------------------------------------------------------------------------
-- The Agda standard library
--
-- Semi-heterogeneous vector equality
------------------------------------------------------------------------

module Data.Vec.Equality where

open import Data.Vec
open import Data.Nat using (suc)
open import Function
open import Level using (_⊔_)
open import Relation.Binary
open import Relation.Binary.PropositionalEquality as P using (_≡_)

module Equality {s₁ s₂} (S : Setoid s₁ s₂) where

  private
    open module SS = Setoid S
      using () renaming (_≈_ to _≊_; Carrier to A)

  infix 4 _≈_

  data _≈_ :  {}  Vec A  
              {}  Vec A   Set (s₁  s₂) where
    []-cong  : []  []
    _∷-cong_ :  { } {xs¹ : Vec A }
                 { } {xs² : Vec A }
                 (x¹≈x² :   ) (xs¹≈xs² : xs¹  xs²) 
                   xs¹    xs²

  length-equal :  {} {xs¹ : Vec A }
                   {} {xs² : Vec A } 
                 xs¹  xs²    
  length-equal []-cong        = P.refl
  length-equal (_ ∷-cong eq₂) = P.cong suc $ length-equal eq₂

  refl :  {n} (xs : Vec A n)  xs  xs
  refl []       = []-cong
  refl (x  xs) = SS.refl ∷-cong refl xs

  sym :  {n m} {xs : Vec A n} {ys : Vec A m} 
        xs  ys  ys  xs
  sym []-cong                = []-cong
  sym (x¹≡x² ∷-cong xs¹≈xs²) = SS.sym x¹≡x² ∷-cong sym xs¹≈xs²

  trans :  {n m l} {xs : Vec A n} {ys : Vec A m} {zs : Vec A l} 
          xs  ys  ys  zs  xs  zs
  trans []-cong            []-cong            = []-cong
  trans (x≈y ∷-cong xs≈ys) (y≈z ∷-cong ys≈zs) =
    SS.trans x≈y y≈z ∷-cong trans xs≈ys ys≈zs

  _++-cong_ :  {n₁¹ n₂¹} {xs₁¹ : Vec A n₁¹} {xs₂¹ : Vec A n₂¹}
                {n₁² n₂²} {xs₁² : Vec A n₁²} {xs₂² : Vec A n₂²} 
              xs₁¹  xs₁²  xs₂¹  xs₂² 
              xs₁¹ ++ xs₂¹  xs₁² ++ xs₂²
  []-cong          ++-cong eq₃ = eq₃
  (eq₁ ∷-cong eq₂) ++-cong eq₃ = eq₁ ∷-cong (eq₂ ++-cong eq₃)

module DecidableEquality {d₁ d₂} (D : DecSetoid d₁ d₂) where

  private module DS = DecSetoid D
  open DS using () renaming (_≟_ to _≟′_ ; Carrier to A)
  open Equality DS.setoid
  open import Relation.Nullary

  _≟_ :  {n m} (xs : Vec A n) (ys : Vec A m)  Dec (xs  ys)
  _≟_ []       []       = yes []-cong
  _≟_ []       (y  ys) = no (λ())
  _≟_ (x  xs) []       = no (λ())
  _≟_ (x  xs) (y  ys) with xs  ys | x ≟′ y
  ... | yes xs≈ys | yes x≊y = yes (x≊y ∷-cong xs≈ys)
  ... | no ¬xs≈ys | _       = no helper
    where
    helper : ¬ (x  xs  y  ys)
    helper (_ ∷-cong xs≈ys) = ¬xs≈ys xs≈ys
  ... | _         | no ¬x≊y = no helper
    where
    helper : ¬ (x  xs  y  ys)
    helper (x≊y ∷-cong _) = ¬x≊y x≊y

module PropositionalEquality {a} {A : Set a} where

  open Equality (P.setoid A)

  to-≡ :  {n} {xs ys : Vec A n}  xs  ys  xs  ys
  to-≡ []-cong                 = P.refl
  to-≡ (P.refl ∷-cong xs¹≈xs²) = P.cong (_∷_ _) $ to-≡ xs¹≈xs²

  from-≡ :  {n} {xs ys : Vec A n}  xs  ys  xs  ys
  from-≡ P.refl = refl _