module Core.Var

import Core.Context
import Core.Thinning

%prefix_record_projections off

-- Variables -------------------------------------------------------------------

data IsVar : Nat -> Context -> Type where
  Here : IsVar 0 (sx :< n)
  There : IsVar k sx -> IsVar (S k) (sx :< n)

export
record Var (sx : Context) where
  constructor MakeVar
  index : Nat
  0 prf : IsVar index sx

%name IsVar prf
%name Var i, j, k

-- Constructors ----------------------------------------------------------------

export
here : Var (sx :< n)
here = MakeVar 0 Here

export
there : Var sx -> Var (sx :< n)
there (MakeVar index prf) = MakeVar (S index) (There prf)

-- Views -----------------------------------------------------------------------

namespace View
  public export
  data View : Var sx -> Type where
    Here : View Core.Var.here
    There : (i : Var sx) -> View (there i)

  %name View.View view

export
view : (i : Var sx) -> View i
view (MakeVar 0 Here) = Here
view (MakeVar (S k) (There prf)) = There (MakeVar k prf)

-- Views are Unique

viewInverse : {0 i : Var sx} -> View i -> Var sx
viewInverse Here = here
viewInverse (There i) = there i

viewInversePrf1 : (view : View i) -> viewInverse view = i
viewInversePrf1 Here = Refl
viewInversePrf1 (There i) = Refl

viewInversePrf2 :
  {0 i : Var sx} ->
  (v : View i) ->
  view (viewInverse v) = (rewrite viewInversePrf1 v in v)
viewInversePrf2 Here = Refl
viewInversePrf2 (There (MakeVar index prf)) = Refl

export
viewUnique : {0 i : Var sx} -> (view1, view2 : View i) -> view1 = view2
viewUnique view1 view2 =
  rewrite sym $ viewInversePrf2 view1 in
  rewrite sym $ viewInversePrf2 view2 in
  rewrite viewInversePrf1 view1 in
  rewrite viewInversePrf1 view2 in
  Refl

-- Weakening -------------------------------------------------------------------

doWkn : Var sx -> {0 thin : sx `Thins` sy} -> View thin -> Var sy
doWkn' : {0 i : Var (sx :< n)} -> View i -> sx `Thins` sy -> Var (sy :< n)

doWkn i (Id sy) = i
doWkn i (Drop thin n) = there $ doWkn i (view thin)
doWkn i (Keep thin n) = doWkn' (view i) thin

doWkn' Here thin = here
doWkn' (There i) thin = there $ doWkn i (view thin)

export
wkn : Var sx -> sx `Thins` sy -> Var sy
wkn i thin = doWkn i (view thin)

-- Is Homomorphic

export
doWknHomo :
  (i : Var sx) ->
  {thin1 : sx `Thins` sy} ->
  {thin2 : sy `Thins` sz} ->
  (view1 : View thin1) ->
  (view2 : View thin2) ->
  doWkn (doWkn i view1) view2 = wkn i (thin2 . thin1)
doWknHomo' :
  {0 i : Var (sx :< n)} ->
  (v : View i) ->
  (thin1 : sx `Thins` sy) ->
  (thin2 : sy `Thins` sz) ->
  doWkn' (view $ doWkn' v thin1) thin2 = doWkn' v (thin2 . thin1)

doWknHomo i view1 (Id sz) =
  rewrite compLeftIdentity thin1 in
  cong (doWkn i) $ viewUnique view1 (view thin1)
doWknHomo i view1 (Drop thin2 n) =
  rewrite compLeftDrop thin2 thin1 n in
  rewrite
    viewUnique
      (view $ drop (thin2 . thin1) n)
      (Drop (thin2 . thin1) n)
  in
  cong there $ doWknHomo i view1 (view thin2)
doWknHomo i (Id _) (Keep thin2 n) =
  rewrite compRightIdentity $ keep thin2 n in
  rewrite
    viewUnique
      (view $ keep thin2 n)
      (Keep thin2 n)
  in
  Refl
doWknHomo i (Drop thin1 n) (Keep thin2 n) =
  rewrite compRightDrop thin2 thin1 n in
  rewrite
    viewUnique
      (view $ drop (thin2 . thin1) n)
      (Drop (thin2 . thin1) n)
  in
  rewrite
    Var.viewUnique
      (view $ there {n} $ doWkn i $ view thin1)
      (There $ doWkn i $ view thin1)
  in
  cong there $ doWknHomo i (view thin1) (view thin2)
doWknHomo i (Keep {prf = prf1} thin1 n) (Keep {prf = prf2} thin2 n) =
  rewrite compKeep thin2 thin1 n in
  rewrite
    viewUnique
      (view $ keep (thin2 . thin1) n)
      (Keep {prf = compPresNotId prf2 prf1} (thin2 . thin1) n)
  in
  doWknHomo' (view i) thin1 thin2

doWknHomo' Here thin1 thin2 = Refl
doWknHomo' (There i) thin1 thin2 =
  rewrite
    viewUnique
      (view $ there {n} $ doWkn i $ view thin1)
      (There $ doWkn i $ view thin1)
  in
  cong there $ doWknHomo i (view thin1) (view thin2)

export
wknHomo :
  (i : Var sx) ->
  (thin1 : sx `Thins` sy) ->
  (thin2 : sy `Thins` sz) ->
  wkn (wkn i thin1) thin2 = wkn i (thin2 . thin1)
wknHomo i thin1 thin2 = doWknHomo i (view thin1) (view thin2)