{-# OPTIONS --safe #-}
module Cubical.Algebra.CommAlgebra.Polynomials where

open import Cubical.Foundations.Prelude
open import Cubical.Foundations.Function using (_$_; _∘_)
open import Cubical.Foundations.Structure using (withOpaqueStr)
open import Cubical.Foundations.Isomorphism using (isoFunInjective)

open import Cubical.Data.Nat
open import Cubical.Data.FinData

open import Cubical.Algebra.CommRing.Base
open import Cubical.Algebra.CommAlgebra.Base
open import Cubical.Algebra.CommRing.Polynomials.Typevariate as Poly hiding (var)
import Cubical.Algebra.CommRing.Polynomials.Typevariate.UniversalProperty
  as Poly

private
  variable
    ℓ ℓ' ℓ'' : Level

_[_]ₐ : (R : CommRing ℓ) (I : Type ℓ') → CommAlgebra R (ℓ-max ℓ ℓ')
R [ I ]ₐ = (R [ I ]) , constPolynomial R I

module _ {R : CommRing ℓ} where
  evPolyIn : {n : ℕ} (A : CommAlgebra R ℓ')
            → ⟨ R [ Fin n ]ₐ ⟩ₐ → FinVec ⟨ A ⟩ₐ n → ⟨ A ⟩ₐ
  evPolyIn {n = n} A P v = Poly.inducedHom (CommAlgebra→CommRing A) (A .snd) v $cr P

module _ {R : CommRing ℓ} {I : Type ℓ'} where
  var : I → ⟨ R [ I ]ₐ ⟩ₐ
  var = Poly.var

  inducedHom : (A : CommAlgebra R ℓ'') (φ : I → ⟨ A ⟩ₐ )
             → CommAlgebraHom (R [ I ]ₐ) A
  inducedHom A ϕ =
    CommAlgebraHomFromCommRingHom
      f
      (λ _ _ → refl)
    where f : CommRingHom _ _
          f = Poly.inducedHom (CommAlgebra→CommRing A) (A .snd) ϕ

  inducedHomUnique :
    (A : CommAlgebra R ℓ'') (φ : I → ⟨ A ⟩ₐ )
    → (f : CommAlgebraHom (R [ I ]ₐ) A) → (⟨ f ⟩ₐ→ ∘ var ≡ φ)
    → inducedHom A φ ≡ f
  inducedHomUnique A φ f p =
    CommAlgebraHom≡ $
    cong fst $
    Poly.inducedHomUnique _ _ φ (CommAlgebraHom→CommRingHom f) (cong fst (CommAlgebraHom→Triangle f)) p

{-
evaluateAt : {R : CommRing ℓ} {I : Type ℓ'} (A : CommAlgebra R ℓ'')
             (f : CommAlgebraHom (R [ I ]) A)
             → (I → fst A)
evaluateAt A f x = f $a (Construction.var x)


homMapIso : {R : CommRing ℓ} {I : Type ℓ''} (A : CommAlgebra R ℓ')
             → Iso (CommAlgebraHom (R [ I ]) A) (I → (fst A))
Iso.fun (homMapIso A) = evaluateAt A
Iso.inv (homMapIso A) = inducedHom A
Iso.rightInv (homMapIso A) = λ ϕ → Theory.mapRetrievable A ϕ
Iso.leftInv (homMapIso {R = R} {I = I} A) =
  λ f → Σ≡Prop (λ f → isPropIsCommAlgebraHom {M = R [ I ]} {N = A} f)
               (Theory.homRetrievable A f)

inducedHomUnique :
  {R : CommRing ℓ} {I : Type ℓ'} (A : CommAlgebra R ℓ'') (φ : I → fst A )
  → (f : CommAlgebraHom (R [ I ]) A) → ((i : I) → fst f (Construction.var i) ≡ φ i)
  → f ≡ inducedHom A φ
inducedHomUnique {I = I} A φ f p =
  isoFunInjective (homMapIso A) f (inducedHom A φ) λ j i → p i j

homMapPath : {R : CommRing ℓ} {I : Type ℓ'} (A : CommAlgebra R (ℓ-max ℓ ℓ'))
             → CommAlgebraHom (R [ I ]) A ≡ (I → fst A)
homMapPath A = isoToPath (homMapIso A)

{- Corollary: Two homomorphisms with the same values on generators are equal -}
equalByUMP : {R : CommRing ℓ} {I : Type ℓ'}
           → (A : CommAlgebra R ℓ'')
           → (f g : CommAlgebraHom (R [ I ]) A)
           → ((i : I) → fst f (Construction.var i) ≡ fst g (Construction.var i))
           → (x : ⟨ R [ I ] ⟩) → fst f x ≡ fst g x
equalByUMP {R = R} {I = I} A f g = funExt⁻ ∘ cong fst ∘ isoFunInjective (homMapIso A) f g ∘ funExt

{- A corollary, which is useful for constructing isomorphisms to
   algebras with the same universal property -}
isIdByUMP : {R : CommRing ℓ} {I : Type ℓ'}
          → (f : CommAlgebraHom (R [ I ]) (R [ I ]))
          → ((i : I) → fst f (Construction.var i) ≡ Construction.var i)
          → (x : ⟨ R [ I ] ⟩) → fst f x ≡ x
isIdByUMP {R = R} {I = I} f p = equalByUMP (R [ I ]) f (idCAlgHom (R [ I ])) p

-- The homomorphism induced by the variables is the identity.
inducedHomVar : (R : CommRing ℓ) (I : Type ℓ')
              → inducedHom (R [ I ]) Construction.var ≡ idCAlgHom (R [ I ])
inducedHomVar R I = isoFunInjective (homMapIso (R [ I ])) _ _ refl

module _ {R : CommRing ℓ} {A B : CommAlgebra R ℓ''} where
  open AlgebraHoms
  A′ = CommAlgebra→Algebra A
  B′ = CommAlgebra→Algebra B
  R′ = (CommRing→Ring R)
  ν : AlgebraHom A′ B′ → (⟨ A ⟩ → ⟨ B ⟩)
  ν φ = φ .fst

  {-
    Hom(R[I],A) → (I → A)
         ↓          ↓ψ
    Hom(R[I],B) → (I → B)
  -}
  naturalEvR : {I : Type ℓ'} (ψ : CommAlgebraHom A B)
             (f : CommAlgebraHom (R [ I ]) A)
             → (fst ψ) ∘ evaluateAt A f ≡ evaluateAt B (ψ ∘a f)
  naturalEvR ψ f = refl

  {-
    Hom(R[I],A) ← (I → A)
         ↓          ↓ψ
    Hom(R[I],B) ← (I → B)
  -}
  natIndHomR : {I : Type ℓ'} (ψ : CommAlgebraHom A B)
               (ϕ : I → ⟨ A ⟩)
               → ψ ∘a inducedHom A ϕ ≡ inducedHom B (fst ψ ∘ ϕ)
  natIndHomR ψ ϕ = isoFunInjective (homMapIso B) _ _
                (evaluateAt B (ψ ∘a (inducedHom A ϕ))        ≡⟨ refl ⟩
                 fst ψ ∘ evaluateAt A (inducedHom A ϕ)       ≡⟨ refl ⟩
                 fst ψ ∘ ϕ                                   ≡⟨ Iso.rightInv (homMapIso B) _ ⟩
                 evaluateAt B (inducedHom B (fst ψ ∘ ϕ))     ∎)

  {-
    Hom(R[I],A) → (I → A)
         ↓          ↓
    Hom(R[J],A) → (J → A)
  -}
  naturalEvL : {I J : Type ℓ'} (φ : J → I)
             (f : CommAlgebraHom (R [ I ]) A)
             → (evaluateAt A f) ∘ φ
               ≡ evaluateAt A (f ∘a (inducedHom (R [ I ]) (λ x → Construction.var (φ x))))
  naturalEvL φ f = refl

module _ {R : CommRing ℓ} where
  {-
    Prove that the FreeCommAlgebra over R on zero generators is
    isomorphic to the initial R-Algebra - R itsself.
  -}
  freeOn⊥ : CommAlgebraEquiv (R [ ⊥ ]) (initialCAlg R)
  freeOn⊥ =
     equivByInitiality
        R (R [ ⊥ ])
          {- Show that R[⊥] has the universal property of the
             initial R-Algbera and conclude that those are isomorphic -}
        λ B →  let to : CommAlgebraHom (R [ ⊥ ]) B → (⊥ → fst B)
                   to = evaluateAt B

                   from :  (⊥ → fst B) → CommAlgebraHom (R [ ⊥ ]) B
                   from = inducedHom B

                   from-to : (x : _) → from (to x) ≡ x
                   from-to x =
                     Σ≡Prop (λ f → isPropIsCommAlgebraHom {M = R [ ⊥ ]} {N = B} f)
                            (Theory.homRetrievable B x)

                   equiv : CommAlgebraHom (R [ ⊥ ]) B ≃ (⊥ → fst B)
                   equiv =
                     isoToEquiv
                       (iso to from (λ x → isContr→isOfHLevel 1 isContr⊥→A _ _) from-to)
               in isOfHLevelRespectEquiv 0 (invEquiv equiv) isContr⊥→A
-}