;;; -*-  Mode: Lisp -*- 
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;;;  Copyright (C) 1999, 2002, 2009, 2015 Marek Rychlik <rychlik@u.arizona.edu>		 
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(defpackage "PAIR-QUEUE"
  (:use :cl :priority-queue :monom :polynomial :symbolic-polynomial :utils)
  (:export "CRITICAL-PAIR"
	   "CRITICAL-PAIR-FIRST"
	   "CRITICAL-PAIR-SECOND"
	   "CRITICAL-PAIR-QUEUE"
	   "SELECTION-STRATEGY"
	   "+NORMAL-STRATEGY+"
	   "+MIN-TOTAL-DEGREE-STRATEGY+"
	   "+MIN-COMBINED-LENGTH-STRATEGY+"
	   "MAKE-CRITICAL-PAIR-QUEUE"
	   )
  (: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, i.e. a pair of two polynomials. The derived
classes may add extra data used in computing the order of critical pairs."))

(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))))

(defclass selection-strategy ()
  ((pair-key-fn :initform (error "Initarg :pair-key-fn must be specified.")
		:initarg :pair-key-fn 
		:accessor pair-key-fn)
   (pair-order-fn :initform (error "Initarg :pair-order-fn must be specified.")
		  :initarg :pair-order-fn 
		  :accessor pair-order-fn))
  (:documentation "The two ingredients of a strategy is a function
PAIR-KEY-FUNCTION which computes a key from the critical pair, which
can be any type of data, and a function PAIR-ORDER-FN used to compaire
the calculated keys to determine which pair is more promising and
should be considered first.  The normal selection strategy for a given
monomial order PAIR-ORDER-FN consists in selecting the pair with the
minimal LCM of leading monomials first."))

(defmethod print-object ((self selection-strategy) stream)
  (print-unreadable-object (self stream :type t :identity t)
    (with-accessors ((pair-key-fn pair-key-fn)
		     (pair-order-fn pair-order-fn))
	self
      (format stream "PAIR-KEY-FN=~A PAIR-ORDER-FN=~A" 
	      pair-key-fn pair-order-fn))))

(defparameter +normal-strategy+
  (make-instance 
   'selection-strategy
   :pair-key-fn #'(lambda (p q) (universal-lcm (leading-monomial p) (leading-monomial q)))
   :pair-order-fn #'lex>)
  "The normal selection strategy where a pair with the largest LCM of
leading monomials is selected.")

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

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

(defclass critical-pair-queue (priority-queue) 
  ()
  (:documentation "Implements critical pair priority queue."))

(defmethod allocate-instance ((self critical-pair-queue) &key strategy poly-list start)
  "Allocates a priority queue SELF for critical pairs from strategy SELF."
  (declare (ignore poly-list start))
  (with-slots (pair-key-fn pair-order-fn)
      strategy
    (reinitialize-instance self
			   :element-type 'critical-pair
			   :element-key #'(lambda (pair) (funcall pair-key-fn 
								  (critical-pair-first pair)
								  (critical-pair-second pair)))
			   :test pair-order-fn)))

(defgeneric enqueue-critical-pairs (self &optional poly-list start)
  (:method ((self critical-pair-queue) &optional (poly-list nil) (start 0))
    "The argument POLY-LIST should the initial list of
polynomials.  and START is the first position beyond the elements
which form a partial Grobner basis, i.e. satisfy the Buchberger
criterion."
    (let* ((s (1- (length poly-list)))
	   (b (nconc (makelist (make-instance 'critical-pair :first (elt poly-list i) :second (elt poly-list j))
			       (i 0 (1- start)) (j start s))
		     (makelist (make-instance 'critical-pair :first (elt poly-list i) :second (elt poly-list j))
			       (i start (1- s)) (j (1+ i) s)))))
      (dolist (pair b self)
	(enqueue self pair)))))