;;; -*-  Mode: Lisp; Package: Maxima; Syntax: Common-Lisp; Base: 10 -*- 
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;;;                                                                              
;;;  Copyright (C) 1999, 2002, 2009, 2015 Marek Rychlik <rychlik@u.arizona.edu>		 
;;;  		       								 
;;;  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	 
;;;  the Free Software Foundation; either version 2 of the License, or		 
;;;  (at your option) any later version.					 
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;;;  This program is distributed in the hope that it will be useful,		 
;;;  but WITHOUT ANY WARRANTY; without even the implied warranty of		 
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;;;  GNU General Public License for more details.				 
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;;;  You should have received a copy of the GNU General Public License		 
;;;  along with this program; if not, write to the Free Software 		 
;;;  Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.	 
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(defpackage "PRIORITY-QUEUE"
  (:use :cl)
  (:export "*PRIORITY-QUEUE-ALLOCATION-SIZE*"
	   "MAKE-PRIORITY-QUEUE"
	   "PRIORITY-QUEUE-INSERT"
	   "PRIORITY-QUEUE-REMOVE"
	   "PRIORITY-QUEUE-EMPTY-P"
	   "PRIORITY-QUEUE-SIZE")

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;;
;; A conventional implementation of priority queues based on heaps
;;
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(defparameter *priority-queue-allocation-size* 16)

(defun priority-queue-make-heap (&key (element-type 'fixnum))
  (make-array *priority-queue-allocation-size* :element-type element-type :fill-pointer 1
	      :adjustable t))

(defstruct (priority-queue (:constructor priority-queue-construct))
  (heap (priority-queue-make-heap))
  test)

(defun make-priority-queue (&key (element-type 'fixnum)
			    (test #'<=)
			    (element-key #'identity))
  (priority-queue-construct
   :heap (priority-queue-make-heap :element-type element-type)
   :test #'(lambda (x y) (funcall test (funcall element-key y) (funcall element-key x)))))
  
(defun priority-queue-insert (pq item)
  (priority-queue-heap-insert (priority-queue-heap pq) item (priority-queue-test pq)))

(defun priority-queue-remove (pq)
  (priority-queue-heap-remove (priority-queue-heap pq) (priority-queue-test pq)))

(defun priority-queue-empty-p (pq)
  (priority-queue-heap-empty-p (priority-queue-heap pq)))

(defun priority-queue-size (pq)
  (fill-pointer (priority-queue-heap pq)))

(defun priority-queue-upheap (a k
	       &optional
	       (test #'<=)
	       &aux  (v (aref a k)))
  (declare (fixnum k))
  (assert (< 0 k (fill-pointer a)))
  (loop
   (let ((parent (ash k -1)))
     (when (zerop parent) (return))
     (unless (funcall test (aref a parent) v)
       (return))
     (setf (aref a k) (aref a parent)
	   k parent)))
  (setf (aref a k) v)
  a)

    
(defun priority-queue-heap-insert (a item &optional (test #'<=))
  (vector-push-extend item a)
  (priority-queue-upheap a (1- (fill-pointer a)) test))

(defun priority-queue-downheap (a k
		 &optional
		 (test #'<=)
		 &aux  (v (aref a k)) (j 0) (n (fill-pointer a)))
  (declare (fixnum k n j))
  (loop
   (unless (<= k (ash n -1))
     (return))
   (setf j (ash k 1))
   (if (and (< j n) (not (funcall test (aref a (1+ j)) (aref a j))))
       (incf j))
   (when (funcall test (aref a j) v)
     (return))
   (setf (aref a k) (aref a j)
	 k j))
  (setf (aref a k) v)
  a)

(defun priority-queue-heap-remove (a &optional (test #'<=) &aux (v (aref a 1)))
  (when (<= (fill-pointer a) 1) (error "Empty queue."))
  (setf (aref a 1) (vector-pop a))
  (priority-queue-downheap a 1 test)
  (values v a))

(defun priority-queue-heap-empty-p (a)
  (<= (fill-pointer a) 1))