From iris.program_logic Require Import atomic.
From iris.heap_lang Require Import notation proofmode.
From iris.algebra.lib Require Import excl_auth.
From iris.base_logic.lib Require Import invariants.
From iris.base_logic.lib Require Import invariants mono_nat mono_list.
From iris_named_props Require Import custom_syntax.

From lmpmc Require Import fin_queue_spec upstream util.

Definition new_node : val := λ: "data" "final",
  let: "ℓ_node" := AllocN #3 #() in
  "ℓ_node"       <- "data";;
  "ℓ_node" +ₗ #1 <- NONE;;
  "ℓ_node" +ₗ #2 <- "final";;
  "ℓ_node".

Definition new : val := λ: <>,
  let: "ℓ_node" := new_node #() #false in
  AllocN #2 "ℓ_node".

Definition set_tail : val := rec: "go" "ℓ_q" "ℓ_node" :=
  let: "ℓ_tail" := !("ℓ_q" +ₗ #1) in
  match: !("ℓ_tail" +ₗ #1) with
    NONE =>
      if: CAS ("ℓ_tail" +ₗ #1) NONE (SOME "ℓ_node") then
        #()
      else
        "go" "ℓ_q" "ℓ_node"
  | SOME "ℓ_next" =>
      CAS ("ℓ_q" +ₗ #1) "ℓ_tail" "ℓ_next";;
      "go" "ℓ_q" "ℓ_node"
  end.

Definition enqueue : val := λ: "ℓ_q" "data",
  set_tail "ℓ_q" (new_node "data" #false).

Definition finalize : val := λ: "ℓ_q" "data",
  set_tail "ℓ_q" (new_node "data" #true).

Definition try_dequeue : val := rec: "go" "ℓ_q" :=
  let: "ℓ_head" := !"ℓ_q" in
  match: !("ℓ_head" +ₗ #1) with
    NONE => InjR NONE
  | SOME "ℓ_next" =>
      if: !("ℓ_next" +ₗ #2) then
        (* The next node is the final node. We refuse to dequeue and
           return the data of the final node. *)
        InjR (SOME !"ℓ_next")
      else (* [ℓ_next] node type = normal *)
        let: "v" := !"ℓ_next" in
        if: CAS "ℓ_q" "ℓ_head" "ℓ_next" then
          InjL "v"
        else
          "go" "ℓ_q"
  end.

Class fin_queueG Σ := FinQueueG
  { fin_queue_mono_natG  :: mono_natG Σ;
    fin_queue_mono_listG :: mono_listG (prodO locO valO) Σ; }.

Definition fin_queueΣ : gFunctors :=
  #[ mono_natΣ; mono_listΣ (prodO locO valO) ].

#[global] Instance subG_fin_queueΣ {Σ} : subG fin_queueΣ Σ → fin_queueG Σ.
Proof. solve_inG. Qed.

Section fin_queue.
  Context `{!heapGS Σ, !fin_queueG Σ}.

  Record gstate :=
    { γ_hist : gname;
      γ_hpos : gname; }.
  Definition gstate_to_pair (γ : gstate) :=
    (γ.(γ_hist), γ.(γ_hpos)).
  Definition gstate_of_pair '(γ_hist, γ_hpos) :=
    {| γ_hist := γ_hist; γ_hpos := γ_hpos |}.
  Instance gstate_eq_dec : EqDecision gstate := ltac:(solve_decision).
  Instance gstate_countable : Countable gstate.
  Proof.
    refine {| encode := encode ∘ gstate_to_pair;
              decode := fmap gstate_of_pair ∘ decode; |}.
    intros []. by rewrite /= decode_encode.
  Qed.

  Variant node_next :=
    | Normal (mℓ_next : option loc)
    | Final.
  Instance node_next_eq_dec : EqDecision node_next.
  Proof. solve_decision. Defined.
  Definition node_final_hl (n : node_next) := #(bool_decide (n = Final)).
  Definition node_next_hl (n : node_next) :=
    match n with
    | Normal mℓ_next => loc_opt_hl mℓ_next
    | Final          => NONEV
    end.
  
  Definition node_repr (ℓ : loc) (data : val) (n : node_next) : iProp Σ :=
    ("Hndata"  ∷ ℓ        ↦□ data) ∗
    ("Hnnext"  ∷ (ℓ +ₗ 1) ↦   node_next_hl n) ∗
    ("Hnfinal" ∷ (ℓ +ₗ 2) ↦□ #(bool_decide (n = Final))).

  Definition loc_at (hist : list (loc * val)) i :=
    hist.*1 !! i.
  Definition val_at (hist : list (loc * val)) i :=
    hist.*2 !! i.

  Lemma loc_at_prefix xs xs' i ℓ :
    loc_at xs i = Some ℓ → xs `prefix_of` xs' → loc_at xs' i = Some ℓ.
  Proof.
    unfold loc_at. intros Hxs Hpre.
    eapply prefix_lookup_Some; first exact Hxs.
    by apply prefix_of_fmap.
  Qed.

  Lemma val_at_prefix xs xs' i ℓ :
    val_at xs i = Some ℓ → xs `prefix_of` xs' → val_at xs' i = Some ℓ.
  Proof.
    unfold val_at. intros Hxs Hpre.
    eapply prefix_lookup_Some; first exact Hxs.
    by apply prefix_of_fmap.
  Qed.

  Lemma loc_at_val_at_Some xs i ℓ :
    loc_at xs i = Some ℓ → ∃ v, val_at xs i = Some v.
  Proof.
    unfold loc_at, val_at. rewrite [xs.*2 !! i]list_lookup_fmap.
    intros [[ℓ' v] [-> ->]]%list_lookup_fmap_Some_1. by exists v.
  Qed.

  Lemma loc_at_val_at_combine {hist i ℓ v} :
    loc_at hist i = Some ℓ →
    val_at hist i = Some v →
    hist !! i = Some (ℓ, v).
  Proof.
    unfold loc_at, val_at. intros Hloc Hval.
    rewrite list_lookup_fmap in Hloc.
    rewrite list_lookup_fmap in Hval.
    destruct (hist !! i) as [[ℓ' v']|]; last done.
    simpl in *. congruence.
  Qed.

  Lemma loc_at_Some_length hist i ℓ :
    loc_at hist i = Some ℓ → i < length hist.
  Proof.
    intros Hloc%lookup_lt_Some.
    by rewrite length_fmap in Hloc.
  Qed.

  Lemma loc_at_drop hist n i ℓ :
    loc_at hist (n + i) = Some ℓ →
    loc_at (drop n hist) i = Some ℓ.
  Proof.
    unfold loc_at. intros Hloc.
    by rewrite list_lookup_fmap lookup_drop -list_lookup_fmap.
  Qed.

  Lemma val_at_drop hist n i ℓ :
    val_at hist (n + i) = Some ℓ →
    val_at (drop n hist) i = Some ℓ.
  Proof.
    unfold val_at. intros Hval.
    by rewrite list_lookup_fmap lookup_drop -list_lookup_fmap.
  Qed.

  Definition next_from (hist : list (loc * val)) fin i :=
    if fin && bool_decide (S i = length hist) then Final else Normal (loc_at hist (S i)).

  Definition next_from_normal hist fin i v :
    next_from hist fin i = Normal v →
    (fin = false ∨ S i ≠ length hist) ∧ loc_at hist (S i) = v.
  Proof.
    rewrite /next_from. intros H.
    destruct fin, (decide (S i = length hist)).
    - rewrite bool_decide_true //= in H.
    - rewrite bool_decide_false //= in H.
      injection H as <-. split; last done. by right.
    - injection H as <-. split; last done. by left.
    - injection H as <-. split; last done. by left.
  Qed.

  Lemma next_from_S ℓv ℓvs fin n :
    next_from (ℓv :: ℓvs) fin (S n) = next_from ℓvs fin n.
  Proof.
    rewrite /next_from /loc_at list_lookup_fmap /= -list_lookup_fmap.
    erewrite bool_decide_ext; first done. lia.
  Qed.

  Lemma node_next_hl_next_from hist fin i :
    node_next_hl (next_from hist fin i) = loc_opt_hl (loc_at hist (S i)).
  Proof.
    unfold node_next_hl, next_from.
    destruct fin, (decide (S i = length hist)) as [Hhist|Hhist]; simpl; try done.
    - by rewrite bool_decide_true // /loc_at list_lookup_fmap lookup_ge_None_2; last lia.
    - by rewrite bool_decide_false.
  Qed.

  Definition hist_repr (hist : list (loc * val)) (fin : bool) : iProp Σ :=
    [∗ list] i ↦ '(ℓ, data) ∈ hist, node_repr ℓ data (next_from hist fin i).

  Lemma hist_repr_peek_1 hist fin i ℓ data :
    loc_at hist i = Some ℓ →
    val_at hist i = Some data → 
    hist_repr hist fin -∗
    node_repr ℓ data (next_from hist fin i) ∗
    (node_repr ℓ data (next_from hist fin i) -∗ hist_repr hist fin).
  Proof.
    iIntros "%Hloc %Hval Hrepr".
    pose proof (loc_at_val_at_combine Hloc Hval) as Hi.
    iPoseProof (big_sepL_lookup_acc with "Hrepr") as "[Hnode Hclose]".
    { apply Hi. }
    iFrame.
  Qed.

  Lemma hist_repr_peek_2 hist fin i ℓ :
    loc_at hist i = Some ℓ →
    hist_repr hist fin -∗
    ∃ data, node_repr ℓ data (next_from hist fin i) ∗
            (node_repr ℓ data (next_from hist fin i) -∗ hist_repr hist fin).
  Proof.
    iIntros "%Hloc Hrepr".
    assert (∃ v, val_at hist i = Some v) as [v Hval].
    { by apply loc_at_val_at_Some in Hloc. }
    iExists v. by iApply hist_repr_peek_1.
  Qed.

  Lemma hist_repr_peek_3 hist fin i ℓ :
    loc_at hist i = Some ℓ →
    hist_repr hist fin -∗
    (ℓ +ₗ 1) ↦ node_next_hl (next_from hist fin i) ∗
    ((ℓ +ₗ 1) ↦ node_next_hl (next_from hist fin i) -∗ hist_repr hist fin).
  Proof.
    iIntros "%Hloc Hrepr".
    iDestruct (hist_repr_peek_2 with "[$]") as "(%v & @ & Hclose)"; first done.
    iFrame. iIntros "Hnnext". iApply "Hclose". iFrame.
  Qed.

  Lemma loc_at_None hist pos :
    loc_at hist pos = None → hist !! pos = None.
  Proof.
    rewrite /loc_at list_lookup_fmap.
    by destruct (hist !! pos).
  Qed.

  Lemma loc_at_length hist pos :
    loc_at hist pos = None → length hist ≤ pos.
  Proof.
    intros H%loc_at_None.
    by apply lookup_ge_None.
  Qed.

  Lemma next_from_drop ℓvs fin n i :
    next_from ℓvs fin (n + i) = next_from (drop n ℓvs) fin i.
  Proof.
    rewrite /next_from /loc_at list_lookup_fmap /= -list_lookup_fmap.
    erewrite bool_decide_ext with (Q:=S i = length (drop n ℓvs)); last first.
    { rewrite length_drop. lia. }
    by rewrite fmap_drop list_lookup_fmap lookup_drop -list_lookup_fmap plus_n_Sm.
  Qed.
  
  Lemma hist_repr_proj hist i ℓ fin :
    loc_at hist i = Some ℓ →
    hist_repr hist fin -∗
    (∃ v, (ℓ +ₗ 1) ↦ v) ∗ hist_repr (drop (S i) hist) fin.
  Proof.
    iIntros "%Hℓi Hrepr".
    iDestruct (big_sepL_take_drop _ _ (S i) with "Hrepr") as "[Hrepr1 Hrepr2]".

    rewrite /loc_at list_lookup_fmap in Hℓi.
    destruct (hist !! i) as [[ℓ' v]|] eqn:Hi; last done.
    simpl in *. injection Hℓi as ->.

    iDestruct (big_sepL_lookup_acc with "Hrepr1") as "[Hrepr Hclose]".
    { rewrite lookup_take_lt //. lia. }
    iSimplifyEq. iDestruct "Hrepr" as "@". iFrame.
    iClear "Hndata Hnfinal Hclose".
    
    iAssert (hist_repr (drop (S i) hist) fin) with "[Hrepr2]" as "Hrepr2".
    { unfold hist_repr. iApply (big_sepL_mono with "Hrepr2").
      iIntros (k [ℓ' v'] Hk) "Hrepr".
      rewrite -next_from_drop; first done. }
    done.
  Qed.

  Lemma loc_at_inj_aux hist i j ℓ fin :
    i < j →
    loc_at hist i = Some ℓ →
    loc_at hist j = Some ℓ →
    hist_repr hist fin -∗
    False.
  Proof.
    iIntros "%ij %Hloci %Hlocj Hrepr".
    assert (i < length hist). { by eapply loc_at_Some_length. }
    assert (j < length hist). { by eapply loc_at_Some_length. }
    assert (∃ n, j = S i + n) as [n ->].
    { exists (j - i - 1). lia. }
    iPoseProof (hist_repr_proj hist i ℓ fin Hloci with "Hrepr") as "[[%ni Hℓi] Hrepr']".
    
    assert (Hlocj' : loc_at (drop (S i) hist) n = Some ℓ).
    { by apply loc_at_drop. }
    
    iPoseProof (hist_repr_proj _ n ℓ fin Hlocj' with "Hrepr'") as "[[%nj Hℓj] _]".
    by iCombine "Hℓi Hℓj" gives %[? _].
  Qed.

  Lemma loc_at_inj {hist i j ℓ fin} :
    loc_at hist i = Some ℓ →
    loc_at hist j = Some ℓ →
    hist_repr hist fin -∗
    ⌜i = j⌝.
  Proof.
    iIntros "%Hloci %Hlocj Hrepr".
    destruct (loc_at_val_at_Some hist i ℓ Hloci) as [vi Hvali].
    destruct (loc_at_val_at_Some hist j ℓ Hlocj) as [vj Hvalj].
    destruct (decide (i < j)) as [Hij|Hij].
    - (* i < j *)
      by iPoseProof (loc_at_inj_aux with "Hrepr") as "H".
    - (* i ≥ j *)
      destruct (decide (j < i)) as [Hji|Hji].
      + (* j < i *)
        by iPoseProof (loc_at_inj_aux with "Hrepr") as "H".
      + (* i = j *)
        iPureIntro. lia.
  Qed.

  Definition queue_γs_dq (hist : list (loc * val)) (hpos : nat) (mfin : option val) : dfrac :=
    match mfin with
    | None => DfracOwn 1
    | Some _ =>
        if decide (S (S hpos) = length hist)
        then DfracDiscarded
        else DfracOwn 1
    end.

  Lemma queue_γs_dq_cases hist hpos mfin :
    queue_γs_dq hist hpos mfin = DfracOwn 1 ∨
    queue_γs_dq hist hpos mfin = DfracDiscarded.
  Proof.
    unfold queue_γs_dq.
    destruct mfin, (decide (S (S hpos) = length hist));
      (by left) || by right.
  Qed.

  Definition queue_repr_1 (γs : gstate) ℓ_q (vs : list val) (mfin : option val) : iProp Σ :=
    ∃ (hist : list (loc * val)) (ℓ_head ℓ_tail : loc) (hpos tpos : nat),
      ("Hhead"   ∷ ℓ_q ↦ #ℓ_head) ∗
      (* Not immediately clear whether this can be discarded when the queue is
         finalized and otherwise emptied *)
      ("Htail"   ∷ (ℓ_q +ₗ 1) ↦ #ℓ_tail) ∗
      ("Hrepr"   ∷ hist_repr hist (bool_decide (is_Some mfin))) ∗
      ("Hhist●"  ∷ γs.(γ_hist) ↪●ML{queue_γs_dq hist hpos mfin} hist) ∗
      ("Hhpos●"  ∷ γs.(γ_hpos) ↪●MN{queue_γs_dq hist hpos mfin} hpos) ∗
      ("%Hℓhpos" ∷ ⌜loc_at hist hpos = Some ℓ_head⌝) ∗
      ("%Hℓtpos" ∷ ⌜loc_at hist tpos = Some ℓ_tail⌝) ∗
      ("%Hvs"    ∷ ⌜vs ++ option_list mfin = (drop (S hpos) hist).*2⌝).

  Definition queue_fin_1 (γs : gstate) (fin : val) : iProp Σ :=
    ∃ (hist : list (loc * val)) (hpos : nat),
      ("Hhist" ∷ γs.(γ_hist) ↪●ML□ hist) ∗
      ("Hhpos" ∷ γs.(γ_hpos) ↪●MN□ hpos) ∗
      ("%Hfin" ∷ ⌜val_at hist (S hpos) = Some fin⌝).

  Definition queue_repr ℓ_q (vs : list val) (mfin : option val) : iProp Σ :=
    ∃ (γs : gstate), meta ℓ_q nroot γs ∗ queue_repr_1 γs ℓ_q vs mfin.

  #[global] Instance queue_repr_timeless ℓ_q vs mfin : Timeless (queue_repr ℓ_q vs mfin) := _.

  Definition queue_fin ℓ_q (fin : val) : iProp Σ :=
    ∃ (γs : gstate), meta ℓ_q nroot γs ∗ queue_fin_1 γs fin.

  #[global] Instance queue_fin_persistent ℓ_q fin : Persistent (queue_fin ℓ_q fin) := _.
  #[global] Instance queue_fin_timeless ℓ_q fin : Timeless (queue_fin ℓ_q fin) := _.

  Lemma queue_fin_obtain ℓ fin : queue_repr ℓ [] (Some fin) -∗ queue_fin ℓ fin.
  Proof.
    iIntros "(%γs & Hγs & @)". simplify_eq/=.
    assert (Hlen : length (drop (S hpos) hist).*2 = 1) by rewrite -Hvs //.
    rewrite fmap_drop length_drop length_fmap in Hlen.
    case_decide; last lia. iFrame. iPureIntro.
    by rewrite /val_at -[S hpos]Nat.add_0_r -lookup_drop -fmap_drop -Hvs.
  Qed.

  Lemma queue_fin_agree ℓ vs fin mfin :
    queue_fin ℓ fin -∗
    queue_repr ℓ vs mfin -∗
    ⌜vs = []⌝ ∗ ⌜mfin = Some fin⌝.
  Proof.
    iIntros "(%γs & Hγs & @) (%γs' & Hγs' & @)".
    iDestruct (meta_agree with "Hγs Hγs'") as "<- {Hγs'}".
    iDestruct (mono_list_auth_own_agree with "Hhist Hhist●") as %[Hfracs <-].
    iDestruct (mono_nat_auth_own_agree with "Hhpos Hhpos●") as %[_ <-].
    iPureIntro.

    unfold queue_γs_dq in Hfracs.
    destruct mfin as [fin'|]; last done.
    destruct (decide (S (S hpos) = length hist)); last done.

    assert (Hlen : S (length vs) = 1).
    { rewrite -Nat.add_1_r -(length_app _ [fin']) Hvs length_fmap length_drop -e. lia. }
    destruct vs; last done. simplify_eq/=.

    by rewrite -Hfin /val_at -[S hpos]Nat.add_0_r -lookup_drop -fmap_drop -Hvs.
  Qed.

  Lemma hist_weaken {γs} hist hpos mfin :
    γs.(γ_hist) ↪●ML hist ==∗ γs.(γ_hist) ↪●ML{queue_γs_dq hist hpos mfin} hist.
  Proof.
    iIntros "Hhist".
    destruct (queue_γs_dq_cases hist hpos mfin) as [-> | ->]; first done.
    by iApply mono_list_auth_own_persist.
  Qed.

  Lemma hpos_weaken {γs hpos} hist mfin :
    γs.(γ_hpos) ↪●MN hpos ==∗ γs.(γ_hpos) ↪●MN{queue_γs_dq hist hpos mfin} hpos.
  Proof.
    iIntros "Hhist".
    destruct (queue_γs_dq_cases hist hpos mfin) as [-> | ->]; first done.
    by iApply mono_nat_own_persist.
  Qed.
  
  Variant new_node_next := NonFinalType | FinalType.
  Definition new_node_next_red (n : new_node_next) :=
    match n with
    | NonFinalType => Normal None
    | FinalType => Final
    end.
  Instance new_node_next_eq_dec : EqDecision new_node_next.
  Proof. solve_decision. Defined.

  Lemma new_node_spec data (final : bool) :
    {{{ True }}}
      new_node data #final
    {{{ (ℓ : loc), RET #ℓ; node_repr ℓ data (new_node_next_red (if final then FinalType else NonFinalType)) }}}.
  Proof.
    iIntros "%Φ _ HΦ". iUnfold new_node. wp_lam.
    wp_alloc ℓ_node as "Hℓ_node"; first done.
    iDestruct (array_cons with "Hℓ_node") as "[Hℓ_node0 Hℓ_node12]".
    iDestruct (array_cons with "Hℓ_node12") as "[Hℓ_node1 Hℓ_node2]".
    iDestruct (array_cons with "Hℓ_node2") as "[Hℓ_node2 _]".
    rewrite Loc.add_assoc.
    wp_let. do 3 wp_store.
    iMod (pointsto_persist with "Hℓ_node0") as "Hℓ_node0".
    iMod (pointsto_persist with "Hℓ_node2") as "Hℓ_node2".
    iModIntro. iApply "HΦ". destruct final; by iFrame.
  Qed.

  Lemma new_spec :
    {{{ True }}}
      new #()
    {{{ (ℓ : loc), RET #ℓ; queue_repr ℓ [] None }}}.
  Proof.
    iIntros "%Φ _ HΦ".  iUnfold new.
    wp_lam. wp_apply (new_node_spec with "[//]") as (ℓ_node) "Hnode". wp_let.

    set ℓ_head := ℓ_node. set ℓ_tail := ℓ_node.
    set hpos := 0. set tpos := 0.
    set hist := [(ℓ_node, #())] : list (loc * val).
    iAssert (hist_repr hist false) with "[$Hnode //]" as "Hrepr".

    iMod (mono_list_own_alloc hist) as "[%γ_hist [Hhist● _]]".
    iMod (mono_nat_own_alloc hpos) as "[%γ_hpos [Hhpos● _]]".
    set γs := {| γ_hist := γ_hist; γ_hpos := γ_hpos |}.

    iApply wp_fupd. iApply wp_allocN; [lia|done|].
    iIntros "!> %ℓ [Hℓ [Htok _]]".
    iDestruct (array_cons with "Hℓ") as "[Hℓ0 Hℓ1]".
    iDestruct (array_cons with "Hℓ1") as "[Hℓ1 _]".
    rewrite Loc.add_0.

    iMod (meta_set ⊤ ℓ γs nroot with "Htok") as "Hmeta"; first done.

    iApply "HΦ". iModIntro. iFrame. by iExists tpos.
  Qed.

  Lemma try_bump_tail_spec hist0 tpos ℓ_old ℓ_new ℓ_q γs :
    loc_at hist0 tpos = Some ℓ_old →
    loc_at hist0 (S tpos) = Some ℓ_new →
    γs.(γ_hist) ↪◯ML hist0 -∗
    <<{ ∀∀ vs mfin, queue_repr_1 γs ℓ_q vs mfin }>>
      CAS #(ℓ_q +ₗ 1) #ℓ_old #ℓ_new @ ∅
    <<{ ∃∃ (b : bool), queue_repr_1 γs ℓ_q vs mfin | RET #b }>>.
  Proof.
    iIntros "%Hold %Hnew #Hhist◯ %Φ AU".
    wp_bind (CmpXchg _ _ _)%E.
    iMod "AU" as "(%vs & %fin & (%hist1 & %ℓ_head & %ℓ_tail & %hpos & %tpos' & @) & [_ Hclose])".
    iAssert ⌜hist0 `prefix_of` hist1⌝%I as %Hhist01.
    { by iDestruct (mono_list_auth_lb_own_valid with "Hhist● Hhist◯") as "[_ ?]". }

    destruct (decide (ℓ_tail = ℓ_old)) as [->|].
    - (* The tail has not been updated yet *)
      wp_cmpxchg_suc.

      iAssert (queue_repr_1 γs ℓ_q vs fin) with "[$Hhead $Htail $Hhist● $Hhpos● $Hrepr]" as "Hq".
      { iPureIntro.
        repeat split; try done || lia.
        exists (S tpos). repeat split; try done.
        by eapply loc_at_prefix. }
      iMod ("Hclose" with "[$Hq]") as "HΦ".
      iModIntro. wp_pures. iApply "HΦ".
    - (* The tail has been updated in the meantime *)
      wp_cmpxchg_fail.
      iAssert (queue_repr_1 γs ℓ_q vs fin) with "[$Hhead $Htail $Hhist● $Hhpos● $Hrepr]" as "Hq".
      { iExists tpos'. by iFrameNamed. }
      iMod ("Hclose" with "[$Hq]") as "HΦ".
      iModIntro. wp_pures. iApply "HΦ".
  Qed.

  (* Given a history that is represented, after updating the [next]
     pointer of the current tail node to a new tail node, we obtain a
     new (extended) history (with the new tail node appended). *)
  Lemma hist_repr_snoc hist tpos (ℓ_tail ℓ_new : loc) data next :
    length hist = S tpos →
    loc_at hist tpos = Some ℓ_tail →
    node_repr ℓ_new data (new_node_next_red next) -∗
    hist_repr hist false -∗
    (ℓ_tail +ₗ 1) ↦ node_next_hl (Normal None) ∗
    ((ℓ_tail +ₗ 1) ↦ node_next_hl (Normal (Some ℓ_new)) -∗ hist_repr (hist ++ [(ℓ_new, data)]) (bool_decide (next = FinalType))).
  Proof.
    unfold loc_at.
    iIntros "%Hhistlen %Hloc @ Hhist".
    rewrite list_lookup_fmap in Hloc.
    destruct (hist !! tpos) as [[ℓ_tail' v_tail]|] eqn:Htpos; last done.
    injection Hloc as ->.
    apply list_elem_of_split_length in Htpos as (hist' & empty & -> & Htpos).
    rewrite length_app length_cons -Htpos in Hhistlen.
    assert (empty = []) as ->. { apply nil_length_inv. lia. }
    clear Hhistlen Htpos.

    iPoseProof (big_sepL_snoc with "Hhist") as "[Hinit Htail]".
    iNamedSuffix "Htail" "_tail".

    iSimplifyEq.
    assert (loc_at (hist' ++ [(ℓ_tail, v_tail)]) (S (length hist')) = None) as ->.
    { apply lookup_ge_None_2. rewrite length_fmap length_app length_cons /=. lia. }
    iFrame "Hnnext_tail". iIntros "Hnnext_tail".
    iSimplifyEq. rewrite /hist_repr.
    iApply big_sepL_snoc.
    iSplitR "Hndata Hnfinal Hnnext".
    - iApply big_sepL_snoc. iSplitL "Hinit".
      + iApply (big_sepL_mono with "Hinit").
        iIntros (k [ℓ v] Hk) "Hrepr".
        unfold next_from. simpl. rewrite [bool_decide (S k = _)]bool_decide_false; last first.
        { apply lookup_lt_Some in Hk. rewrite !length_app /=. lia. }
        rewrite andb_false_r /=.
        assert (loc_at ((hist' ++ [(ℓ_tail, v_tail)]) ++ [(ℓ_new, data)]) (S k) = loc_at (hist' ++ [(ℓ_tail, v_tail)]) (S k)) as ->.
        { rewrite /loc_at !list_lookup_fmap lookup_app_l // length_app /=.
          apply lookup_lt_Some in Hk. lia. }
        done.
      + iFrame.
        rewrite /next_from /= [bool_decide (S (length hist') = _)]bool_decide_false; last first.
        { rewrite !length_app /=. lia. }
        rewrite andb_false_r /= /loc_at list_lookup_fmap lookup_app_r; last first.
        { rewrite length_app /=. lia. }
        rewrite length_app /= Nat.add_1_r Nat.sub_diag /=.
        by iFrame.
    - iFrame.
      rewrite /next_from /= /loc_at length_app /= Nat.add_1_r list_lookup_fmap lookup_app_r.
      * rewrite !length_app /= !Nat.add_1_r [bool_decide (S _ = S _)]bool_decide_true //
          andb_true_r Nat.sub_succ -Arith_base.minus_Sn_m_stt // Nat.sub_diag /=.
        destruct next; simpl; iFrame.
      * rewrite length_app /= Nat.add_1_r. lia.
  Qed.

  Definition iffinal {A} (n : new_node_next) (a b : A) :=
    match n with
    | FinalType => a
    | NonFinalType => b
    end.
  
  Lemma set_tail_spec (ℓ_q ℓ_node : loc) data next :
    node_repr ℓ_node data (new_node_next_red next) -∗
    <<{ ∀∀ vs, queue_repr ℓ_q vs None }>>
      set_tail #ℓ_q #ℓ_node @ ∅
    <<{ queue_repr ℓ_q (vs ++ iffinal next [] [data]) (iffinal next (Some data) None) | RET #() }>>.
  Proof.
    iIntros "@ %Φ AU". iLöb as "IH". wp_rec.

    wp_pures. wp_bind (! _)%E.
    iMod "AU" as "(%vs & (%γs & #Hγs & %hist0 & %ℓ_head & %ℓ_tail & %hpos & %tpos & @) & [Hclose _])".
    iDestruct (mono_list_lb_own_get with "Hhist●") as "#Hhist0◯".
    wp_load. iSimpl in "Hrepr".
    rename Hℓtpos into Hℓtpos0.
    iAssert (queue_repr_1 γs ℓ_q vs None) with "[$Hhead $Htail $Hhist● $Hhpos● $Hrepr]" as "Hq".
    { iExists tpos. by iFrameNamed. }
    iMod ("Hclose" with "[$]") as "AU". clear Hvs Hℓhpos hpos vs ℓ_head.
    iModIntro. wp_let. wp_pure.

    wp_bind (! _)%E.
    iMod "AU" as "(%vs & (%γs' & Hγs' & %hist2 & %ℓ_head & %ℓ_tail'' & %hpos & %tpos'' & @) & [Hclose _])".
    iDestruct (meta_agree with "Hγs Hγs'") as "<- {Hγs'}".
    iDestruct (mono_list_auth_lb_own_valid with "Hhist● Hhist0◯") as %Hhist02.
    iDestruct (mono_list_lb_own_get with "Hhist●") as "#Hhist2◯".
    destruct Hhist02 as [_ Hhist02]. rename Hℓtpos into Hℓtpos2.
    assert (Hℓtpos0'' : loc_at hist2 tpos = Some ℓ_tail).
    { by eapply loc_at_prefix. } clear Hℓtpos0.
    wp_pures.
    iDestruct (hist_repr_peek_2 with "[$Hrepr]") as "(%w & Hnrepr & Hrepr)"; first done.
    iNamedSuffix "Hnrepr" "_tail".
    wp_load.
    iDestruct "Hnfinal_tail" as "#Hnfinal_tail2".
    iPoseProof ("Hrepr" with "[$]") as "Hrepr".
    iAssert (queue_repr_1 γs ℓ_q vs None) with "[$Hhead $Htail $Hhist● $Hhpos● $Hrepr]" as "Hq".
    { iExists tpos''. by iFrameNamed. }
    iMod ("Hclose" with "[$]") as "AU". clear Hvs Hℓhpos hpos vs ℓ_head.
    iModIntro.

    destruct (next_from hist2 false tpos) as [[ℓ_next|]|] eqn:Htpos2; last done.
    + (* it is not the last node *)
      simpl. apply next_from_normal in Htpos2 as [Hnext HStpos2]. rewrite HStpos2 /=.
      
      wp_pures. wp_bind (Snd (CmpXchg _ _ _))%E.
      awp_apply (try_bump_tail_spec with "[//]").
      { apply Hℓtpos0''. }
      { done. }
      rewrite /atomic_acc /=.
      iMod "AU" as "(%vs & (%γs' & Hγs' & Hrepr) & [Hclose _])".
      iDestruct (meta_agree with "Hγs Hγs'") as "<- {Hγs'}".
      iModIntro. iExists vs. iFrame "Hrepr". iSplit.
      * iFrame. iIntros "Hrepr". by iApply ("Hclose" with "[$Hrepr]").
      * iIntros "%b Hrepr".
        iMod ("Hclose" with "[$Hrepr //]") as "AU".
        iModIntro. wp_pures.
        by iApply ("IH" with "[$] [$] [$]").
    + (* it is the last (non-final) node *)
      simpl. apply next_from_normal in Htpos2 as [Hnext HStpos2]. rewrite HStpos2 /=.
      wp_pures. wp_bind (CmpXchg _ _ _)%E.

      iMod "AU" as "(%vs & (%γs' & Hγs' & %hist3 & %ℓ_head & %ℓ_tail''' & %hpos & %tpos''' & @) & Hclose)".
      iDestruct (meta_agree with "Hγs Hγs'") as "<- {Hγs'}".
      iDestruct (mono_list_auth_lb_own_valid with "Hhist● Hhist2◯") as %Hhist23.
      destruct Hhist23 as [_ Hhist23]. rename Hℓtpos into Hℓtpos3.

      destruct (loc_at hist3 (S tpos)) as [ℓ_tail''''|] eqn:Hrace.
      * (* Another item has been enqueued in the meantime, so the CAS is poised to fail. *)
        iDestruct "Hclose" as "[Hclose _]".

        iDestruct (hist_repr_peek_3 with "[$Hrepr]") as "[Hℓ_tail1 Hrepr]".
        { eapply prefix_lookup_Some; first apply Hℓtpos0''. by apply prefix_of_fmap. }
        rewrite /next_from [bool_decide (S _ = _)]bool_decide_false; last first.
        { apply lookup_lt_Some in Hrace. rewrite length_fmap in Hrace. lia. }
        rewrite andb_false_r Hrace /=.
        wp_cmpxchg_fail.
        iSpecialize ("Hrepr" with "Hℓ_tail1").
        iAssert (queue_repr_1 γs ℓ_q vs None) with "[$Hhead $Htail $Hhist● $Hhpos● $Hrepr]" as "Hq".
        { iExists tpos'''. by iFrameNamed. }
        iMod ("Hclose" with "[$]") as "AU". iModIntro.
        wp_pures. iApply ("IH" with "Hndata Hnnext Hnfinal AU").
      * (* This node is still the tail, so the CAS will succeed. *)
        iDestruct "Hclose" as "[_ Hclose]".

        iAssert ⌜length hist3 = S tpos⌝%I as %Hhist3.
        { apply loc_at_length in Hrace as Hhist3.
          iPureIntro. apply loc_at_prefix with (xs':=hist3) in Hℓtpos0''; last done.
          apply lookup_lt_Some in Hℓtpos0''. rewrite length_fmap in Hℓtpos0''. lia. }

        iDestruct (hist_repr_peek_2 with "[$Hrepr]") as "(%w' & Hℓ_tail3 & Hrepr)".
        { eapply prefix_lookup_Some; first apply Hℓtpos0''. by apply prefix_of_fmap. }
        rewrite /next_from [bool_decide (S _ = _)]bool_decide_true // andb_true_r Hrace /=.
        iNamedSuffix "Hℓ_tail3" "_tail3".
        iPoseProof ("Hrepr" with "[$]") as "Hrepr". clear w.
        
        iPoseProof (hist_repr_snoc with "[$Hndata $Hnfinal $Hnnext] [$Hrepr]") as "[Htnode Henq]".
        { apply Hhist3. }
        { eapply prefix_lookup_Some. apply Hℓtpos0''. by apply prefix_of_fmap. }
        wp_cmpxchg_suc. iSpecialize ("Henq" with "Htnode").

        pose mfin := iffinal next (Some data) None.

        iMod (mono_list_auth_own_update_app [(ℓ_node, data)] with "Hhist●") as "[Hhist● _]".
        iMod (hpos_weaken (hist3 ++ [(ℓ_node, data)]) mfin with "Hhpos●") as "Hhpos●".
        iMod (hist_weaken (hist3 ++ [(ℓ_node, data)]) hpos mfin with "Hhist●") as "Hhist●".
        replace (bool_decide (next = FinalType)) with (bool_decide (is_Some mfin)); last first.
        { apply bool_decide_ext. destruct next; by cbn. }
        
        iAssert (queue_repr_1 γs ℓ_q (vs ++ iffinal next [] [data]) mfin) with "[$Hhead $Htail $Hhist● $Hhpos● $Henq]" as "Hq".
        { iExists tpos'''. iPureIntro.
          repeat split; try done.
          - eapply loc_at_prefix; first done.
            by apply prefix_app_r.
          - eapply loc_at_prefix; first done.
            by apply prefix_app_r.
          - rewrite drop_app fmap_app -Hvs. rewrite /= app_nil_r -app_assoc.
            f_equal.
            assert (Hhpos : hpos < length hist3).
            { apply lookup_lt_Some in Hℓhpos.
              by rewrite length_fmap in Hℓhpos. }
            assert (S hpos - length hist3 = 0) as -> by lia.
            simpl. by destruct next. }
        iMod ("Hclose" with "[$]") as "HΦ".
        iModIntro. wp_pures. done.
  Qed.

  Lemma enqueue_spec (ℓ_q : loc) (data : val) :
    ⊢ <<{ ∀∀ vs, queue_repr ℓ_q vs None }>>
        enqueue #ℓ_q data @ ∅
      <<{ queue_repr ℓ_q (vs ++ [data]) None | RET #() }>>.
  Proof.
    iIntros "%Φ AU". wp_lam. wp_pures.
    wp_apply (new_node_spec with "[//]") as "%ℓ_node Hnode".
    awp_apply (set_tail_spec with "[$Hnode]").
    rewrite /atomic_acc /=. iMod "AU" as "(%vs & Hrepr & Hclose)".
    iModIntro. iFrame.
  Qed.

  Lemma finalize_spec (ℓ_q : loc) (data : val) :
    ⊢ <<{ ∀∀ vs, queue_repr ℓ_q vs None }>>
        finalize #ℓ_q data @ ∅
      <<{ queue_repr ℓ_q vs (Some data) | RET #() }>>.
  Proof.
    iIntros "%Φ AU". wp_lam. wp_pures.
    wp_apply (new_node_spec with "[//]") as "%ℓ_node Hnode".
    awp_apply (set_tail_spec with "[$Hnode]").
    rewrite /atomic_acc /=. iMod "AU" as "(%vs & Hrepr & Hclose)".
    iModIntro. iFrame. by rewrite app_nil_r.
  Qed.

  Lemma try_dequeue_spec (ℓ_q : loc) :
    ⊢ <<{ ∀∀ vs mfin, queue_repr ℓ_q vs mfin }>>
        try_dequeue #ℓ_q @ ∅
      <<{ queue_repr ℓ_q (tail vs) mfin
        | RET (if head vs is Some v then InjLV v else InjRV (val_opt_hl mfin)) }>>.
  Proof.
    iIntros "%Φ AU". iLöb as "IH". wp_rec.

    wp_bind (! _)%E.
    iMod "AU" as "(%vs0 & %fin0 & (%γs & #Hγs & %hist0 & %ℓ_head0 & %ℓ_tail0 & %hpos0 & %tpos0 & @) & [Hclose _])".
    iDestruct (mono_list_lb_own_get with "Hhist●") as "#Hhist0◯".
    iDestruct (mono_nat_lb_own_get with "Hhpos●") as "#Hhpos0◯".
    rename Hℓhpos into Hℓhpos0.
    wp_load.
    iMod ("Hclose" with "[$Hhead $Htail $Hhist● $Hhpos● $Hrepr $Hγs]") as "AU".
    { iExists tpos0. by iFrameNamed. }
    iModIntro. clear fin0 vs0 Hvs tpos0 Hℓtpos ℓ_tail0.

    wp_pures. wp_bind (! _)%E.
    iMod "AU" as "(%vs1 & %fin1 & (%γs' & Hγs' & %hist1 & %ℓ_head1 & %ℓ_tail1 & %hpos1 & %tpos1 & @) & Hclose)".
    iDestruct (meta_agree with "Hγs Hγs'") as "<- {Hγs'}".
    iDestruct (mono_list_lb_own_get with "Hhist●") as "#Hhist1◯".
    iDestruct (mono_nat_auth_lb_own_valid with "Hhpos● Hhpos0◯") as %Hhpos01.
    iDestruct (mono_list_auth_lb_own_valid with "Hhist● Hhist0◯") as %Hhist01.
    destruct Hhpos01 as [_ Hhpos01]. destruct Hhist01 as [_ Hhist01].
    assert (Hℓhpos0' : loc_at hist1 hpos0 = Some ℓ_head0).
    { by eapply loc_at_prefix. } clear Hℓhpos0.
    iDestruct (hist_repr_peek_2 with "[$Hrepr]") as "(%u & @ & Hrepr)"; first done.
    wp_load.
    iSpecialize ("Hrepr" with "[$]").

    rewrite node_next_hl_next_from.
    destruct (loc_at hist1 (S hpos0)) as [ℓ_headS0|] eqn:Hℓ_headS0.
    - simpl. iDestruct "Hclose" as "[Hclose _]".
      iMod ("Hclose" with "[$Hhead $Htail $Hhist● $Hhpos● $Hrepr $Hγs]") as "AU".
      { iExists tpos1. by iFrameNamed. }
      iModIntro. clear u fin1 vs1 Hvs tpos1 Hℓtpos ℓ_tail1.

      wp_pures. wp_bind (! _)%E.
      iMod "AU" as "(%vs2 & %fin2 & (%γs' & Hγs' & %hist2 & %ℓ_head2 & %ℓ_tail2 & %hpos2 & %tpos2 & @) & Hclose)".
      iDestruct (meta_agree with "Hγs Hγs'") as "<- {Hγs'}".
      iDestruct (mono_list_lb_own_get with "Hhist●") as "#Hhist2◯".
      iDestruct (mono_nat_auth_lb_own_valid with "Hhpos● Hhpos0◯") as %Hhpos02.
      iDestruct (mono_list_auth_lb_own_valid with "Hhist● Hhist1◯") as %Hhist12.
      destruct Hhpos02 as [_ Hhpos02]. destruct Hhist12 as [_ Hhist12].
      assert (Hℓ_headS0' : loc_at hist2 (S hpos0) = Some ℓ_headS0).
      { by eapply loc_at_prefix. } clear Hℓ_headS0.
      apply loc_at_val_at_Some in Hℓ_headS0' as Hval_headS0'.
      destruct Hval_headS0' as [w Hval_headS0'].
      iDestruct (hist_repr_peek_1 with "[$Hrepr]") as "(@ & Hrepr)"; try done.
      iDestruct "Hndata" as "#Hndata_head".
      wp_load.

      destruct (next_from hist2 (bool_decide (is_Some fin2)) (S hpos0)) as [mℓ_next|] eqn:Hnext2.
      + iDestruct "Hclose" as "[Hclose _]".
        iEval (rewrite bool_decide_false //).

        iDestruct "Hnfinal" as "#Hnfinal_head".
        iEval (rewrite bool_decide_false //) in "Hnfinal_head".

        iSpecialize ("Hrepr" with "[$]").
        iMod ("Hclose" with "[$Hhead $Htail $Hhist● $Hhpos● $Hrepr $Hγs]") as "AU".
        { iExists tpos2. by iFrameNamed. }
        iModIntro.
        clear fin2 vs2 Hvs tpos2 Hℓtpos ℓ_tail2 Hnext2.

        wp_pures. wp_load. wp_pures.

        wp_bind (CmpXchg _ _ _)%E.
        iMod "AU" as "(%vs3 & %fin3 & (%γs' & Hγs' & %hist3 & %ℓ_head3 & %ℓ_tail3 & %hpos3 & %tpos3 & @) & Hclose)".
        iDestruct (meta_agree with "Hγs Hγs'") as "<- {Hγs'}".
        iDestruct (mono_list_auth_lb_own_valid with "Hhist● Hhist2◯") as %Hhist23.
        destruct Hhist23 as [_ Hhist23].
        assert (Hhist13 : hist1 `prefix_of` hist3).
        { by trans hist2. }

        destruct (decide (ℓ_head0 = ℓ_head3)) as [->|Hhead03].
        * iDestruct "Hclose" as "[_ Hclose]".

          wp_cmpxchg_suc.

          assert (Hℓhpos0'' : loc_at hist3 hpos0 = Some ℓ_head3).
          { by eapply loc_at_prefix. } clear Hℓhpos0'.
          iAssert ⌜hpos0 = hpos3⌝%I as %->.
          { by iApply (loc_at_inj with "Hrepr"). }

          destruct (decide (length (vs3 ++ option_list fin3) = 1)) as [Hvs3|Hvs3].
          -- destruct fin3 eqn:Hfin3.
             ++ assert (Hℓ_headS0'' : loc_at hist3 (S hpos3) = Some ℓ_headS0).
                { by eapply loc_at_prefix. } clear Hℓ_headS0'.
                assert (Hval_headS0'' : val_at hist3 (S hpos3) = Some w).
                { by eapply val_at_prefix. } clear Hval_headS0'.

                simplify_eq/=.
                destruct vs3; last first.
                { rewrite length_app /= Nat.add_1_r in Hvs3. lia. }

                assert (length hist3 = S (S hpos3)).
                { apply (f_equal length), symmetry in Hvs.
                  rewrite /= length_fmap length_drop in Hvs. lia. }

                iDestruct (hist_repr_peek_1 with "[$Hrepr]") as "(@ & Hrepr)"; try done.
                iClear "Hnnext".
                rewrite /next_from !andb_true_l [bool_decide (S _ = _)]bool_decide_true //=.
                by iCombine "Hnfinal_head Hnfinal" gives "[_ %contra]".
             ++ simplify_eq/=. rewrite app_nil_r in Hvs Hvs3.

                iMod (mono_nat_own_update (S hpos3) with "Hhpos●") as "[Hhpos● _]". { lia. }
                iMod ("Hclose" with "[$Hhead $Htail $Hhist● $Hhpos● $Hrepr $Hγs]") as "HΦ".
                { iExists tpos3. iFrameNamed.
                  iPureIntro. repeat split; try done.
                  - by eapply loc_at_prefix.
                  - by rewrite /= fmap_drop /= -Nat.add_1_r -drop_drop -fmap_drop -Hvs app_nil_r. }

                iModIntro. wp_pures.

                assert (Hval_headS0'' : val_at hist3 (S hpos3) = Some w).
                { by eapply val_at_prefix. } clear Hval_headS0'.
                unfold val_at in Hval_headS0''.
                destruct vs3; try done.
                rewrite -[S hpos3]Nat.add_0_r -lookup_drop -fmap_drop -Hvs /= in Hval_headS0''.
                by simplify_eq/=.
          -- assert (S (S hpos3) ≠ length hist3).
             { rewrite Hvs length_fmap length_drop in Hvs3. lia. }

             assert (queue_γs_dq hist3 hpos3 fin3 = DfracOwn 1) as ->.
             { unfold queue_γs_dq. by repeat case_match. }
             iMod (mono_nat_own_update (S hpos3) with "Hhpos●") as "[Hhpos● _]". { lia. }
             iMod (hpos_weaken hist3 fin3 with "Hhpos●") as "Hhpos●".
             iMod (hist_weaken hist3 (S hpos3) fin3 with "Hhist●") as "Hhist●".

             iMod ("Hclose" with "[$Hhead $Htail $Hhist● $Hhpos● $Hrepr $Hγs]") as "HΦ".
             { iExists tpos3. iFrameNamed.
               iPureIntro. repeat split; try done.
               - by eapply loc_at_prefix.
               - rewrite -Nat.add_1_r -drop_drop fmap_drop -Hvs.
                 destruct fin3; last by rewrite /= !app_nil_r.
                 by destruct vs3. }

             iModIntro. wp_pures.

             assert (Hval_headS0'' : val_at hist3 (S hpos3) = Some w).
             { by eapply val_at_prefix. } clear Hval_headS0'.
             unfold val_at in Hval_headS0''.
             rewrite -[S hpos3]Nat.add_0_r -lookup_drop -fmap_drop -Hvs in Hval_headS0''.

             destruct vs3; simplify_eq/=; last done.
             by destruct fin3.

        * iDestruct "Hclose" as "[Hclose _]".

          wp_cmpxchg_fail.

          iMod ("Hclose" with "[$Hhead $Htail $Hhist● $Hhpos● $Hrepr $Hγs]") as "AU".
          { iExists tpos3. by iFrameNamed. }

          iModIntro. wp_pures. iApply ("IH" with "AU").
        
      + iDestruct "Hclose" as "[_ Hclose]".

        destruct fin2 as [fin2|]; last done. simplify_eq/=.
        assert (Hhist2 : length hist2 = S (S hpos0)).
        { destruct (decide (length hist2 = S (S hpos0))); first done.
          by rewrite /next_from bool_decide_false in Hnext2. }
        assert (S hpos2 < length hist2).
        { rewrite -(length_fmap snd).
          apply lookup_lt_is_Some_1.
          rewrite -[S hpos2]Nat.add_0_r -lookup_drop -fmap_drop -Hvs.
          by destruct vs2, fin2. }
        assert (hpos0 = hpos2) as -> by lia.
        assert (Hvs2 : length vs2 = 0).
        { apply eq_add_S. rewrite -Nat.add_1_r -(length_app _ [fin2]) Hvs length_fmap length_drop Hhist2. lia. }
        destruct vs2; last done. clear Hvs2.

        iSpecialize ("Hrepr" with "[$]").
        iMod ("Hclose" with "[$Hhead $Htail $Hhist● $Hhpos● $Hrepr $Hγs]") as "HΦ".
        { iExists tpos2. by iFrameNamed. }

        iModIntro. clear tpos2 Hℓhpos Hℓtpos ℓ_tail2.
        
        wp_pures. wp_load.
        rewrite /val_at in Hval_headS0'.
        rewrite -[S hpos2]Nat.add_0_r -lookup_drop -fmap_drop -Hvs in Hval_headS0'.
        simplify_eq/=. wp_pures. iApply "HΦ".
    - apply loc_at_length in Hℓ_headS0 as Hhistlen.
      rewrite drop_ge /= in Hvs; last lia. destruct vs1, fin1; try done.
      simplify_eq/=.

      iDestruct "Hclose" as "[_ Hclose]".
      iMod ("Hclose" with "[$Hhead $Htail $Hhist● $Hhpos● $Hrepr $Hγs]") as "HΦ".
      { iExists tpos1. iFrameNamed. iPureIntro.
        rewrite drop_ge //=. lia. }

      iModIntro. wp_pures. iApply "HΦ".
  Qed.

End fin_queue.

Definition fin_queue_hl :=
  {| fin_queue_spec.new := new;
     fin_queue_spec.enqueue := enqueue;
     fin_queue_spec.finalize := finalize;
     fin_queue_spec.try_dequeue := try_dequeue; |}.

Definition fin_queue_iris `{!heapGS Σ, !fin_queueG Σ} : fin_queue_spec.fin_queue_iris Σ fin_queue_hl.
Proof.
  refine (FinQueueIris _ _
            fin_queue_hl _ _ _ _ _
            queue_fin_obtain
            queue_fin_agree
            new_spec
            enqueue_spec
            finalize_spec
            try_dequeue_spec).
Defined.

#[global] Typeclasses Opaque queue_repr queue_fin.