149 lines
5.4 KiB
Text
149 lines
5.4 KiB
Text
/-
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Copyright (c) 2015 Leonardo de Moura. All rights reserved.
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Released under Apache 2.0 license as described in the file LICENSE.
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Authors: Leonardo de Moura
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Naive sort for lists
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-/
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import data.list.comb data.list.set data.list.perm logic.connectives
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namespace list
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open decidable nat
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variable {A : Type}
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variable (R : A → A → Prop)
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variable [decR : decidable_rel R]
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include decR
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definition min_core : list A → A → A
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| [] a := a
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| (b::l) a := if R b a then min_core l b else min_core l a
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definition min : Π (l : list A), l ≠ nil → A
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| [] h := absurd rfl h
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| (a::l) h := min_core R l a
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variable [decA : decidable_eq A]
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include decA
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variable {R}
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variables (to : total R) (tr : transitive R) (rf : reflexive R)
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lemma min_core_lemma : ∀ {b l} a, b ∈ l ∨ b = a → R (min_core R l a) b
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| b [] a h := or.elim h
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(suppose b ∈ [], absurd this !not_mem_nil)
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(suppose b = a,
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assert R a a, from rf a,
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begin subst b, unfold min_core, assumption end)
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| b (c::l) a h := or.elim h
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(suppose b ∈ c :: l, or.elim (eq_or_mem_of_mem_cons this)
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(suppose b = c,
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or.elim (em (R c a))
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(suppose R c a,
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assert R (min_core R l b) b, from min_core_lemma _ (or.inr rfl),
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begin unfold min_core, rewrite [if_pos `R c a`], subst c, assumption end)
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(suppose ¬ R c a,
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assert R a c, from or_resolve_right (to c a) this,
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assert R (min_core R l a) a, from min_core_lemma _ (or.inr rfl),
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assert R (min_core R l a) c, from tr this `R a c`,
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begin unfold min_core, rewrite [if_neg `¬ R c a`], subst b, exact `R (min_core R l a) c` end))
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(suppose b ∈ l,
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or.elim (em (R c a))
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(suppose R c a,
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assert R (min_core R l c) b, from min_core_lemma _ (or.inl `b ∈ l`),
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begin unfold min_core, rewrite [if_pos `R c a`], assumption end)
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(suppose ¬ R c a,
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assert R (min_core R l a) b, from min_core_lemma _ (or.inl `b ∈ l`),
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begin unfold min_core, rewrite [if_neg `¬ R c a`], assumption end)))
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(suppose b = a,
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assert R (min_core R l a) b, from min_core_lemma _ (or.inr this),
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or.elim (em (R c a))
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(suppose R c a,
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assert R (min_core R l c) c, from min_core_lemma _ (or.inr rfl),
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assert R (min_core R l c) a, from tr this `R c a`,
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begin unfold min_core, rewrite [if_pos `R c a`], subst b, exact `R (min_core R l c) a` end)
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(suppose ¬ R c a, begin unfold min_core, rewrite [if_neg `¬ R c a`], assumption end))
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lemma min_core_le_of_mem {b : A} {l : list A} (a : A) : b ∈ l → R (min_core R l a) b :=
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assume h : b ∈ l, min_core_lemma to tr rf a (or.inl h)
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lemma min_core_le {l : list A} (a : A) : R (min_core R l a) a :=
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min_core_lemma to tr rf a (or.inr rfl)
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lemma min_lemma : ∀ {a : A} {l} (h : l ≠ nil), all l (R (min R l h))
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| a [] h := absurd rfl h
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| a (b::l) h :=
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all_of_forall (take x, suppose x ∈ b::l,
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or.elim (eq_or_mem_of_mem_cons this)
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(suppose x = b,
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assert R (min_core R l b) b, from min_core_le to tr rf b,
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begin subst x, unfold min, assumption end)
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(suppose x ∈ l,
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assert R (min_core R l b) x, from min_core_le_of_mem to tr rf _ this,
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begin unfold min, assumption end))
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variable (R)
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lemma min_core_mem : ∀ l a, min_core R l a ∈ l ∨ min_core R l a = a
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| [] a := or.inr rfl
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| (b::l) a := or.elim (em (R b a))
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(suppose R b a,
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begin
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unfold min_core, rewrite [if_pos `R b a`],
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apply or.elim (min_core_mem l b),
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suppose min_core R l b ∈ l, or.inl (mem_cons_of_mem _ this),
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suppose min_core R l b = b, by rewrite this; exact or.inl !mem_cons
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end)
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(suppose ¬ R b a,
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begin
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unfold min_core, rewrite [if_neg `¬ R b a`],
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apply or.elim (min_core_mem l a),
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suppose min_core R l a ∈ l, or.inl (mem_cons_of_mem _ this),
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suppose min_core R l a = a, or.inr this
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end)
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lemma min_mem : ∀ (l : list A) (h : l ≠ nil), min R l h ∈ l
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| [] h := absurd rfl h
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| (a::l) h :=
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begin
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unfold min,
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apply or.elim (min_core_mem R l a),
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suppose min_core R l a ∈ l, mem_cons_of_mem _ this,
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suppose min_core R l a = a, by rewrite this; apply mem_cons
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end
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omit decR
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private lemma ne_nil {l : list A} {n : nat} : length l = succ n → l ≠ nil :=
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assume h₁ h₂, by rewrite h₂ at h₁; contradiction
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include decR
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lemma sort_aux_lemma {l n} (h : length l = succ n) : length (erase (min R l (ne_nil h)) l) = n :=
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have min R l _ ∈ l, from min_mem R l (ne_nil h),
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assert length (erase (min R l _) l) = pred (length l), from length_erase_of_mem this,
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by rewrite h at this; exact this
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definition sort_aux : Π (n : nat) (l : list A), length l = n → list A
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| 0 l h := []
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| (succ n) l h :=
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let m := min R l (ne_nil h) in
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let l₁ := erase m l in
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m :: sort_aux n l₁ (sort_aux_lemma R h)
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definition sort (l : list A) : list A :=
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sort_aux R (length l) l rfl
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open perm
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lemma sort_aux_perm : ∀ {n : nat} {l : list A} (h : length l = n), sort_aux R n l h ~ l
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| 0 l h := by rewrite [↑sort_aux, eq_nil_of_length_eq_zero h]
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| (succ n) l h :=
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let m := min R l (ne_nil h) in
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assert leq : length (erase m l) = n, from sort_aux_lemma R h,
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calc m :: sort_aux R n (erase m l) leq
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~ m :: erase m l : perm.skip m (sort_aux_perm leq)
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... ~ l : perm_erase (min_mem _ _ _)
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lemma sort_perm (l : list A) : sort R l ~ l :=
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sort_aux_perm R rfl
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end list
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