(* (c) Copyright 2006-2016 Microsoft Corporation and Inria.                  *)
(* Distributed under the terms of CeCILL-B.                                  *)
From mathcomp
From mathcomp
(******************************************************************************)
(* This file provides a library for the basic theory of Euclidean and pseudo- *)
(* Euclidean division for polynomials over ring structures.                   *)
(* The library defines two versions of the pseudo-euclidean division: one for *)
(* coefficients in a (not necessarily commutative) ring structure and one for *)
(* coefficients equipped with a structure of integral domain. From the latter *)
(* we derive the definition of the usual Euclidean division for coefficients  *)
(* in a field. Only the definition of the pseudo-division for coefficients in *)
(* an integral domain is exported by default and benefits from notations.     *)
(* Also, the only theory exported by default is the one of division for       *)
(* polynomials with coefficients in a field.                                  *)
(* Other definitions and facts are qualified using name spaces indicating the *)
(* hypotheses made on the structure of coefficients and the properties of the *)
(* polynomial one divides with.                                               *)
(*                                                                            *)
(* Pdiv.Field (exported by the present library):                              *)
(*          edivp p q == pseudo-division of p by q with p q : {poly R} where  *)
(*                       R is an idomainType.                                 *)
(*                       Computes (k, quo, rem) : nat * {poly r} * {poly R},  *)
(*                       such that size rem < size q and:                     *)
(*                       + if lead_coef q is not a unit, then:                *)
(*                         (lead_coef q ^+ k) *: p = q * quo + rem            *)
(*                       + else if lead_coef q is a unit, then:               *)
(*                         p = q * quo + rem and k = 0                        *)
(*             p %/ q == quotient (second component) computed by (edivp p q). *)
(*             p %% q == remainder (third component) computed by (edivp p q). *)
(*          scalp p q == exponent (first component) computed by (edivp p q).  *)
(*             p %| q == tests the nullity of the remainder of the            *)
(*                       pseudo-division of p by q.                           *)
(*         rgcdp p q  == Pseudo-greater common divisor obtained by performing *)
(*                       the Euclidean algorithm on p and q using redivp as   *)
(*                       Euclidean division.                                  *)
(*             p %= q == p and q are associate polynomials, i.e., p %| q and  *)
(*                       q %| p, or equivalently, p = c *: q for some nonzero *)
(*                       constant c.                                          *)
(*           gcdp p q == Pseudo-greater common divisor obtained by performing *)
(*                       the Euclidean algorithm on p and q using  edivp as   *)
(*                       Euclidean division.                                  *)
(*          egcdp p q == The pair of Bezout coefficients: if e := egcdp p q,  *)
(*                       then size e.1 <= size q, size e.2 <= size p, and     *)
(*                       gcdp p q %= e.1 * p + e.2 * q                        *)
(*       coprimep p q == p and q are coprime, i.e., (gcdp p q) is a nonzero   *)
(*                       constant.                                            *)
(*          gdcop q p == greatest divisor of p which is coprime to q.         *)
(* irreducible_poly p <-> p has only trivial (constant) divisors.             *)
(*                                                                            *)
(* Pdiv.Idomain: theory available for edivp and the related operation under   *)
(*    the sole assumption that the ring of coefficients is canonically an     *)
(*    integral domain (R : idomainType).                                      *)
(*                                                                            *)
(* Pdiv.IdomainMonic:  theory available for edivp and the related operations  *)
(*    under the assumption that the ring of coefficients is canonically       *)
(*    and integral domain (R : idomainType) an the divisor is monic.          *)
(*                                                                            *)
(* Pdiv.IdomainUnit: theory available for edivp and the related operations    *)
(*    under the assumption that the ring of coefficients is canonically an    *)
(*    integral domain (R : idomainType) and the leading coefficient of the    *)
(*    divisor is a unit.                                                      *)
(*                                                                            *)
(* Pdiv.ClosedField: theory available for edivp and the related operation     *)
(*    under the sole assumption that the ring of coefficients is canonically  *)
(*    an algebraically closed field (R : closedField).                        *)
(*                                                                            *)
(*  Pdiv.Ring :                                                               *)
(*   redivp p q == pseudo-division of p by q with p q : {poly R} where R is   *)
(*                 a ringType.                                                *)
(*                 Computes (k, quo, rem) : nat * {poly r} * {poly R},        *)
(*                 such that if rem = 0 then quo * q = p * (lead_coef q ^+ k) *)
(*                                                                            *)
(*   rdivp p q  == quotient (second component) computed by (redivp p q).      *)
(*   rmodp p q  == remainder (third component) computed by (redivp p q).      *)
(*   rscalp p q == exponent (first component) computed by (redivp p q).       *)
(*   rdvdp p q  == tests the nullity of the remainder of the pseudo-division  *)
(*                 of p by q.                                                 *)
(*   rgcdp p q  == analogue of gcdp for coefficients in a ringType.           *)
(*   rgdcop p q == analogue of gdcop for coefficients in a ringType.          *)
(*rcoprimep p q == analogue of coprimep p q for coefficients in a ringType.   *)
(*                                                                            *)
(* Pdiv.RingComRreg : theory of the operations defined in Pdiv.Ring, when the *)
(*   ring of coefficients is canonically commutative (R : comRingType) and    *)
(*   the leading coefficient of the divisor is both right regular and         *)
(*   commutes as a constant polynomial with the divisor itself                *)
(*                                                                            *)
(* Pdiv.RingMonic : theory of the operations defined in Pdiv.Ring, under the  *)
(*   assumption that the divisor is monic.                                    *)
(*                                                                            *)
(* Pdiv.UnitRing: theory of the operations defined in Pdiv.Ring, when the     *)
(*   ring R of coefficients is canonically with units (R : unitRingType).     *)
(*                                                                            *)
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Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.
Local Open Scope ring_scope.
Reserved Notation "p %= q" (at level 70, no associativity).
Local Notation simp := Monoid.simpm.
Module Pdiv.
Module CommonRing.
Section RingPseudoDivision.
Variable R : ringType.
Implicit Types d p q r : {poly R}.
(* Pseudo division, defined on an arbitrary ring *)
Definition redivp_rec (q : {poly R})  :=
let sq := size q in
let cq := lead_coef q in
fix loop (k : nat) (qq r : {poly R})(n : nat) {struct n} :=
if size r < sq then (k, qq, r) else
let m := (lead_coef r) *: 'X^(size r - sq) in
let qq1 := qq * cq%:P + m in
let r1 := r * cq%:P - m * q in
if n is n1.+1 then loop k.+1 qq1 r1 n1 else (k.+1, qq1, r1).
Definition redivp_expanded_def p q :=
if q == 0 then (0%N, 0, p) else redivp_rec q 0 0 p (size p).
Fact redivp_key : unit. Proof. by []. Qed.
Definition redivp : {poly R} -> {poly R} -> nat * {poly R} * {poly R} :=
locked_with redivp_key redivp_expanded_def.
Canonical redivp_unlockable := [unlockable fun redivp].
Definition rdivp p q := ((redivp p q).1).2.
Definition rmodp p q := (redivp p q).2.
Definition rscalp p q := ((redivp p q).1).1.
Definition rdvdp p q := rmodp q p == 0.

Lemma rgcdpE p q :
rgcdp p q = if size p < size q
then rgcdp (rmodp q p) p else rgcdp (rmodp p q) q.