open import Category -- https://github.com/konn/category-agda                                                                                     
open import Level

module freyd {c₁ c₂ ℓ : Level} (A : Category c₁ c₂ ℓ) 
  where

open import cat-utility
open import HomReasoning
open import Relation.Binary.Core
open Functor

-- C is small full subcategory of A ( C is image of F )

record SmallFullSubcategory {c₁ c₂ ℓ : Level} (A : Category c₁ c₂ ℓ) 
           : Set  (suc ℓ ⊔ (suc c₁ ⊔ suc c₂)) where
   field
      SFSF : Functor A A  
      SFSFMap← : { a b : Obj A } → Hom A (FObj SFSF a) (FObj SFSF b ) → Hom A a b 
      full→ : { a b : Obj A } { x : Hom A (FObj SFSF a) (FObj SFSF b) } → A [ FMap SFSF ( SFSFMap← x ) ≈ x ]

      -- ≈→≡ : {a b : Obj A } →  { x y : Hom A (FObj SFSF a) (FObj SFSF b) } → 
      --          (x≈y : A [ FMap SFSF x ≈ FMap SFSF y  ]) → FMap SFSF x ≡ FMap SFSF y   -- codomain of FMap is local small

-- pre-initial

record PreInitial {c₁ c₂ ℓ : Level} (A : Category c₁ c₂ ℓ) 
      (F : Functor A A )  : Set  (suc ℓ ⊔ (suc c₁ ⊔ suc c₂)) where
   field
      preinitialObj : ∀{  a : Obj A } →  Obj A 
      preinitialArrow : ∀{  a : Obj A } →  Hom A ( FObj F (preinitialObj {a} )) a 

-- initial object

record HasInitialObject {c₁ c₂ ℓ : Level} (A : Category c₁ c₂ ℓ) (i : Obj A) : Set  (suc ℓ ⊔ (suc c₁ ⊔ suc c₂)) where
   field
      initial :  ∀( a : Obj A ) →  Hom A i a
      uniqueness  : ( a : Obj A ) →  ( f : Hom A i a ) → A [ f ≈  initial a ]

-- A complete catagory has initial object if it has PreInitial-SmallFullSubcategory

open NTrans
open Limit
open SmallFullSubcategory
open PreInitial
open Complete
open Equalizer
open IsEqualizer

initialFromPreInitialFullSubcategory : {c₁ c₂ ℓ : Level} (A : Category c₁ c₂ ℓ)
      (comp : Complete A A)
      (SFS : SmallFullSubcategory A ) → 
      (PI : PreInitial A (SFSF SFS )) → { a0 : Obj A } → HasInitialObject A (limit-c comp (SFSF SFS))
initialFromPreInitialFullSubcategory A comp SFS PI = record {
      initial = initialArrow ; 
      uniqueness  = λ a f → lemma1 a f
    } where
      F : Functor A A 
      F = SFSF SFS   
      FMap← : { a b : Obj A } → Hom A (FObj F a) (FObj F b ) → Hom A a b 
      FMap←  = SFSFMap←  SFS
      lim : ( Γ : Functor A A ) → Limit A A Γ (limit-c comp Γ)  (limit-u comp Γ)
      lim Γ =  isLimit comp Γ 
      equ : {a b : Obj A} → (f g : Hom A a b)  → IsEqualizer A (equalizer-e comp f g ) f g
      equ f g = Complete.isEqualizer comp f g 
      a0 : Obj A
      a0 = limit-c comp F
      u : NTrans A A (K A A a0) F
      u = limit-u comp F
      ep : {a b : Obj A} → {f g : Hom A a b}  → Obj A 
      ep {a} {b} {f} {g} = equalizer-p comp f g
      ee :  {a b : Obj A} → {f g : Hom A a b}  → Hom A (ep {a} {b} {f} {g} ) a 
      ee {a} {b} {f} {g} = equalizer-e comp f g  
      f : {a : Obj A} → Hom A  a0 (FObj F  (preinitialObj PI {a} ) ) 
      f {a} = TMap u (preinitialObj PI {a} ) 
      initialArrow :  ∀( a : Obj A )  →  Hom A a0 a
      initialArrow a  = A [ preinitialArrow PI {a}  o f ]
      equ-fi : { a : Obj A} → {f' : Hom A a0 a} → 
          IsEqualizer A ee ( A [ preinitialArrow PI {a} o  f ] ) f'
      equ-fi  {a} {f'} = equ ( A [ preinitialArrow PI {a} o  f ] ) f'
      e=id : {e : Hom A a0 a0} → ( {c : Obj A} → A [ A [ TMap u  c o  e ]  ≈  TMap u c ] ) → A [ e  ≈ id1 A a0 ]
      e=id  {e} uce=uc =  let open ≈-Reasoning (A) in
            begin
              e
            ≈↑⟨ limit-uniqueness  (lim F) e ( λ {i} → uce=uc ) ⟩
              limit (lim F) a0 u 
            ≈⟨ limit-uniqueness (lim F) (id1 A a0) ( λ {i} → idR ) ⟩
              id1 A a0
            ∎
      kfuc=uc : {c k1 : Obj A} →  {p : Hom A k1 a0} → A [ A [ TMap u  c  o  
              A [ p o A [ preinitialArrow PI {k1} o TMap u (preinitialObj PI) ] ] ]  
                      ≈ TMap u c ]
      kfuc=uc {c} {k1} {p} =  let open ≈-Reasoning (A) in
            begin
                 TMap u  c  o ( p o ( preinitialArrow PI {k1} o TMap u (preinitialObj PI)  ))
            ≈⟨ cdr assoc  ⟩
                 TMap u c o ((p o preinitialArrow PI) o TMap u (preinitialObj PI))
            ≈⟨ assoc ⟩
                 (TMap u  c  o ( p o ( preinitialArrow PI {k1} ))) o TMap u (preinitialObj PI)  
            ≈↑⟨ car  ( full→ SFS ) ⟩
                  FMap F (FMap← (TMap u c o p o preinitialArrow PI)) o TMap u (preinitialObj PI)
            ≈⟨ nat u  ⟩
                  TMap u c o FMap (K A A a0) (FMap← (TMap u c o p o preinitialArrow PI)) 
            ≈⟨⟩
                  TMap u c o id1 A a0
            ≈⟨ idR ⟩
                 TMap u  c  
            ∎
      kfuc=1 : {k1 : Obj A} →  {p : Hom A k1 a0} → A [ A [ p o A [ preinitialArrow PI {k1} o TMap u (preinitialObj PI) ] ] ≈ id1 A a0 ]
      kfuc=1 {k1} {p} = e=id ( kfuc=uc )
      -- if equalizer has right inverse, f = g
      lemma2 :  (a b : Obj A) {c : Obj A} ( f g : Hom A a b ) 
            {e : Hom A c a } {e' : Hom A a c } ( equ : IsEqualizer A e f g ) (inv-e : A [ A [ e o e' ] ≈ id1 A a ] )
           → A [ f ≈ g ]
      lemma2 a b {c} f g {e} {e'} equ inv-e = let open ≈-Reasoning (A) in
            let open Equalizer in
            begin
                f
               ≈↑⟨ idR ⟩
                 f o  id1 A a 
               ≈↑⟨ cdr inv-e ⟩
                 f o  (  e o e'  ) 
               ≈⟨ assoc  ⟩
                 ( f o  e ) o e'  
               ≈⟨ car ( fe=ge equ ) ⟩ ( g o  e ) o e'  
               ≈↑⟨ assoc  ⟩
                 g o  (  e o e'  ) 
               ≈⟨ cdr inv-e   ⟩
                 g o  id1 A a
               ≈⟨ idR ⟩
                g
            ∎
      lemma1 : (a : Obj A) (f' : Hom A a0 a) → A [ f' ≈ initialArrow a ]
      lemma1 a f' = let open ≈-Reasoning (A) in 
             sym (
             begin
                 initialArrow a
             ≈⟨⟩
                 preinitialArrow PI {a} o  f
             ≈⟨ lemma2 a0 a (A [ preinitialArrow PI {a} o  f ]) f' {ee {a0} {a} {A [ preinitialArrow PI {a} o  f ]} {f'} } (equ-fi )
                      (kfuc=1 {ep} {ee} ) ⟩
                 f'
             ∎  )