source: branches/f4grobner/term.lisp@ 2704

;;; -*-  Mode: Lisp -*- 
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;;;  Copyright (C) 1999, 2002, 2009, 2015 Marek Rychlik <rychlik@u.arizona.edu>          
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;;;  This program is free software; you can redistribute it and/or modify        
;;;  it under the terms of the GNU General Public License as published by        
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(defpackage "TERM"
  (:use :cl :monom :ring)
  (:export "TERM" "MAKE-TERM-VARIABLE")
  (:documentation "This package implements class TERM. A term is a
product of a scalar and powers of some variables, such as
5*X^2*Y^3. The part of the term without the coefficient is a monomial
X^2*Y^3, which is represented by class MONOM, provided by the :MONOM
package. In this implementation, a TERM specializes MONOL. Also, a
monomial can be considered a TERM whose coefficient is the unit
element (1) of the underlying ring. The generic method CHANGE-CLASS
can be used to convert between a MONOM and a TERM, observing this
convention."))

(in-package :term)

(proclaim '(optimize (speed 3) (space 0) (safety 0) (debug 0)))

(defclass term (monom)
  ((coeff :initarg :coeff :accessor term-coeff
          :documentation "The coefficient."))
  (:default-initargs :dimension nil :exponents nil :coeff nil)
  (:documentation "Implements a term, i.e. a product of a ring element
and powers of some variables, such as 5*X^2*Y^3."))

(defmethod r-coeff ((self term))
  (term-coeff self))

(defmethod (setf r-coeff) (new-value (self term))
  (setf (term-coeff self) new-value))


(defmethod print-object ((self term) stream)
  (format stream "#<TERM DIMENSION=~A EXPONENTS=~A COEFF=~A>"
          (r-dimension self)
          (r-exponents self)
          (r-coeff self)))

(defmethod shared-initialize ((self term) slot-names
                              &rest 
                                initargs 
                              &key
                                coeff
                                &allow-other-keys)
  (declare (ignore initargs))
  (if (eq slot-names t) (setf slot-names '(coeff)))
  (dolist (slot-name slot-names)
    (case slot-name
      (coeff
       (setf (slot-value self 'coeff) coeff)))))

(defmethod update-instance-for-different-class :after ((old monom) 
                                                       (new term)
                                                       &key)
  ;; Changing an instance of class MONOM to class TERM may also
  ;; happen when OLD is an instance of TERM, in which case the
  ;; value of the coefficient should be preserved.
  ;; This implementation calls R-COEFF, which satisfies
  ;; this requirement.
  (setf (slot-value new 'coeff) (r-coeff old)))


#|
(defun make-term-variable (nvars pos
                           &optional
                             (power 1)
                             (coeff 1))
  "Construct a term in the polynomial ring RING[X[0],X[1],X[2],...X[NVARS-1]]
over the ring RING which represents a single variable. It assumes
number of variables NVARS and the variable is at position
POS. Optionally, the variable may appear raised to power POWER.
Optionally, the term may appear with an arbitrary coefficient, which
defaults to the unit of the RING."
  (declare (type fixnum nvars pos))
  (make-term :monom (make-monom-variable nvars pos power)
             :coeff coeff))

|#

(defmethod r* :around ((term1 term) (term2 term))
  "Returns the product of the terms TERM1 and TERM2,
or NIL when the product is 0. This definition takes care of divisors of 0
in the coefficient ring."
  (let ((result (change-class (call-next-method) 'term)))
    (setf (r-coeff result) (r* (r-coeff term1) (r-coeff term2)))
    result))

(defmethod multiply-by ((self term) (other monom))
  "Returns the product of the terms TERM1 and TERM2,
or NIL when the product is 0. This definition takes care of divisors of 0
in the coefficient ring."
  (setf (r-coeff self) (multiply-by (r-coeff self) (r-coeff other)))
  self)

(defmethod multiply-by ((self term) (other scalar))
  "Returns the product of the terms TERM1 and TERM2,
or NIL when the product is 0. This definition takes care of divisors of 0
in the coefficient ring."
  (setf (r-coeff self) (multiply-by (r-coeff self) other))
  self)

(defmethod unary-minus ((self term))
  (setf (term-coeff self) (unary-minus (term-coeff self)))
  self)

#|

(defun term->cons (term) 
  "A human-readable representation of a term as a cons (MONOM . COEFF)."
  (declare (type term term))
  (cons (monom->list (term-monom term)) (term-coeff term)))

|#