{-# OPTIONS --without-K --safe #-}

open import Axiom.Extensionality.Propositional

-- Cumulativity of the gluing models for terms and types
module Mints.Soundness.Cumulativity (fext :  { ℓ′}  Extensionality  ℓ′) where

open import Lib
open import Data.Nat.Properties as ℕₚ

open import Mints.Statics.Properties
open import Mints.Semantics.Readback
open import Mints.Semantics.Properties.PER fext
open import Mints.Soundness.LogRel
open import Mints.Soundness.Realizability fext
open import Mints.Soundness.Properties.LogRel fext


-- Similar to cumulativity of the PER model, we also need a lowering lemma in order to
-- establish cumulativity (®El-lower).  Unlike the PER model, lowering in the gluing
-- model is a bit more difficult because we need to have one extra assumption of
-- syntactic and semantic types glued in the lower level. By exploiting this
-- assumption, we replace everything about types to the lower level in the gluing of
-- terms.
mutual

  ®-cumu-step :  {i} (A≈B : A  B  𝕌 i) 
                Γ  T ®[ i ] A≈B 
                -----------------------------
                Γ  T ®[ suc i ] 𝕌-cumu-step i A≈B
  ®-cumu-step (ne C≈C′) (⊢T , rel) = cumu ⊢T , λ ⊢σ  ≈-cumu (rel ⊢σ)
  ®-cumu-step N T∼A                = ≈-cumu T∼A
  ®-cumu-step (U′ j<i) T∼A         = ≈-cumu T∼A
  ®-cumu-step ( A≈B) T∼A          = record
    { GT   = GT
    ; T≈   = ≈-cumu T≈
    ; krip = λ {_} {σ} Ψs ⊢ΨsΔ ⊢σ  ®-cumu-step (A≈B (ins (mt σ) (len Ψs))) (krip Ψs ⊢ΨsΔ ⊢σ)
    }
    where open Glu□ T∼A
  ®-cumu-step (Π iA RT) T∼A        = record
    { IT   = IT
    ; OT   = OT
    ; ⊢OT  = cumu ⊢OT
    ; T≈   = ≈-cumu T≈
    ; krip = λ {_} {σ} ⊢σ 
      let open ΠRel (krip ⊢σ)
      in record
      { IT-rel = ®-cumu-step (iA (mt σ)) IT-rel
      ; OT-rel = λ s∼a a∈  ®-cumu-step (ΠRT.T≈T′ (RT (mt σ) (El-lower _ (iA (mt σ)) a∈))) (OT-rel (®El-lower (iA (mt σ)) IT-rel s∼a) (El-lower _ (iA (mt σ)) a∈))
      }
    }
    where open GluΠ T∼A


  ®El-cumu-step :  {i} (A≈B : A  B  𝕌 i) 
                  Γ  t  T ®[ i ] a ∈El A≈B 
                  ------------------------------------------
                  Γ  t  T ®[ suc i ] a ∈El 𝕌-cumu-step i A≈B
  ®El-cumu-step (ne C≈C′) (ne c∈ , rel)    = ne c∈ , record
    { t∶T  = t∶T
    ; ⊢T   = cumu ⊢T
    ; krip = λ ⊢σ  let Tσ≈ , tσ≈ = krip ⊢σ in ≈-cumu Tσ≈ , tσ≈
    }
    where open GluNe rel
  ®El-cumu-step N (t∼a , T≈N)              = t∼a , ≈-cumu T≈N
  ®El-cumu-step (U′ j<i) t∼a
    rewrite Glu-wellfounded-≡ j<i
          | Glu-wellfounded-≡ (≤-step j<i) = record
    { t∶T = t∶T
    ; T≈  = ≈-cumu T≈
    ; A∈𝕌 = A∈𝕌
    ; rel = rel
    }
    where open GluU t∼a
  ®El-cumu-step ( A≈B) t∼a                = record
    { GT   = GT
    ; t∶T  = t∶T
    ; a∈El = El-cumu-step _ ( A≈B) a∈El
    ; T≈   = ≈-cumu T≈
    ; krip = λ {_} {σ} Ψs ⊢ΨsΔ ⊢σ 
      let open □Krip (krip Ψs ⊢ΨsΔ ⊢σ)
      in record
      { ua  = ua
      ; ↘ua = ↘ua
      ; rel = ®El-cumu-step (A≈B (ins (mt σ) (len Ψs))) rel
      }
    }
    where open Glubox t∼a
  ®El-cumu-step (Π iA RT) t∼a              = record
    { t∶T  = t∶T
    ; a∈El = El-cumu-step _ (Π iA RT) a∈El
    ; IT   = IT
    ; OT   = OT
    ; ⊢OT  = cumu ⊢OT
    ; T≈   = ≈-cumu T≈
    ; krip = λ {_} {σ} ⊢σ 
      let open ΛRel (krip ⊢σ)
      in record
      { IT-rel = ®-cumu-step (iA (mt σ)) IT-rel
      ; ap-rel = λ s∼b b∈ 
        let open ΛKripke (ap-rel (®El-lower (iA (mt σ)) IT-rel s∼b) (El-lower _ (iA (mt σ)) b∈))
        in record
        { fa  = fa
        ; ↘fa = ↘fa
        ; ®fa = ®El-cumu-step (ΠRT.T≈T′ (RT (mt σ) (El-lower _ (iA (mt σ)) b∈))) ®fa
        }
      }
    }
    where open GluΛ t∼a


  -- This is tricky! We need to pass on the knowledge that T is related to A in a
  -- lower level such that ®El can be lowered! It cannot be done without this extra
  -- piece of knowledge.
  ®El-lower :  {i} (A≈B : A  B  𝕌 i) 
              Γ  T ®[ i ] A≈B   --  This assumption is critically needed.
              Γ  t  T ®[ suc i ] a ∈El 𝕌-cumu-step i A≈B 
              -----------------------------------------
              Γ  t  T ®[ i ] a ∈El A≈B
  ®El-lower (ne C≈C′) (⊢T , rel) (ne c∈⊥ , rel′) = ne c∈⊥ , record
    { t∶T  = t∶T
    ; ⊢T   = ⊢T
    ; krip = λ ⊢σ  rel ⊢σ , proj₂ (krip ⊢σ)
    }
    where open GluNe rel′ hiding (⊢T)
  ®El-lower N T∼A (t∼a , _)                      = t∼a , T∼A
  ®El-lower (U′ j<i) T∼A t∼a
    rewrite Glu-wellfounded-≡ j<i
          | Glu-wellfounded-≡ (≤-step j<i)       = record
    { t∶T = t∶T
    ; T≈  = T∼A
    ; A∈𝕌 = A∈𝕌
    ; rel = rel
    }
    where open GluU t∼a
  ®El-lower ( A≈B) T∼A t∼a                      = record
    { GT   = T.GT
    ; t∶T  = t∶T
    ; a∈El = El-lower _ ( A≈B) a∈El
    ; T≈   = T.T≈
    ; krip = λ {_} {σ} Ψs ⊢ΨsΔ ⊢σ 
      let open □Krip (krip Ψs ⊢ΨsΔ ⊢σ)
          A≈Bcu = A≈B (ins (mt σ) (len Ψs))
      in record
      { ua  = ua
      ; ↘ua = ↘ua
      ; rel = ®El-lower (A≈B (ins (mt σ) (len Ψs)))
                        (T.krip Ψs ⊢ΨsΔ ⊢σ)
                        (®El-resp-T≈ (𝕌-cumu-step _ (A≈B (ins (mt σ) (len Ψs))))
                                     rel
                                     (≈-sym (®⇒≈ (𝕌-cumu-step _ A≈Bcu) (®-cumu-step A≈Bcu (T.krip Ψs ⊢ΨsΔ ⊢σ)) (®El⇒® (𝕌-cumu-step _ A≈Bcu) rel))))
      }
    }
    where module T = Glu□ T∼A
          open Glubox t∼a
  ®El-lower (Π iA RT) T∼A t∼a                    = record
    { t∶T  = t∶T
    ; a∈El = El-lower _ (Π iA RT) a∈El
    ; IT   = T.IT
    ; OT   = T.OT
    ; ⊢OT  = T.⊢OT
    ; T≈   = T.T≈
    ; krip = λ {_} {σ} ⊢σ 
      let open ΛRel (krip ⊢σ)
          module Π = ΠRel (T.krip ⊢σ)
          iAcu = 𝕌-cumu-step _ (iA (mt σ))
      in record
      { IT-rel = Π.IT-rel
      ; ap-rel = λ s∼b b∈ 
        let open ΛKripke (ap-rel (®El-resp-T≈ iAcu
                                              (®El-cumu-step (iA (mt σ)) s∼b)
                                              (®⇒≈ iAcu (®-cumu-step (iA (mt σ)) Π.IT-rel) IT-rel))
                                 (El-cumu-step _ (iA (mt σ)) b∈))
            RT₁      = RT (mt σ) b∈
            RT₂      = RT (mt σ) (El-lower _ (iA (mt σ)) (El-cumu-step _ (iA (mt σ)) b∈))
            T≈T′     = ΠRT.T≈T′ RT₁
            T≈T′cumu = 𝕌-cumu-step _ T≈T′
            ®fa′     = ®El-≡ (𝕌-cumu-step _ (ΠRT.T≈T′ RT₂))
                             T≈T′cumu
                             ®fa
                             (⟦⟧-det (ΠRT.↘⟦T⟧ RT₂) (ΠRT.↘⟦T⟧ RT₁))
        in record
        { fa  = fa
        ; ↘fa = ↘fa
        ; ®fa = ®El-lower T≈T′ (Π.OT-rel s∼b b∈)
                          (®El-resp-T≈ T≈T′cumu ®fa′
                                       (®⇒≈ T≈T′cumu (®El⇒® T≈T′cumu ®fa′) (®-cumu-step T≈T′ (Π.OT-rel s∼b b∈))))
        }
      }
    }
    where module T = GluΠ T∼A
          open GluΛ t∼a

-- Push cumulativity to any higher level.
®-cumu-steps :  {i} j
               (A≈B : A  B  𝕌 i) 
               Γ  T ®[ i ] A≈B 
               -----------------------------
               Γ  T ®[ j + i ] 𝕌-cumu-steps i j A≈B
®-cumu-steps zero A≈B T∼A    = T∼A
®-cumu-steps (suc j) A≈B T∼A = ®-cumu-step (𝕌-cumu-steps _ j A≈B) (®-cumu-steps j A≈B T∼A)


®El-cumu-steps :  {i} j
                 (A≈B : A  B  𝕌 i) 
                 Γ  t  T ®[ i ] a ∈El A≈B 
                 ------------------------------------------
                 Γ  t  T ®[ j + i ] a ∈El 𝕌-cumu-steps i j A≈B
®El-cumu-steps zero A≈B t∼a    = t∼a
®El-cumu-steps (suc j) A≈B t∼a = ®El-cumu-step (𝕌-cumu-steps _ j A≈B) (®El-cumu-steps j A≈B t∼a)


®-cumu :  {i j}
         (A≈B : A  B  𝕌 i) 
         Γ  T ®[ i ] A≈B 
         (i≤j : i  j) 
         -----------------------------
         Γ  T ®[ j ] 𝕌-cumu i≤j A≈B
®-cumu {i = i} A≈B T∼A i≤j
  with ®-cumu-steps (≤-diff i≤j) A≈B T∼A
...  | rel = helper (𝕌-cumu-steps i (≤-diff i≤j) A≈B) (𝕌-cumu i≤j A≈B) rel (trans (ℕₚ.+-comm (≤-diff i≤j) i) (≤-diff-+ i≤j))
  where helper :  {i j} (A≈B : A  B  𝕌 i) (A≈B′ : A  B  𝕌 j)  Γ  T ®[ i ] A≈B  i  j  Γ  T ®[ j ] A≈B′
        helper A≈B A≈B′ T∼A refl = ®-one-sided A≈B A≈B′ T∼A

®El-cumu :  {i j}
           (A≈B : A  B  𝕌 i) 
           Γ  t  T ®[ i ] a ∈El A≈B 
           (i≤j : i  j) 
           -----------------------------
           Γ  t  T ®[ j ] a ∈El 𝕌-cumu i≤j A≈B
®El-cumu {i = i} A≈B t∼a i≤j
  with ®El-cumu-steps (≤-diff i≤j) A≈B t∼a
...  | rel = helper (𝕌-cumu-steps i (≤-diff i≤j) A≈B) (𝕌-cumu i≤j A≈B) rel (trans (ℕₚ.+-comm (≤-diff i≤j) i) (≤-diff-+ i≤j))
  where helper :  {i j} (A≈B : A  B  𝕌 i) (A≈B′ : A  B  𝕌 j)  Γ  t  T ®[ i ] a ∈El A≈B  i  j  Γ  t  T ®[ j ] a ∈El A≈B′
        helper A≈B A≈B′ t∼a refl = ®El-one-sided A≈B A≈B′ t∼a

®El-lowers :  {i} j
             (A≈B : A  B  𝕌 i) 
             Γ  T ®[ i ] A≈B 
             Γ  t  T ®[ j + i ] a ∈El 𝕌-cumu-steps i j A≈B 
             -----------------------------
             Γ  t  T ®[ i ] a ∈El A≈B
®El-lowers 0       A≈B T∼A t∼a = t∼a
®El-lowers (suc j) A≈B T∼A t∼a = ®El-lowers j A≈B T∼A (®El-lower (𝕌-cumu-steps _ j A≈B) (®-cumu-steps j A≈B T∼A) t∼a)


-- Given cumulativity and lowering, we can obtain a generalization of both, stating
-- that if types are related in any level, then we can adjust the gluing for terms to
-- this level, regardless of the original level.
®El-irrel :  {i j}
            (A≈B : A  B  𝕌 i) 
            (A≈B′ : A  B′  𝕌 j) 
            Γ  T ®[ j ] A≈B′ 
            Γ  t  T ®[ i ] a ∈El A≈B 
            -----------------------------
            Γ  t  T ®[ j ] a ∈El A≈B′
®El-irrel {i = i} {j} A≈B A≈B′ T∼A t∼a
  with i ≤? j
...  | yes i≤j = ®El-one-sided (𝕌-cumu i≤j A≈B) A≈B′ (®El-cumu A≈B t∼a i≤j)
...  | no  i≰j
    with ≰⇒≥ i≰j
...    | i≥j
      rewrite sym (m∸n+n≡m i≥j) = ®El-lowers (i  j) A≈B′ T∼A (®El-one-sided A≈B (𝕌-cumu-steps _ (i  j) A≈B′) t∼a)


-- The master lemma which handles everything you need to deal with universe levels.
--
-- This proof is not very straightforward. We have:
--
--     A ≈ A′ at level i
--     B ≈ B′ at level j
--     A ≈ B at level k
--     t : T ∼ a ∈ El A at level i
--     T ≈ T′ at level k
--
-- Our goal is t : T′ ∼ a ∈ El B at level j
--
-- We proceed as follows:
--
-- t : T  ∼ a ∈ El A at level max i k          (by cumulativity)
-- t : T′ ∼ a ∈ El A at level max i k          (T ≈ T′ at level max i k)
-- t : T′ ∼ a ∈ El A at level max j (max i k)  (cumulativity)
--
-- The previous step lifts the gluing relation to a high enough level so that we can
-- move a to B
--
-- t : T′ ∼ a ∈ El B at level max j (max i k)  (by transportation due to A ≈ B)
-- t : T′ ∼ a ∈ El B at level j                (by lowering)
®El-master :  {i j k} 
             (A≈A′ : A  A′  𝕌 i)
             (B≈B′ : B  B′  𝕌 j)
             (A≈B : A  B  𝕌 k) 
             Γ  T′ ®[ j ] B≈B′ 
             Γ  t  T ®[ i ] a ∈El A≈A′ 
             Γ  T  T′  Se k 
             ------------------------------
             Γ  t  T′ ®[ j ] a ∈El B≈B′
®El-master {i = i} {j} {k} A≈A′ B≈B′ A≈B T′∼B t∼a T≈T′
  = ®El-irrel B≈B′↑ B≈B′ T′∼B
    (®El-transport A≈A′↑↑ B≈B′↑ (𝕌-cumu k≤m′ A≈B)
    (®El-cumu A≈A′↑
    (®El-resp-T≈ A≈A′↑
    (®El-cumu A≈A′ t∼a i≤m) (lift-⊢≈-Se-max′ T≈T′))
              m≤m′))
  where m      = max i k
        i≤m    = m≤m⊔n i k
        k≤m    = m≤n⊔m i k
        m′     = max j m
        j≤m′   = m≤m⊔n j m
        m≤m′   = m≤n⊔m j m
        k≤m′   = ≤-trans k≤m m≤m′
        A≈A′↑  = 𝕌-cumu i≤m A≈A′
        A≈A′↑↑ = 𝕌-cumu m≤m′ A≈A′↑
        B≈B′↑  = 𝕌-cumu j≤m′ B≈B′