open import Relation.Binary.PropositionalEquality as Eq using (_≡_; refl; sym; trans; cong)
open import Relation.Binary.Definitions using (Decidable)
open import Relation.Binary.Core using (Rel)
open import Relation.Nullary using (Dec; yes; no)
open import Agda.Primitive using (Level; _⊔_)

module Map {a b : Level} (A : Set a) (B : Set b)
    (≡-dec-A : Decidable (_≡_ {a} {A}))
    where

import Data.List.Membership.Propositional as MemProp

open import Relation.Nullary using (¬_)
open import Data.Nat using (ℕ)
open import Data.List using (List; []; _∷_; _++_)
open import Data.List.Relation.Unary.All using (All; []; _∷_)
open import Data.List.Relation.Unary.Any using (Any; here; there) -- TODO: re-export these with nicer names from map
open import Data.Product using (_×_; _,_; Σ; proj₁ ; proj₂)
open import Data.Empty using (⊥)

keys : List (A × B) → List A
keys [] = []
keys ((k , v) ∷ xs) = k ∷ keys xs

data Unique {c} {C : Set c} : List C → Set c where
    empty : Unique []
    push : forall {x : C} {xs : List C}
        → All (λ x' → ¬ x ≡ x') xs
        → Unique xs
        → Unique (x ∷ xs)

Unique-append : ∀ {c} {C : Set c} {x : C} {xs : List C} → ¬ MemProp._∈_ x xs → Unique xs →  Unique (xs ++ (x ∷ []))
Unique-append {c} {C} {x} {[]} _ _ = push [] empty
Unique-append {c} {C} {x} {x' ∷ xs'} x∉xs (push x'≢ uxs') = push (help x'≢) (Unique-append (λ x∈xs' → x∉xs (there x∈xs')) uxs')
    where
        x'≢x : ¬ x' ≡ x
        x'≢x x'≡x = x∉xs (here (sym x'≡x))

        help : {l : List C} → All (λ x'' → ¬ x' ≡ x'') l → All (λ x'' → ¬ x' ≡ x'') (l ++ (x ∷ []))
        help {[]} _ = x'≢x ∷ []
        help {e ∷ es} (x'≢e ∷ x'≢es) = x'≢e ∷ help x'≢es

absurd : ∀ {a} {A : Set a} →  ⊥ → A
absurd ()

private module _ where
    open MemProp using (_∈_)

    unique-not-in : ∀ {k : A} {v : B} {l : List (A × B)}  → ¬ (All (λ k' → ¬ k ≡ k') (keys l) × (k , v) ∈ l)
    unique-not-in {l = (k' , _) ∷ xs} (k≢k' ∷ _ , here k',≡x) = k≢k' (cong proj₁ k',≡x)
    unique-not-in {l = _ ∷ xs} (_ ∷ rest , there k,v'∈xs) = unique-not-in (rest , k,v'∈xs)

    ListAB-functional : ∀ {k : A} {v v' : B} {l : List (A × B)} → Unique (keys l) → (k , v) ∈ l → (k , v') ∈ l → v ≡ v'
    ListAB-functional _ (here k,v≡x) (here k,v'≡x) = cong proj₂ (trans k,v≡x (sym k,v'≡x))
    ListAB-functional (push k≢xs _) (here k,v≡x) (there k,v'∈xs) rewrite sym k,v≡x = absurd (unique-not-in (k≢xs , k,v'∈xs))
    ListAB-functional (push k≢xs _) (there k,v∈xs) (here k,v'≡x) rewrite sym k,v'≡x = absurd (unique-not-in (k≢xs , k,v∈xs))
    ListAB-functional {l = _ ∷ xs } (push _ uxs) (there k,v∈xs) (there k,v'∈xs) = ListAB-functional uxs k,v∈xs k,v'∈xs

private module ImplRelation (_≈_ : B → B → Set b) where
    open MemProp using (_∈_)

    subset : List (A × B) → List (A × B) → Set (a ⊔ b)
    subset m₁ m₂ = ∀ (k : A) (v : B) → (k , v) ∈ m₁ → Σ B (λ v' → v ≈ v' × ((k , v') ∈ m₂))

private module ImplInsert (f : B → B → B) where
    open MemProp using (_∈_)

    private
        _∈k_ : A → List (A × B) → Set a
        _∈k_ k m = k ∈ (keys m)

        foldr : ∀ {c} {C : Set c} → (A → B → C → C) -> C -> List (A × B) -> C
        foldr f b [] = b
        foldr f b ((k , v) ∷ xs) = f k v (foldr f b xs)

    insert : A → B → List (A × B) → List (A × B)
    insert k v [] = (k , v) ∷ []
    insert k v (x@(k' , v') ∷ xs) with ≡-dec-A k k'
    ...                             | yes _ = (k' , f v v') ∷ xs
    ...                             | no _ = x ∷ insert k v xs

    merge : List (A × B) → List (A × B) → List (A × B)
    merge m₁ m₂ = foldr insert m₂ m₁

    insert-keys-∈ : ∀ (k : A) (v : B) (l : List (A × B)) → k ∈k l → keys l ≡ keys (insert k v l)
    insert-keys-∈ k v ((k' , v') ∷ xs) (here k≡k') with (≡-dec-A k k')
    ...                                                | yes _ = refl
    ...                                                | no k≢k' = absurd (k≢k' k≡k')
    insert-keys-∈ k v ((k' , _) ∷ xs) (there k∈kxs) with (≡-dec-A k k')
    ...                                                | yes _ = refl
    ...                                                | no _ = cong (λ xs' → k' ∷ xs') (insert-keys-∈ k v xs k∈kxs)

    insert-keys-∉ : ∀ (k : A) (v : B) (l : List (A × B)) → ¬ (k ∈k l) → (keys l ++ (k ∷ [])) ≡ keys (insert k v l)
    insert-keys-∉ k v [] _ = refl
    insert-keys-∉ k v ((k' , v') ∷ xs) k∉kl with (≡-dec-A k k')
    ...                                         | yes k≡k' = absurd (k∉kl (here k≡k'))
    ...                                         | no _ = cong (λ xs' → k' ∷ xs') (insert-keys-∉ k v xs (λ k∈kxs → k∉kl (there k∈kxs)))

    ∈k-dec : ∀ (k : A) (l : List (A × B)) → Dec (k ∈k l)
    ∈k-dec k [] = no (λ ())
    ∈k-dec k ((k' , v) ∷ xs) with (≡-dec-A k k')
    ...                        | yes k≡k' = yes (here k≡k')
    ...                        | no k≢k' with (∈k-dec k xs)
    ...                                    | yes k∈kxs = yes (there k∈kxs)
    ...                                    | no k∉kxs = no witness
                                               where
                                                   witness : ¬ k ∈k ((k' , v) ∷ xs)
                                                   witness (here k≡k') = k≢k' k≡k'
                                                   witness (there k∈kxs) = k∉kxs k∈kxs

    insert-preserves-Unique : ∀ (k : A) (v : B) (l : List (A × B)) → Unique (keys l) → Unique (keys (insert k v l))
    insert-preserves-Unique k v l u with (∈k-dec k l)
    ...                               | yes k∈kl rewrite insert-keys-∈ k v l k∈kl = u
    ...                               | no k∉kl rewrite sym (insert-keys-∉ k v l k∉kl) = Unique-append k∉kl u

    merge-preserves-Unique : ∀ (l₁ l₂ : List (A × B)) → Unique (keys l₂) → Unique (keys (merge l₁ l₂))
    merge-preserves-Unique [] l₂ u₂ = u₂
    merge-preserves-Unique ((k₁ , v₁) ∷ xs₁) l₂ u₂ = insert-preserves-Unique k₁ v₁ (merge xs₁ l₂) (merge-preserves-Unique xs₁ l₂ u₂)

Map : Set (a ⊔ b)
Map = Σ (List (A × B)) (λ l → Unique (keys l))

_∈_ : (A × B) → Map → Set (a ⊔ b)
_∈_ p (kvs , _) = MemProp._∈_ p kvs

_∈k_ : A → Map → Set a
_∈k_ k (kvs , _) = MemProp._∈_ k (keys kvs)

Map-functional : ∀ {k : A} {v v' : B} {m : Map} → (k , v) ∈ m → (k , v') ∈ m → v ≡ v'
Map-functional {m = (l , ul)} k,v∈m k,v'∈m = ListAB-functional ul k,v∈m k,v'∈m

data Provenance (k : A) (m₁ m₂ : Map) : Set (a ⊔ b) where
    both : (v₁ v₂ : B) → (k , v₁) ∈ m₁ → (k , v₂) ∈ m₂ → Provenance k m₁ m₂
    in₁ : (v₁ : B) → (k , v₁) ∈ m₁ → ¬ k ∈k m₂ → Provenance k m₁ m₂
    in₂ : (v₂ : B) → ¬ k ∈k m₁ → (k , v₂) ∈ m₂ → Provenance k m₁ m₂

module _ (f : B → B → B) where
    open ImplInsert f renaming
        ( insert to insert-impl
        ; merge to merge-impl
        )

    insert : A → B → Map → Map
    insert k v (kvs , uks) = (insert-impl k v kvs , insert-preserves-Unique k v kvs uks)

    merge : Map → Map → Map
    merge (kvs₁ , _) (kvs₂ , uks₂) = (merge-impl kvs₁ kvs₂ , merge-preserves-Unique kvs₁ kvs₂ uks₂)

    MergeResult : {k : A} {m₁ m₂ : Map} → Provenance k m₁ m₂ → Set (a ⊔ b)
    MergeResult {k} {m₁} {m₂} (both v₁ v₂ _ _) = (k , f v₁ v₂) ∈ merge m₁ m₂
    MergeResult {k} {m₁} {m₂} (in₁ v₁ _ _) = (k , v₁) ∈ merge m₁ m₂
    MergeResult {k} {m₁} {m₂} (in₂ v₂ _ _) = (k , v₂) ∈ merge m₁ m₂

    merge-provenance : ∀ (m₁ m₂ : Map) (k : A) → k ∈k merge m₁ m₂ → Σ (Provenance k m₁ m₂) MergeResult
    merge-provenance = {!!}

module _ (_≈_ : B → B → Set b) where
    open ImplRelation _≈_ renaming (subset to subset-impl)

    subset : Map → Map → Set (a ⊔ b)
    subset (kvs₁ , _) (kvs₂ , _) = subset-impl kvs₁ kvs₂

    lift : Map → Map → Set (a ⊔ b)
    lift m₁ m₂ = subset m₁ m₂ × subset m₂ m₁