source: branches/f4grobner/order.lisp@ 1541

;;; -*-  Mode: Lisp -*- 
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;;;  Copyright (C) 1999, 2002, 2009, 2015 Marek Rychlik <rychlik@u.arizona.edu>          
;;;                                                                              
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;;
;; Implementations of various admissible monomial orders
;; Implementation of order-making functions/closures.
;;
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(defpackage "ORDER"
  (:use :cl :monomial)
  (:export "LEX>"
           "GRLEX>"
           "REVLEX>"
           "GREVLEX>"
           "INVLEX>"
           "REVERSE-MONOMIAL-ORDER"
           "MAKE-ELIMINATION-ORDER-FACTORY"))

(in-package :order)

;; pure lexicographic
(defun lex> (p q &optional (start 0) (end (monom-dimension  p)))
  "Return T if P>Q with respect to lexicographic order, otherwise NIL.
The second returned value is T if P=Q, otherwise it is NIL."
  (do ((i start (1+ i)))
      ((>= i end) (values nil t))
    (cond
     ((> (monom-elt p i) (monom-elt q i))
      (return-from lex> (values t nil)))
     ((< (monom-elt p i) (monom-elt q i))
      (return-from lex> (values nil nil))))))

;; total degree order , ties broken by lexicographic
(defun grlex> (p q &optional (start 0) (end (monom-dimension  p)))
  "Return T if P>Q with respect to graded lexicographic order, otherwise NIL.
The second returned value is T if P=Q, otherwise it is NIL."
  (let ((d1 (monom-total-degree p start end))
        (d2 (monom-total-degree q start end)))
    (cond
      ((> d1 d2) (values t nil))
      ((< d1 d2) (values nil nil))
      (t
        (lex> p q start end)))))


;; reverse lexicographic
(defun revlex> (p q &optional (start 0) (end (monom-dimension  p)))
  "Return T if P>Q with respect to reverse lexicographic order, NIL
otherwise.  The second returned value is T if P=Q, otherwise it is
NIL. This is not and admissible monomial order because some sets do
not have a minimal element. This order is useful in constructing other
orders."
  (do ((i (1- end) (1- i)))
      ((< i start) (values nil t))
    (cond
     ((< (monom-elt p i) (monom-elt q i))
      (return-from revlex> (values t nil)))
     ((> (monom-elt p i) (monom-elt q i))
      (return-from revlex> (values nil nil))))))


;; total degree, ties broken by reverse lexicographic
(defun grevlex> (p q &optional (start 0) (end (monom-dimension  p)))
  "Return T if P>Q with respect to graded reverse lexicographic order,
NIL otherwise. The second returned value is T if P=Q, otherwise it is NIL."
  (let ((d1 (monom-total-degree p start end))
        (d2 (monom-total-degree q start end)))
    (cond
     ((> d1 d2) (values t nil))
     ((< d1 d2) (values nil nil))
     (t
      (revlex> p q start end)))))

(defun invlex> (p q &optional (start 0) (end (monom-dimension  p)))
  "Return T if P>Q with respect to inverse lexicographic order, NIL otherwise
The second returned value is T if P=Q, otherwise it is NIL."
  (do ((i (1- end) (1- i)))
      ((< i start) (values nil t))
      (cond
         ((> (monom-elt p i) (monom-elt q i))
          (return-from invlex> (values t nil)))
         ((< (monom-elt p i) (monom-elt q i))
          (return-from invlex> (values nil nil))))))


(defun reverse-monomial-order (order)
  "Create the inverse monomial order to the given monomial order ORDER."
  #'(lambda (p q &optional (start 0) (end (monom-dimension q))) 
      (funcall order q p start end)))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;
;; Order making functions
;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

;; This returns a closure with the same signature
;; as all orders such as #'LEX>.
(defun make-elimination-order-factory-1 (&optional (secondary-elimination-order #'lex>))
  "It constructs an elimination order used for the 1-st elimination ideal,
i.e. for eliminating the first variable. Thus, the order compares the degrees of the
first variable in P and Q first, with ties broken by SECONDARY-ELIMINATION-ORDER."
  #'(lambda (p q &optional (start 0) (end (monom-dimension p)))
      (cond
        ((> (monom-elt p start) (monom-elt q start))
         (values t nil))
        ((< (monom-elt p start) (monom-elt q start))
         (values nil nil))
        (t 
         (funcall secondary-elimination-order p q (1+ start) end)))))

;; This returns a closure which is called with an integer argument.
;; The result is *another closure* with the same signature as all
;; orders such as #'LEX>.
(defun make-elimination-order-factory (&optional 
                                         (primary-elimination-order #'lex>)
                                         (secondary-elimination-order #'lex>))
  "Return a function with a single integer argument K. This should be
the number of initial K variables X[0],X[1],...,X[K-1], which precede
remaining variables.  The call to the closure creates a predicate
which compares monomials according to the K-th elimination order. The
monomial orders PRIMARY-ELIMINATION-ORDER and
SECONDARY-ELIMINATION-ORDER are used to compare the first K and the
remaining variables, respectively, with ties broken by lexicographical
order. That is, if PRIMARY-ELIMINATION-ORDER yields (VALUES NIL T),
which indicates that the first K variables appear with identical
powers, then the result is that of a call to
SECONDARY-ELIMINATION-ORDER applied to the remaining variables
X[K],X[K+1],..."
  #'(lambda (k) 
      (cond 
        ((<= k 0) 
         (error "K must be at least 1"))
        ((= k 1)
         (make-elimination-order-factory-1 secondary-elimination-order))
        (t
         #'(lambda (p q &optional (start 0) (end (monom-dimension  p)))
             (multiple-value-bind (primary equal)
                 (funcall primary-elimination-order p q start k)
               (if equal
                   (funcall secondary-elimination-order p q k end)
                   (values primary nil))))))))