(** This file collects general purpose tactics that are used throughout
the development. *)
From Coq Require Export Lia.
From stdpp Require Export decidable.
From stdpp Require Import options.

Lemma f_equal_dep {A B} (f g : ∀ x : A, B x) x : f = g → f x = g x.
Proof. intros ->; reflexivity. Qed.
Lemma f_equal_help {A B} (f g : A → B) x y : f = g → x = y → f x = g y.
Proof. intros -> ->; reflexivity. Qed.
Ltac f_equal :=
  let rec go :=
    match goal with
    | _ => reflexivity
    | _ => apply f_equal_help; [go|try reflexivity]
    | |- ?f ?x = ?g ?x => apply (f_equal_dep f g); go
    end in
  try go.

(** We declare hint databases [f_equal], [congruence] and [lia] and containing
solely the tactic corresponding to its name. These hint database are useful in
to be combined in combination with other hint database. *)
Global Hint Extern 998 (_ = _) => f_equal : f_equal.
Global Hint Extern 999 => congruence : congruence.
Global Hint Extern 1000 => lia : lia.
Global Hint Extern 1001 => progress subst : subst. (** backtracking on this one will
be very bad, so use with care! *)

(** The tactic [intuition] expands to [intuition auto with *] by default. This
is rather efficient when having big hint databases, or expensive [Hint Extern]
declarations as the ones above. *)
Tactic Notation "intuition" := intuition auto.

(** [done] can get slow as it calls "trivial". [fast_done] can solve way less
goals, but it will also always finish quickly.  We do 'reflexivity' last because
for goals of the form ?x = y, if we have x = y in the context, we will typically
want to use the assumption and not reflexivity *)
Ltac fast_done :=
  solve
    [ eassumption
    | symmetry; eassumption
    | apply not_symmetry; eassumption
    | reflexivity ].
Tactic Notation "fast_by" tactic(tac) :=
  tac; fast_done.

(** A slightly modified version of Ssreflect's finishing tactic [done]. It
also performs [reflexivity] and uses symmetry of negated equalities. Compared
to Ssreflect's [done], it does not compute the goal's [hnf] so as to avoid
unfolding setoid equalities. Note that this tactic performs much better than
Coq's [easy] tactic as it does not perform [inversion]. *)
Ltac done :=
  solve
  [ repeat first
    [ fast_done
    | solve [trivial]
    (* All the tactics below will introduce themselves anyway, or make no sense
       for goals of product type. So this is a good place for us to do it. *)
    | progress intros
    | solve [symmetry; trivial]
    | solve [apply not_symmetry; trivial]
    | discriminate
    | contradiction
    | split
    | match goal with H : ¬_ |- _ => case H; clear H; fast_done end ]
  ].
Tactic Notation "by" tactic(tac) :=
  tac; done.

Ltac done_if b :=
  match b with
  | true => done
  | false => idtac
  end.

(** Aliases for trans and etrans that are easier to type *)
Tactic Notation "trans" constr(A) := transitivity A.
Tactic Notation "etrans" := etransitivity.

(** Tactics for splitting conjunctions:

- [split_and] : split the goal if is syntactically of the shape [_ ∧ _]
- [split_and?] : split the goal repeatedly (perhaps zero times) while it is
  of the shape [_ ∧ _].
- [split_and!] : works similarly, but at least one split should succeed. In
  order to do so, it will head normalize the goal first to possibly expose a
  conjunction.

Note that [split_and] differs from [split] by only splitting conjunctions. The
[split] tactic splits any inductive with one constructor.


- [destruct_and? H] : destruct assumption [H] repeatedly (perhaps zero times)
  while it is of the shape [_ ∧ _].
- [destruct_and! H] : works similarly, but at least one destruct should succeed.
  In order to do so, it will head normalize the goal first to possibly expose a
  conjunction.
- [destruct_and?] iterates [destruct_or? H] on every matching assumption [H].
- [destruct_and!] works similarly, but at least one destruct should succeed.
*)
Tactic Notation "split_and" :=
  match goal with
  | |- _ ∧ _ => split
  | |- Is_true (_ && _) => apply andb_True; split
  end.
Tactic Notation "split_and" "?" := repeat split_and.
Tactic Notation "split_and" "!" := hnf; split_and; split_and?.

Ltac destruct_and_go H :=
  try lazymatch type of H with
  | True => clear H
  | _ ∧ _ =>
    let H1 := fresh in
    let H2 := fresh in
    destruct H as [ H1 H2 ];
    destruct_and_go H1; destruct_and_go H2
  | Is_true (bool_decide _) =>
    apply (bool_decide_unpack _) in H;
    destruct_and_go H
  | Is_true (_ && _) =>
    apply andb_True in H;
    destruct_and_go H
  end.

Tactic Notation "destruct_and" "?" ident(H) :=
  destruct_and_go H.
Tactic Notation "destruct_and" "!" ident(H) :=
  hnf in H; progress (destruct_and? H).

Tactic Notation "destruct_and" "?" :=
  repeat match goal with H : _ |- _ => progress (destruct_and? H) end.
Tactic Notation "destruct_and" "!" :=
  progress destruct_and?.

(** Tactics for splitting disjunctions in an assumption:

- [destruct_or? H] : destruct the assumption [H] repeatedly (perhaps zero times)
  while it is of the shape [_ ∨ _].
- [destruct_or! H] : works similarly, but at least one destruct should succeed.
  In order to do so, it will head normalize the goal first to possibly
  expose a disjunction.
- [destruct_or?] iterates [destruct_or? H] on every matching assumption [H].
- [destruct_or!] works similarly, but at least one destruct should succeed.
*)
Tactic Notation "destruct_or" "?" ident(H) :=
  repeat match type of H with
  | False => destruct H
  | _ ∨ _ => destruct H as [H|H]
  | Is_true (bool_decide _) => apply (bool_decide_unpack _) in H
  | Is_true (_ || _) => apply orb_True in H; destruct H as [H|H]
  end.
Tactic Notation "destruct_or" "!" ident(H) := hnf in H; progress (destruct_or? H).

Tactic Notation "destruct_or" "?" :=
  repeat match goal with H : _ |- _ => progress (destruct_or? H) end.
Tactic Notation "destruct_or" "!" :=
  progress destruct_or?.


(** The tactic [case_match] destructs an arbitrary match in the conclusion or
assumptions, and generates a corresponding equality. This tactic is best used
together with the [repeat] tactical. *)
Ltac case_match :=
  match goal with
  | H : context [ match ?x with _ => _ end ] |- _ => destruct x eqn:?
  | |- context [ match ?x with _ => _ end ] => destruct x eqn:?
  end.

(** The tactic [unless T by tac_fail] succeeds if [T] is not provable by
the tactic [tac_fail]. *)
Tactic Notation "unless" constr(T) "by" tactic3(tac_fail) :=
  first [assert T by tac_fail; fail 1 | idtac].

(** The tactic [repeat_on_hyps tac] repeatedly applies [tac] in unspecified
order on all hypotheses until it cannot be applied to any hypothesis anymore. *)
Tactic Notation "repeat_on_hyps" tactic3(tac) :=
  repeat match goal with H : _ |- _ => progress tac H end.

(** The tactic [clear dependent H1 ... Hn] clears the hypotheses [Hi] and
their dependencies. *)
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) :=
  clear dependent H1; clear dependent H2.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) :=
  clear dependent H1 H2; clear dependent H3.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) hyp(H4) :=
  clear dependent H1 H2 H3; clear dependent H4.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) hyp(H4)
  hyp(H5) := clear dependent H1 H2 H3 H4; clear dependent H5.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) hyp(H4) hyp(H5)
  hyp (H6) := clear dependent H1 H2 H3 H4 H5; clear dependent H6.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) hyp(H4) hyp(H5)
  hyp (H6) hyp(H7) := clear dependent H1 H2 H3 H4 H5 H6; clear dependent H7.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) hyp(H4) hyp(H5)
  hyp (H6) hyp(H7) hyp(H8) :=
  clear dependent H1 H2 H3 H4 H5 H6 H7; clear dependent H8.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) hyp(H4) hyp(H5)
  hyp (H6) hyp(H7) hyp(H8) hyp(H9) :=
  clear dependent H1 H2 H3 H4 H5 H6 H7 H8; clear dependent H9.
Tactic Notation "clear" "dependent" hyp(H1) hyp(H2) hyp(H3) hyp(H4) hyp(H5)
  hyp (H6) hyp(H7) hyp(H8) hyp(H9) hyp(H10) :=
  clear dependent H1 H2 H3 H4 H5 H6 H7 H8 H9; clear dependent H10.

(** The tactic [is_non_dependent H] determines whether the goal's conclusion or
hypotheses depend on [H]. *)
Tactic Notation "is_non_dependent" constr(H) :=
  match goal with
  | _ : context [ H ] |- _ => fail 1
  | |- context [ H ] => fail 1
  | _ => idtac
  end.

(** The tactic [var_eq x y] fails if [x] and [y] are unequal, and [var_neq]
does the converse. *)
Ltac var_eq x1 x2 := match x1 with x2 => idtac | _ => fail 1 end.
Ltac var_neq x1 x2 := match x1 with x2 => fail 1 | _ => idtac end.

(** The tactic [eunify x y] succeeds if [x] and [y] can be unified, and fails
otherwise. If it succeeds, it will instantiate necessary evars in [x] and [y].

Contrary to Coq's standard [unify] tactic, which uses [constr] for the arguments
[x] and [y], [eunify] uses [open_constr] so that one can use holes (i.e., [_]s).
For example, it allows one to write [eunify x (S _)], which will test if [x]
unifies a successor. *)
Tactic Notation "eunify" open_constr(x) open_constr(y) :=
  unify x y.

(** Operational type class projections in recursive calls are not folded back
appropriately by [simpl]. The tactic [csimpl] uses the [fold_classes] tactics
to refold recursive calls of [fmap], [mbind], [omap] and [alter]. A
self-contained example explaining the problem can be found in the following
Coq-club message:

https://sympa.inria.fr/sympa/arc/coq-club/2012-10/msg00147.html *)
Ltac fold_classes :=
  repeat match goal with
  | |- context [ ?F ] =>
    progress match type of F with
    | FMap _ =>
       change F with (@fmap _ F);
       repeat change (@fmap _ (@fmap _ F)) with (@fmap _ F)
    | MBind _ =>
       change F with (@mbind _ F);
       repeat change (@mbind _ (@mbind _ F)) with (@mbind _ F)
    | OMap _ =>
       change F with (@omap _ F);
       repeat change (@omap _ (@omap _ F)) with (@omap _ F)
    | Alter _ _ _ =>
       change F with (@alter _ _ _ F);
       repeat change (@alter _ _ _ (@alter _ _ _ F)) with (@alter _ _ _ F)
    end
  end.
Ltac fold_classes_hyps H :=
  repeat match type of H with
  | context [ ?F ] =>
    progress match type of F with
    | FMap _ =>
       change F with (@fmap _ F) in H;
       repeat change (@fmap _ (@fmap _ F)) with (@fmap _ F) in H
    | MBind _ =>
       change F with (@mbind _ F) in H;
       repeat change (@mbind _ (@mbind _ F)) with (@mbind _ F) in H
    | OMap _ =>
       change F with (@omap _ F) in H;
       repeat change (@omap _ (@omap _ F)) with (@omap _ F) in H
    | Alter _ _ _ =>
       change F with (@alter _ _ _ F) in H;
       repeat change (@alter _ _ _ (@alter _ _ _ F)) with (@alter _ _ _ F) in H
    end
  end.
Tactic Notation "csimpl" "in" hyp(H) :=
  try (progress simpl in H; fold_classes_hyps H).
Tactic Notation "csimpl" := try (progress simpl; fold_classes).
Tactic Notation "csimpl" "in" "*" :=
  repeat_on_hyps (fun H => csimpl in H); csimpl.

(** The tactic [simplify_eq] repeatedly substitutes, discriminates,
and injects equalities, and tries to contradict impossible inequalities. *)
Tactic Notation "simplify_eq" := repeat
  match goal with
  | H : _ ≠ _ |- _ => by case H; try clear H
  | H : _ = _ → False |- _ => by case H; try clear H
  | H : ?x = _ |- _ => subst x
  | H : _ = ?x |- _ => subst x
  | H : _ = _ |- _ => discriminate H
  | H : _ ≡ _ |- _ => apply leibniz_equiv in H
  | H : ?f _ = ?f _ |- _ => apply (inj f) in H
  | H : ?f _ _ = ?f _ _ |- _ => apply (inj2 f) in H; destruct H
    (* before [injection] to circumvent bug #2939 in some situations *)
  | H : ?f _ = ?f _ |- _ => progress injection H as H
    (* first hyp will be named [H], subsequent hyps will be given fresh names *)
  | H : ?f _ _ = ?f _ _ |- _ => progress injection H as H
  | H : ?f _ _ _ = ?f _ _ _ |- _ => progress injection H as H
  | H : ?f _ _ _ _ = ?f _ _ _ _ |- _ => progress injection H as H
  | H : ?f _ _ _ _ _ = ?f _ _ _ _ _ |- _ => progress injection H as H
  | H : ?f _ _ _ _ _ _ = ?f _ _ _ _ _ _ |- _ => progress injection H as H
  | H : ?x = ?x |- _ => clear H
    (* unclear how to generalize the below *)
  | H1 : ?o = Some ?x, H2 : ?o = Some ?y |- _ =>
    assert (y = x) by congruence; clear H2
  | H1 : ?o = Some ?x, H2 : ?o = None |- _ => congruence
  | H : @existT ?A _ _ _ = existT _ _ |- _ =>
     apply (Eqdep_dec.inj_pair2_eq_dec _ (decide_rel (=@{A}))) in H
  end.
Tactic Notation "simplify_eq" "/=" :=
  repeat (progress csimpl in * || simplify_eq).
Tactic Notation "f_equal" "/=" := csimpl in *; f_equal.

Ltac setoid_subst_aux R x :=
  match goal with
  | H : R x ?y |- _ =>
     is_var x;
     try match y with x _ => fail 2 end;
     repeat match goal with
     | |- context [ x ] => setoid_rewrite H
     | H' : context [ x ] |- _ =>
        try match H' with H => fail 2 end;
        setoid_rewrite H in H'
     end;
     clear x H
  end.
Ltac setoid_subst :=
  repeat match goal with
  | _ => progress simplify_eq/=
  | H : @equiv ?A ?e ?x _ |- _ => setoid_subst_aux (@equiv A e) x
  | H : @equiv ?A ?e _ ?x |- _ => symmetry in H; setoid_subst_aux (@equiv A e) x
  end.

(** f_equiv works on goals of the form [f _ = f _], for any relation and any
number of arguments. It looks for an appropriate [Proper] instance, and applies
it. The tactic is somewhat limited, since it cannot be used to backtrack on
the Proper instances that has been found. To that end, we try to avoid the
trivial instance in which the resulting goals have an [eq]. More generally,
we try to "maintain" the relation of the current goal. For example,
when having [Proper (equiv ==> dist) f] and [Proper (dist ==> dist) f], it will
favor the second because the relation (dist) stays the same. *)
Ltac f_equiv :=
  match goal with
  | |- pointwise_relation _ _ _ _ => intros ?
  (* We support matches on both sides, *if* they concern the same variable, or
     variables in some relation. *)
  | |- ?R (match ?x with _ => _ end) (match ?x with _ => _ end) =>
    destruct x
  | H : ?R ?x ?y |- ?R2 (match ?x with _ => _ end) (match ?y with _ => _ end) =>
     destruct H
  (* First assume that the arguments need the same relation as the result *)
  | |- ?R (?f _) _ => simple apply (_ : Proper (R ==> R) f)
  | |- ?R (?f _ _) _ => simple apply (_ : Proper (R ==> R ==> R) f)
  | |- ?R (?f _ _ _) _ => simple apply (_ : Proper (R ==> R ==> R ==> R) f)
  | |- ?R (?f _ _ _ _) _ => simple apply (_ : Proper (R ==> R ==> R ==> R ==> R) f)
  | |- ?R (?f _ _ _ _ _) _ => simple apply (_ : Proper (R ==> R ==> R ==> R ==> R ==> R) f)
  (* For the case in which R is polymorphic, or an operational type class,
  like equiv. *)
  | |- (?R _) (?f _) _ => simple apply (_ : Proper (R _ ==> _) f)
  | |- (?R _ _) (?f _) _ => simple apply (_ : Proper (R _ _ ==> _) f)
  | |- (?R _ _ _) (?f _) _ => simple apply (_ : Proper (R _ _ _ ==> _) f)
  | |- (?R _) (?f _ _) _ => simple apply (_ : Proper (R _ ==> R _ ==> _) f)
  | |- (?R _ _) (?f _ _) _ => simple apply (_ : Proper (R _ _ ==> R _ _ ==> _) f)
  | |- (?R _ _ _) (?f _ _) _ => simple apply (_ : Proper (R _ _ _ ==> R _ _ _ ==> _) f)
  | |- (?R _) (?f _ _ _) _ => simple apply (_ : Proper (R _ ==> R _ ==> R _ ==> _) f)
  | |- (?R _ _) (?f _ _ _) _ => simple apply (_ : Proper (R _ _ ==> R _ _ ==> R _ _ ==> _) f)
  | |- (?R _ _ _) (?f _ _ _) _ => simple apply (_ : Proper (R _ _ _ ==> R _ _ _ R _ _ _ ==> _) f)
  | |- (?R _) (?f _ _ _ _) _ => simple apply (_ : Proper (R _ ==> R _ ==> R _ ==> R _ ==> _) f)
  | |- (?R _ _) (?f _ _ _ _) _ => simple apply (_ : Proper (R _ _ ==> R _ _ ==> R _ _ ==> R _ _ ==> _) f)
  | |- (?R _ _ _) (?f _ _ _ _) _ => simple apply (_ : Proper (R _ _ _ ==> R _ _ _ ==> R _ _ _ ==> R _ _ _ ==> _) f)
  | |- (?R _) (?f _ _ _ _ _) _ => simple apply (_ : Proper (R _ ==> R _ ==> R _ ==> R _ ==> R _ ==> _) f)
  | |- (?R _ _) (?f _ _ _ _ _) _ => simple apply (_ : Proper (R _ _ ==> R _ _ ==> R _ _ ==> R _ _ ==> R _ _ ==> _) f)
  | |- (?R _ _ _) (?f _ _ _ _ _) _ => simple apply (_ : Proper (R _ _ _ ==> R _ _ _ ==> R _ _ _ ==> R _ _ _ ==> R _ _ _ ==> _) f)
  (* Next, try to infer the relation. Unfortunately, very often, it will turn
     the goal into a Leibniz equality so we get stuck. *)
  (* TODO: Can we exclude that instance? *)
  | |- ?R (?f _) _ => simple apply (_ : Proper (_ ==> R) f)
  | |- ?R (?f _ _) _ => simple apply (_ : Proper (_ ==> _ ==> R) f)
  | |- ?R (?f _ _ _) _ => simple apply (_ : Proper (_ ==> _ ==> _ ==> R) f)
  | |- ?R (?f _ _ _ _) _ => simple apply (_ : Proper (_ ==> _ ==> _ ==> _ ==> R) f)
  | |- ?R (?f _ _ _ _ _) _ => simple apply (_ : Proper (_ ==> _ ==> _ ==> _ ==> _ ==> R) f)
  (* In case the function symbol differs, but the arguments are the same,
     maybe we have a pointwise_relation in our context. *)
  (* TODO: If only some of the arguments are the same, we could also
     query for "pointwise_relation"'s. But that leads to a combinatorial
     explosion about which arguments are and which are not the same. *)
  | H : pointwise_relation _ ?R ?f ?g |- ?R (?f ?x) (?g ?x) => simple apply H
  | H : pointwise_relation _ (pointwise_relation _ ?R) ?f ?g |- ?R (?f ?x ?y) (?g ?x ?y) => simple apply H
  end;
  try simple apply reflexivity.
Tactic Notation "f_equiv" "/=" := csimpl in *; f_equiv.

(** The tactic [solve_proper_unfold] unfolds the first head symbol, so that
we proceed by repeatedly using [f_equiv]. *)
Ltac solve_proper_unfold :=
  (* Try unfolding the head symbol, which is the one we are proving a new property about *)
  lazymatch goal with
  | |- ?R (?f _ _ _ _ _ _ _ _ _ _) (?f _ _ _ _ _ _ _ _ _ _) => unfold f
  | |- ?R (?f _ _ _ _ _ _ _ _ _) (?f _ _ _ _ _ _ _ _ _) => unfold f
  | |- ?R (?f _ _ _ _ _ _ _ _) (?f _ _ _ _ _ _ _ _) => unfold f
  | |- ?R (?f _ _ _ _ _ _ _) (?f _ _ _ _ _ _ _) => unfold f
  | |- ?R (?f _ _ _ _ _ _) (?f _ _ _ _ _ _) => unfold f
  | |- ?R (?f _ _ _ _ _) (?f _ _ _ _ _) => unfold f
  | |- ?R (?f _ _ _ _) (?f _ _ _ _) => unfold f
  | |- ?R (?f _ _ _) (?f _ _ _) => unfold f
  | |- ?R (?f _ _) (?f _ _) => unfold f
  | |- ?R (?f _) (?f _) => unfold f
  end.
(** [solve_proper_prepare] does some preparation work before the main
[solve_proper] loop.  Having this as a separate tactic is useful for debugging
[solve_proper] failure. *)
Ltac solve_proper_prepare :=
  (* Introduce everything *)
  intros;
  repeat lazymatch goal with
  | |- Proper _ _ => intros ???
  | |- (_ ==> _)%signature _ _ => intros ???
  | |- pointwise_relation _ _ _ _ => intros ?
  | |- ?R ?f _ =>
     (* Deal with other cases where we have an equivalence relation on functions
     (e.g. a [pointwise_relation] that is hidden in some form in [R]). We do
     this by checking if the arguments of the relation are actually functions,
     and then forcefully introduce one ∀ and introduce the remaining ∀s that
     show up in the goal. To check that we actually have an equivalence relation
     on functions, we try to eta expand [f], which will only succeed if [f] is
     actually a function. *)
     let f' := constr:(λ x, f x) in
     (* Now forcefully introduce the first ∀ and other ∀s that show up in the
     goal afterwards. *)
     intros ?; intros
  end; simplify_eq;
  (* We try with and without unfolding. We have to backtrack on
     that because unfolding may succeed, but then the proof may fail. *)
  (solve_proper_unfold + idtac); simpl.
(** The tactic [solve_proper_core tac] solves goals of the form "Proper (R1 ==> R2)", for
any number of relations. The actual work is done by repeatedly applying
[tac]. *)
Ltac solve_proper_core tac :=
  solve_proper_prepare;
  (* Now do the job. *)
  solve [repeat first [eassumption | tac ()] ].

(** Finally, [solve_proper] tries to apply [f_equiv] in a loop. *)
Ltac solve_proper := solve_proper_core ltac:(fun _ => f_equiv).

(** The tactic [intros_revert tac] introduces all foralls/arrows, performs tac,
and then reverts them. *)
Ltac intros_revert tac :=
  lazymatch goal with
  | |- ∀ _, _ => let H := fresh in intro H; intros_revert tac; revert H
  | |- _ => tac
  end.

(** The tactic [iter tac l] runs [tac x] for each element [x ∈ l] until [tac x]
succeeds. If it does not suceed for any element of the generated list, the whole
tactic wil fail. *)
Tactic Notation "iter" tactic(tac) tactic(l) :=
  let rec go l :=
  match l with ?x :: ?l => tac x || go l end in go l.

(** Given [H : A_1 → ... → A_n → B] (where each [A_i] is non-dependent), the
tactic [feed tac H tac_by] creates a subgoal for each [A_i] and calls [tac p]
with the generated proof [p] of [B]. *)
Tactic Notation "feed" tactic(tac) constr(H) :=
  let rec go H :=
  let T := type of H in
  lazymatch eval hnf in T with
  | ?T1 → ?T2 =>
    (* Use a separate counter for fresh names to make it more likely that
    the generated name is "fresh" with respect to those generated before
    calling the [feed] tactic. In particular, this hack makes sure that
    tactics like [let H' := fresh in feed (fun p => pose proof p as H') H] do
    not break. *)
    let HT1 := fresh "feed" in assert T1 as HT1;
      [| go (H HT1); clear HT1 ]
  | ?T1 => tac H
  end in go H.

(** The tactic [efeed tac H] is similar to [feed], but it also instantiates
dependent premises of [H] with evars. *)
Tactic Notation "efeed" constr(H) "using" tactic3(tac) "by" tactic3 (bytac) :=
  let rec go H :=
  let T := type of H in
  lazymatch eval hnf in T with
  | ?T1 → ?T2 =>
    let HT1 := fresh "feed" in assert T1 as HT1;
      [bytac | go (H HT1); clear HT1 ]
  | ?T1 → _ =>
    let e := fresh "feed" in evar (e:T1);
    let e' := eval unfold e in e in
    clear e; go (H e')
  | ?T1 => tac H
  end in go H.
Tactic Notation "efeed" constr(H) "using" tactic3(tac) :=
  efeed H using tac by idtac.

(** The following variants of [pose proof], [specialize], [inversion], and
[destruct], use the [feed] tactic before invoking the actual tactic. *)
Tactic Notation "feed" "pose" "proof" constr(H) "as" ident(H') :=
  feed (fun p => pose proof p as H') H.
Tactic Notation "feed" "pose" "proof" constr(H) :=
  feed (fun p => pose proof p) H.

Tactic Notation "efeed" "pose" "proof" constr(H) "as" ident(H') :=
  efeed H using (fun p => pose proof p as H').
Tactic Notation "efeed" "pose" "proof" constr(H) :=
  efeed H using (fun p => pose proof p).

Tactic Notation "feed" "specialize" hyp(H) :=
  feed (fun p => specialize p) H.
Tactic Notation "efeed" "specialize" hyp(H) :=
  efeed H using (fun p => specialize p).

Tactic Notation "feed" "inversion" constr(H) :=
  feed (fun p => let H':=fresh in pose proof p as H'; inversion H') H.
Tactic Notation "feed" "inversion" constr(H) "as" simple_intropattern(IP) :=
  feed (fun p => let H':=fresh in pose proof p as H'; inversion H' as IP) H.

Tactic Notation "feed" "destruct" constr(H) :=
  feed (fun p => let H':=fresh in pose proof p as H'; destruct H') H.
Tactic Notation "feed" "destruct" constr(H) "as" simple_intropattern(IP) :=
  feed (fun p => let H':=fresh in pose proof p as H'; destruct H' as IP) H.

(** The block definitions are taken from [Coq.Program.Equality] and can be used
by tactics to separate their goal from hypotheses they generalize over. *)
Definition block {A : Type} (a : A) := a.

Ltac block_goal := match goal with [ |- ?T ] => change (block T) end.
Ltac unblock_goal := unfold block in *.


(** The tactic [select pat tac] finds the last (i.e., bottommost) hypothesis
matching [pat] and passes it to the continuation [tac]. Its main advantage over
using [match goal with ] directly is that it is shorter. If [pat] matches
multiple hypotheses and [tac] fails, then [select tac] will not backtrack on
subsequent matching hypotheses.

The tactic [select] is written in CPS and does not return the name of the
hypothesis due to limitations in the Ltac1 tactic runtime (see
https://gitter.im/coq/coq?at=5e96c82f85b01628f04bbb89). *)
Tactic Notation "select" open_constr(pat) tactic3(tac) :=
  lazymatch goal with
  (** Before running [tac] on the hypothesis [H] we must first unify the
      pattern [pat] with the term it matched against. This forces every evar
      coming from [pat] (and in particular from the holes [_] it contains and
      from the implicit arguments it uses) to be instantiated. If we do not do
      so then shelved goals are produced for every such evar. *)
  | H : pat |- _ => let T := (type of H) in unify T pat; tac H
  end.
(** [select_revert] reverts the first hypothesis matching [pat]. *)
Tactic Notation "revert" "select" open_constr(pat) := select pat (fun H => revert H).

Tactic Notation "rename" "select" open_constr(pat) "into" ident(name) :=
  select pat (fun H => rename H into name).

(** Coq's [firstorder] tactic fails or loops on rather small goals already. In
particular, on those generated by the tactic [unfold_elem_ofs] which is used
to solve propositions on sets. The [naive_solver] tactic implements an
ad-hoc and incomplete [firstorder]-like solver using Ltac's backtracking
mechanism. The tactic suffers from the following limitations:
- It might leave unresolved evars as Ltac provides no way to detect that.
- To avoid the tactic becoming too slow, we allow a universally quantified
  hypothesis to be instantiated only once during each search path.
- It does not perform backtracking on instantiation of universally quantified
  assumptions.

We use a counter to make the search breath first. Breath first search ensures
that a minimal number of hypotheses is instantiated, and thus reduced the
posibility that an evar remains unresolved.

Despite these limitations, it works much better than Coq's [firstorder] tactic
for the purposes of this development. This tactic either fails or proves the
goal. *)
Lemma forall_and_distr (A : Type) (P Q : A → Prop) :
  (∀ x, P x ∧ Q x) ↔ (∀ x, P x) ∧ (∀ x, Q x).
Proof. firstorder. Qed.

(** The tactic [no_new_unsolved_evars tac] executes [tac] and fails if it
creates any new evars. This trick is by Jonathan Leivent, see:
https://coq.inria.fr/bugs/show_bug.cgi?id=3872 *)

Ltac no_new_unsolved_evars tac := exact ltac:(tac).

Tactic Notation "naive_solver" tactic(tac) :=
  unfold iff, not in *;
  repeat match goal with
  | H : context [∀ _, _ ∧ _ ] |- _ =>
    repeat setoid_rewrite forall_and_distr in H; revert H
  end;
  let rec go n :=
  repeat match goal with
  (**i solve the goal *)
  | |- _ => fast_done
  (**i intros *)
  | |- ∀ _, _ => intro
  (**i simplification of assumptions *)
  | H : False |- _ => destruct H
  | H : _ ∧ _ |- _ =>
     (* Work around bug https://coq.inria.fr/bugs/show_bug.cgi?id=2901 *)
     let H1 := fresh in let H2 := fresh in
     destruct H as [H1 H2]; try clear H
  | H : ∃ _, _  |- _ =>
     let x := fresh in let Hx := fresh in
     destruct H as [x Hx]; try clear H
  | H : ?P → ?Q, H2 : ?P |- _ => specialize (H H2)
  | H : Is_true (bool_decide _) |- _ => apply (bool_decide_unpack _) in H
  | H : Is_true (_ && _) |- _ => apply andb_True in H; destruct H
  (**i simplify and solve equalities *)
  | |- _ => progress simplify_eq/=
  (**i operations that generate more subgoals *)
  | |- _ ∧ _ => split
  | |- Is_true (bool_decide _) => apply (bool_decide_pack _)
  | |- Is_true (_ && _) => apply andb_True; split
  | H : _ ∨ _ |- _ =>
     let H1 := fresh in destruct H as [H1|H1]; try clear H
  | H : Is_true (_ || _) |- _ =>
     apply orb_True in H; let H1 := fresh in destruct H as [H1|H1]; try clear H
  (**i solve the goal using the user supplied tactic *)
  | |- _ => solve [tac]
  end;
  (**i use recursion to enable backtracking on the following clauses. *)
  match goal with
  (**i instantiation of the conclusion *)
  | |- ∃ x, _ => no_new_unsolved_evars ltac:(eexists; go n)
  | |- _ ∨ _ => first [left; go n | right; go n]
  | |- Is_true (_ || _) => apply orb_True; first [left; go n | right; go n]
  | _ =>
    (**i instantiations of assumptions. *)
    lazymatch n with
    | S ?n' =>
      (**i we give priority to assumptions that fit on the conclusion. *)
      match goal with
      | H : _ → _ |- _ =>
        is_non_dependent H;
        no_new_unsolved_evars
          ltac:(first [eapply H | efeed pose proof H]; clear H; go n')
      end
    end
  end
  in iter (fun n' => go n') (eval compute in (seq 1 6)).
Tactic Notation "naive_solver" := naive_solver eauto.