math_toolkit.perf.perf_test_app performance test application] before
changing the default settings.

[endsect][/section:rational Polynomial and Rational Function Evaluation]

[/ 
  Copyright 2006 John Maddock and Paul A. Bristow.
  Distributed under the Boost Software License, Version 1.0.
  (See accompanying file LICENSE_1_0.txt or copy at
  http://www.boost.org/LICENSE_1_0.txt).
]