Library prosa.util.search_arg

From mathcomp Require Import ssreflect ssrbool eqtype ssrnat seq fintype.
Require Import prosa.util.tactics.

This file introduces a function called search_arg that allows finding the argument within a given range for which a function is minimal w.r.t. to a given order while satisfying a given predicate, along with lemmas establishing the basic properties of search_arg.
Note that while this is quite similar to arg min ... / arg max ... in ssreflect (fintype), this function is subtly different in that it possibly returns None and that it does not require the last element in the given range to satisfy the predicate. In contrast, ssreflect's notion of extremum in fintype uses the upper bound of the search space as the default value, which is rather unnatural when searching through a schedule.

Section ArgSearch.

  (* Given a function f that maps the naturals to elements of type T... *)
  Context {T : Type}.
  Variable f: nat T.

  (* ... a predicate P on T ... *)
  Variable P: pred T.

  (* ... and an order R on T ... *)
  Variable R: rel T.

  (* ... we define the procedure search_arg to iterate a given search space
     a, b), while checking each element whether [f] satisfies [P] at that point and returning the extremum as defined by [R]. *)

  Fixpoint search_arg (a b : nat) : option nat :=
    if a < b then
      match b with
      | 0 ⇒ None
      | S b'match search_arg a b' with
                | Noneif P (f b') then Some b' else None
                | Some xif P (f b') && R (f b') (f x) then Some b' else Some x
    else None.

In the following, we establish basic properties of search_arg.

  (* To begin, we observe that the search yields None iff predicate P does
     not hold for any of the points in the search interval. *)

  Lemma search_arg_none:
     a b,
      search_arg a b = None x, a x < b ~~ P (f x).

  (* Conversely, if we know that f satisfies P for at least one point in
     the search space, then search_arg yields some point. *)

  Lemma search_arg_not_none:
     a b,
      ( x, (a x < b) P (f x))
       y, search_arg a b = Some y.

  (* Since search_arg considers only points at which f satisfies P, if it
     returns a point, then that point satisfies P. *)

  Lemma search_arg_pred:
     a b x,
      search_arg a b = Some x P (f x).

  (* Since search_arg considers only points within a given range, if it
     returns a point, then that point lies within the given range. *)

  Lemma search_arg_in_range:
     a b x,
      search_arg a b = Some x a x < b.

  (* Let us assume that R is a reflexive and transitive total order... *)
  Hypothesis R_reflexive: reflexive R.
  Hypothesis R_transitive: transitive R.
  Hypothesis R_total: total R.

  (* ...then search_arg yields an extremum w.r.t. to a, b), that is, if [search_arg] yields a point x, then [R (f x) (f y)] holds for any [y] in the search range [a, b) that satisfies [P]. *)
  Lemma search_arg_extremum:
     a b x,
      search_arg a b = Some x
        a y < b
        P (f y)
        R (f x) (f y).

End ArgSearch.

Section ExMinn.

  (* We show that the fact that the minimal satisfying argument ex_minn ex of 
     a predicate pred satisfies another predicate P implies the existence
     of a minimal element that satisfies both pred and P. *)

  Lemma prop_on_ex_minn:
     (P : nat Prop) (pred : nat bool) (ex : n, pred n),
      P (ex_minn ex)
       n, P n pred n ( n', pred n' n n').

  (* As a corollary, we show that if there is a constant c such 
     that P c, then the minimal satisfying argument ex_minn ex 
     of a predicate P is less than or equal to c. *)

  Corollary ex_minn_le_ex:
     (P : nat bool) (exP : n, P n) (c : nat),
      P c
      ex_minn exP c.

End ExMinn.