;;; -*-  Mode: Lisp -*- 
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;;;                                                                              
;;;  Copyright (C) 1999, 2002, 2009, 2015 Marek Rychlik <rychlik@u.arizona.edu>		 
;;;  		       								 
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(defpackage "MONOM"
  (:use :cl :ring)
  (:export "MONOM"
	   "EXPONENT"
	   "MONOM-DIMENSION"
	   "MONOM-EXPONENTS"
	   "MAKE-MONOM-VARIABLE")
  (:documentation
   "This package implements basic operations on monomials. 
DATA STRUCTURES: Conceptually, monomials can be represented as lists:

 	monom: (n1 n2 ... nk) where ni are non-negative integers

However, lists may be implemented as other sequence types, so the
flexibility to change the representation should be maintained in the
code to use general operations on sequences whenever possible. The
optimization for the actual representation should be left to
declarations and the compiler.

EXAMPLES: Suppose that variables are x and y. Then

 	Monom x*y^2 ---> (1 2) "))

(in-package :monom)

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

(deftype exponent ()
  "Type of exponent in a monomial."
  'fixnum)

(defclass monom ()
  ((exponents :initarg :exponents :accessor monom-exponents
	      :documentation "The powers of the variables."))
  ;; default-initargs are not needed, they are handled by SHARED-INITIALIZE
  ;;(:default-initargs :dimension 'foo :exponents 'bar :exponent 'baz)
  (:documentation 
   "Implements a monomial, i.e. a product of powers
of variables, like X*Y^2."))

(defmethod print-object ((self monom) stream)
  (print-unreadable-object (self stream :type t :identity t)
    (with-accessors ((exponents monom-exponents))
	self
      (format stream "EXPONENTS=~A"
	      exponents))))

(defmethod initialize-instance :after ((self monom)
				       &key 
					 (dimension 0 dimension-supplied-p)
					 (exponents nil exponents-supplied-p)
					 (exponent  0)
				       &allow-other-keys
				       )
  "The following INITIALIZE-INSTANCE method allows instance initialization
of a MONOM in a style similar to MAKE-ARRAY, e.g.:

 (MAKE-INSTANCE :EXPONENTS '(1 2 3))      --> #<MONOM EXPONENTS=#(1 2 3)>
 (MAKE-INSTANCE :DIMENSION 3)             --> #<MONOM EXPONENTS=#(0 0 0)>
 (MAKE-INSTANCE :DIMENSION 3 :EXPONENT 7) --> #<MONOM EXPONENTS=#(7 7 7)>

If both DIMENSION and EXPONENTS are supplied, they must be compatible,
i.e. the length of EXPONENTS must be equal DIMENSION. If EXPONENTS
is not supplied, a monom with repeated value EXPONENT is created.
By default EXPONENT is 0, which results in a constant monomial.
"
  (cond 
    (exponents-supplied-p
     (when (and dimension-supplied-p
		(/= dimension (length exponents)))
       (error "EXPONENTS (~A) must have supplied length DIMENSION (~A)"
	      exponents dimension))
     (let ((dim (length exponents)))
       (setf (slot-value self 'exponents) (make-array dim :initial-contents exponents))))
    (dimension-supplied-p
     ;; when all exponents are to be identical
     (setf (slot-value self 'exponents) (make-array (list dimension) 
						    :initial-element exponent
						    :element-type 'exponent)))
    (t  
     (error "Initarg DIMENSION or EXPONENTS must be supplied."))))

(defmacro monom-dimension (m)
  `(length (monom-exponents ,m)))

(defmethod r-equalp ((m1 monom) (m2 monom))
  "Returns T iff monomials M1 and M2 have identical
EXPONENTS."
  (equalp (monom-exponents m1) (monom-exponents m2)))

(defmethod r-coeff ((m monom))
  "A MONOM can be treated as a special case of TERM,
where the coefficient is 1."
  1)
  
(defmethod r-elt ((m monom) index)
  "Return the power in the monomial M of variable number INDEX."
  (with-slots (exponents)
      m
    (elt exponents index)))

(defmethod (setf r-elt) (new-value (m monom) index)
  "Return the power in the monomial M of variable number INDEX."
  (with-slots (exponents)
      m
    (setf (elt exponents index) new-value)))

(defmethod r-total-degree ((m monom) &optional (start 0) (end (monom-dimension m)))
  "Return the todal degree of a monomoal M. Optinally, a range
of variables may be specified with arguments START and END."
  (declare (type fixnum start end))
  (with-slots (exponents)
      m
    (reduce #'+ exponents :start start :end end)))


(defmethod r-sugar ((m monom) &aux (start 0) (end (monom-dimension m)))
  "Return the sugar of a monomial M. Optinally, a range
of variables may be specified with arguments START and END."
  (declare (type fixnum start end))
    (r-total-degree m start end))

(defmethod multiply-by ((self monom) (other monom))
  (with-slots ((exponents1 exponents))
      self
    (with-slots ((exponents2 exponents))
	other
      (unless (= (length exponents1) (length exponents2))
	(error "Incompatible dimensions"))
      (map-into exponents1 #'+ exponents1 exponents2)))
  self)

(defmethod divide-by ((self monom) (other monom))
  (with-slots ((exponents1 exponents))
      self
    (with-slots ((exponents2 exponents))
	other
      (unless (= (length exponents1) (length exponents2))
	(error "divide-by: Incompatible dimensions."))
      (unless (every #'>= exponents1 exponents2)
	(error "divide-by: Negative power would result."))
      (map-into exponents1 #'- exponents1 exponents2)))
  self)

(defmethod copy-instance :around ((object monom)  &rest initargs &key &allow-other-keys)
  "An :AROUNT method for COPY-INSTANCE. The primary method is a shallow copy,
  while for monomials we typically need a fresh copy of the
  exponents."
  (declare (ignore object initargs))
  (let ((copy (call-next-method)))
    (setf (monom-exponents copy) (copy-seq (monom-exponents copy)))
    copy))

(defmethod r* ((m1 monom) (m2 monom))
  "Non-destructively multiply monomial M1 by M2."
  (multiply-by (copy-instance m1) (copy-instance m2)))

(defmethod r/ ((m1 monom) &rest m2)
  "Non-destructively divide monomial M1 by monomial M2."
  (divide-by (copy-instance m1) (copy-instance m2)))

(defmethod r-divides-p ((m1 monom) (m2 monom))
  "Returns T if monomial M1 divides monomial M2, NIL otherwise."
  (with-slots ((exponents1 exponents))
      m1
    (with-slots ((exponents2 exponents))
	m2
      (every #'<= exponents1 exponents2))))


(defmethod r-divides-lcm-p ((m1 monom) (m2 monom) (m3 monom))
  "Returns T if monomial M1 divides LCM(M2,M3), NIL otherwise."
  (every #'(lambda (x y z) (<= x (max y z))) 
	 m1 m2 m3))


(defmethod r-lcm-divides-lcm-p ((m1 monom) (m2 monom) (m3 monom) (m4 monom))
  "Returns T if monomial MONOM-LCM(M1,M2) divides MONOM-LCM(M3,M4), NIL otherwise."
  (declare (type monom m1 m2 m3 m4))
  (every #'(lambda (x y z w) (<= (max x y) (max z w))) 
	 m1 m2 m3 m4))
	 
(defmethod r-lcm-equal-lcm-p (m1 m2 m3 m4)
  "Returns T if monomial LCM(M1,M2) equals LCM(M3,M4), NIL otherwise."
  (with-slots ((exponents1 exponents))
      m1
    (with-slots ((exponents2 exponents))
	m2
      (with-slots ((exponents3 exponents))
	  m3
	(with-slots ((exponents4 exponents))
	    m4
	  (every 
	   #'(lambda (x y z w) (= (max x y) (max z w)))
	   exponents1 exponents2 exponents3 exponents4))))))

(defmethod r-divisible-by-p ((m1 monom) (m2 monom))
  "Returns T if monomial M1 is divisible by monomial M2, NIL otherwise."
  (with-slots ((exponents1 exponents))
      m1
    (with-slots ((exponents2 exponents))
	m2
      (every #'>= exponents1 exponents2))))

(defmethod r-rel-prime-p ((m1 monom) (m2 monom))
  "Returns T if two monomials M1 and M2 are relatively prime (disjoint)."
  (with-slots ((exponents1 exponents))
      m1
    (with-slots ((exponents2 exponents))
	m2
      (every #'(lambda (x y) (zerop (min x y))) exponents1 exponents2))))


(defmethod r-lcm ((m1 monom) (m2 monom)) 
  "Returns least common multiple of monomials M1 and M2."
  (with-slots ((exponents1 exponents))
      m1
    (with-slots ((exponents2 exponents))
	m2
      (let* ((exponents (copy-seq exponents1)))
	(map-into exponents #'max exponents1 exponents2)
	(make-instance 'monom :exponents exponents)))))


(defmethod r-gcd ((m1 monom) (m2 monom))
  "Returns greatest common divisor of monomials M1 and M2."
  (with-slots ((exponents1 exponents))
      m1
    (with-slots ((exponents2 exponents))
	m2
      (let* ((exponents (copy-seq exponents1)))
	(map-into exponents #'min exponents1 exponents2)
	(make-instance 'monom :exponents exponents)))))

(defmethod r-depends-p ((m monom) k)
  "Return T if the monomial M depends on variable number K."
  (declare (type fixnum k))
  (with-slots (exponents)
      m
    (plusp (elt exponents k))))

(defmethod left-tensor-product-by ((self monom) (other monom))
  (with-slots ((exponents1 exponents))
      self
    (with-slots ((exponents2 exponents))
	other
      (setf exponents1 (concatenate 'vector exponents2 exponents1))))
  self)

(defmethod right-tensor-product-by ((self monom) (other monom))
  (with-slots ((exponents1 exponents))
      self
    (with-slots ((exponents2 exponents))
	other
      (setf exponents1 (concatenate 'vector exponents1 exponents2))))
  self)

(defmethod left-contract ((self monom) k)
  "Drop the first K variables in monomial M."
  (declare (fixnum k))
  (with-slots (exponents) 
      self
    (setf exponents (subseq exponents k)))
  self)

(defun make-monom-variable (nvars pos &optional (power 1)
			    &aux (m (make-instance 'monom :dimension nvars)))
  "Construct a monomial in the polynomial ring
RING[X[0],X[1],X[2],...X[NVARS-1]] over the (unspecified) 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. "
  (declare (type fixnum nvars pos power) (type monom m))
  (with-slots (exponents)
      m
    (setf (elt exponents pos) power)
    m))

(defmethod r->list ((m monom))
  "A human-readable representation of a monomial M as a list of exponents."  
  (coerce (monom-exponents m) 'list))

(defmethod r-dimension ((self monom))
  (monom-dimension self))

(defmethod r-exponents ((self monom))
  (monom-exponents self))