;;; -*-  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 "PAIR-QUEUE"
  (:use :cl :priority-queue :monom :polynomial :utils)
  (:export "CRITICAL-PAIR"
	   "CRITICAL-PAIR-FIRST"
	   "CRITICAL-PAIR-SECOND"
	   "MAKE-PAIR-QUEUE"
	   "PAIR-QUEUE-INITIALIZE"
	   "PAIR-QUEUE-INSERT"
	   "PAIR-QUEUE-REMOVE"
	   "PAIR-QUEUE-SIZE"
	   "PAIR-QUEUE-EMPTY-P"
	   "SET-PAIR-HEURISTIC"
	   )
  (:documentation "Critical pair queue implementation. The pair queue is a list of critical
pairs, ordered by some partial order. Pair queue is a kind of priority queue.")
  )
	   

(in-package :pair-queue)

(defclass critical-pair ()
  ((first :initform nil  :initarg :first :accessor critical-pair-first :type poly)
   (second :initform nil :initarg :second :accessor critical-pair-second :type poly))
  (:documentation "Represents a critical pair."))

(defmethod print-object ((self critical-pair) stream)
  (print-unreadable-object (self stream :type t :identity t)
    (with-accessors ((first critical-pair-first)
		     (second critical-pair-second))
	self
      (format stream "FIRST=~A SECOND=~A" 
	      first second))))

(defun make-pair-queue (pair-key-fn pair-order-fn)
  "Constructs a priority queue for critical pairs."
  (make-priority-queue
   :element-type 'critical-pair
   :element-key #'(lambda (pair) (funcall pair-key-fn (critical-pair-first pair) (critical-pair-second pair)))
   :test pair-order-fn))

(defun pair-queue-initialize (pq f start
			      &aux
			      (s (1- (length f)))
			      (b (nconc (makelist (make-pair (elt f i) (elt f j))
						 (i 0 (1- start)) (j start s))
					(makelist (make-pair (elt f i) (elt f j))
						 (i start (1- s)) (j (1+ i) s)))))
  "Initializes the priority for critical pairs. F is the initial list of polynomials.
START is the first position beyond the elements which form a partial
grobner basis, i.e. satisfy the Buchberger criterion."
  (declare (type priority-queue pq) (type fixnum start))
  (dolist (pair b pq)
    (priority-queue-insert pq pair)))

(defun pair-queue-insert (b pair)
  (priority-queue-insert b pair))

(defun pair-queue-remove (b)
  (priority-queue-remove b))

(defun pair-queue-size (b)
  (priority-queue-size b))

(defun pair-queue-empty-p (b)
  (priority-queue-empty-p b))

(defclass selection-strategy ()
  (:documentation "Sets up variables *PAIR-KEY-FUNCTION* and *PAIR-ORDER* used
to determine the priority of critical pairs in the priority queue.")
  ((pair-key-fn :initform nil :accessor pair-key-fn)
   (pair-order-fn :initform nil :accessor monomial-order-fn)))

(defun make-normal-strategy (monomial-order-fn)
  "Make a normal selection strategy for a given monomial order. A pair
with the minimal LCM of leading monomials is selected."
  (make-instance 'selection-strategy
		 :pair-key-fn #'(lambda (p q) (universal-lcm (leading-monomial p) (leading-monomial q)))
		 :pair-order-fn #'monomial-order-fn))

(defun make-min-total-degree-strategy ()
  "Make a selection strategy where a pair with a minimum total degree of LCM 
of leading monomials is selected."
  (make-instance 'selection-strategy
		 :pair-key-fn #'(lambda (p q) (total-degree (universal-lcm (leading-monomial p) (leading-monomial q))))
		 :pair-order* #'<))

(defun make-minimal-length-strategy ()
  "Make a selection strategy where a pair with the minimum combined length of both
polynomials is selected."
  (make-instance 'selection-strategy
		 :pair-key-fn #'(lambda (p q) (+ (poly-length p) (poly-length q)))
		 :pair-order #'<))