------------------------------------------------------------------------ -- The Agda standard library -- -- A data structure which keeps track of an upper bound on the number -- of elements /not/ in a given list ------------------------------------------------------------------------ import Level open import Relation.Binary module Data.List.Countdown (D : DecSetoid Level.zero Level.zero) where open import Data.Empty open import Data.Fin using (Fin; zero; suc) open import Function open import Function.Equality using (_⟨$⟩_) open import Function.Injection using (Injection; module Injection) open import Data.List open import Data.List.Any as Any using (here; there) open import Data.Nat using (ℕ; zero; suc) open import Data.Product open import Data.Sum open import Relation.Nullary open import Relation.Binary.PropositionalEquality as PropEq using (_≡_; _≢_; refl; cong) open PropEq.≡-Reasoning private open module D = DecSetoid D hiding (refl) renaming (Carrier to Elem) open Any.Membership D.setoid ------------------------------------------------------------------------ -- Helper functions private drop-suc : ∀ {n} {i j : Fin n} → _≡_ {A = Fin (suc n)} (suc i) (suc j) → i ≡ j drop-suc refl = refl drop-inj₂ : ∀ {A B : Set} {x y} → inj₂ {A = A} {B = B} x ≡ inj₂ y → x ≡ y drop-inj₂ refl = refl -- The /first/ occurrence of x in xs. first-occurrence : ∀ {xs} x → x ∈ xs → x ∈ xs first-occurrence x (here x≈y) = here x≈y first-occurrence x (there {x = y} x∈xs) with x ≟ y ... | yes x≈y = here x≈y ... | no _ = there $ first-occurrence x x∈xs -- The index of the first occurrence of x in xs. first-index : ∀ {xs} x → x ∈ xs → Fin (length xs) first-index x x∈xs = Any.index $ first-occurrence x x∈xs -- first-index preserves equality of its first argument. first-index-cong : ∀ {x₁ x₂ xs} (x₁∈xs : x₁ ∈ xs) (x₂∈xs : x₂ ∈ xs) → x₁ ≈ x₂ → first-index x₁ x₁∈xs ≡ first-index x₂ x₂∈xs first-index-cong {x₁} {x₂} x₁∈xs x₂∈xs x₁≈x₂ = helper x₁∈xs x₂∈xs where helper : ∀ {xs} (x₁∈xs : x₁ ∈ xs) (x₂∈xs : x₂ ∈ xs) → first-index x₁ x₁∈xs ≡ first-index x₂ x₂∈xs helper (here x₁≈x) (here x₂≈x) = refl helper (here x₁≈x) (there {x = x} x₂∈xs) with x₂ ≟ x ... | yes x₂≈x = refl ... | no x₂≉x = ⊥-elim (x₂≉x (trans (sym x₁≈x₂) x₁≈x)) helper (there {x = x} x₁∈xs) (here x₂≈x) with x₁ ≟ x ... | yes x₁≈x = refl ... | no x₁≉x = ⊥-elim (x₁≉x (trans x₁≈x₂ x₂≈x)) helper (there {x = x} x₁∈xs) (there x₂∈xs) with x₁ ≟ x | x₂ ≟ x ... | yes x₁≈x | yes x₂≈x = refl ... | yes x₁≈x | no x₂≉x = ⊥-elim (x₂≉x (trans (sym x₁≈x₂) x₁≈x)) ... | no x₁≉x | yes x₂≈x = ⊥-elim (x₁≉x (trans x₁≈x₂ x₂≈x)) ... | no x₁≉x | no x₂≉x = cong suc $ helper x₁∈xs x₂∈xs -- first-index is injective in its first argument. first-index-injective : ∀ {x₁ x₂ xs} (x₁∈xs : x₁ ∈ xs) (x₂∈xs : x₂ ∈ xs) → first-index x₁ x₁∈xs ≡ first-index x₂ x₂∈xs → x₁ ≈ x₂ first-index-injective {x₁} {x₂} = helper where helper : ∀ {xs} (x₁∈xs : x₁ ∈ xs) (x₂∈xs : x₂ ∈ xs) → first-index x₁ x₁∈xs ≡ first-index x₂ x₂∈xs → x₁ ≈ x₂ helper (here x₁≈x) (here x₂≈x) _ = trans x₁≈x (sym x₂≈x) helper (here x₁≈x) (there {x = x} x₂∈xs) _ with x₂ ≟ x helper (here x₁≈x) (there {x = x} x₂∈xs) _ | yes x₂≈x = trans x₁≈x (sym x₂≈x) helper (here x₁≈x) (there {x = x} x₂∈xs) () | no x₂≉x helper (there {x = x} x₁∈xs) (here x₂≈x) _ with x₁ ≟ x helper (there {x = x} x₁∈xs) (here x₂≈x) _ | yes x₁≈x = trans x₁≈x (sym x₂≈x) helper (there {x = x} x₁∈xs) (here x₂≈x) () | no x₁≉x helper (there {x = x} x₁∈xs) (there x₂∈xs) _ with x₁ ≟ x | x₂ ≟ x helper (there {x = x} x₁∈xs) (there x₂∈xs) _ | yes x₁≈x | yes x₂≈x = trans x₁≈x (sym x₂≈x) helper (there {x = x} x₁∈xs) (there x₂∈xs) () | yes x₁≈x | no x₂≉x helper (there {x = x} x₁∈xs) (there x₂∈xs) () | no x₁≉x | yes x₂≈x helper (there {x = x} x₁∈xs) (there x₂∈xs) eq | no x₁≉x | no x₂≉x = helper x₁∈xs x₂∈xs (drop-suc eq) -- If there are at least two elements in Fin (suc n), then Fin n is -- inhabited. This is a variant of the thick function from Conor -- McBride's "First-order unification by structural recursion". thick : ∀ {n} (i j : Fin (suc n)) → i ≢ j → Fin n thick zero zero i≢j = ⊥-elim (i≢j refl) thick zero (suc j) _ = j thick {zero} (suc ()) _ _ thick {suc n} (suc i) zero _ = zero thick {suc n} (suc i) (suc j) i≢j = suc (thick i j (i≢j ∘ cong suc)) -- thick i is injective in one of its arguments. thick-injective : ∀ {n} (i j k : Fin (suc n)) {i≢j : i ≢ j} {i≢k : i ≢ k} → thick i j i≢j ≡ thick i k i≢k → j ≡ k thick-injective zero zero _ {i≢j = i≢j} _ = ⊥-elim (i≢j refl) thick-injective zero _ zero {i≢k = i≢k} _ = ⊥-elim (i≢k refl) thick-injective zero (suc j) (suc k) j≡k = cong suc j≡k thick-injective {zero} (suc ()) _ _ _ thick-injective {suc n} (suc i) zero zero _ = refl thick-injective {suc n} (suc i) zero (suc k) () thick-injective {suc n} (suc i) (suc j) zero () thick-injective {suc n} (suc i) (suc j) (suc k) {i≢j} {i≢k} eq = cong suc $ thick-injective i j k {i≢j ∘ cong suc} {i≢k ∘ cong suc} (drop-suc eq) ------------------------------------------------------------------------ -- The countdown data structure -- If counted ⊕ n is inhabited then there are at most n values of type -- Elem which are not members of counted (up to _≈_). You can read the -- symbol _⊕_ as partitioning Elem into two parts: counted and -- uncounted. infix 4 _⊕_ record _⊕_ (counted : List Elem) (n : ℕ) : Set where field -- An element can be of two kinds: -- ⑴ It is provably in counted. -- ⑵ It is one of at most n elements which may or may not be in -- counted. The "at most n" part is guaranteed by the field -- "injective". kind : ∀ x → x ∈ counted ⊎ Fin n injective : ∀ {x y i} → kind x ≡ inj₂ i → kind y ≡ inj₂ i → x ≈ y -- A countdown can be initialised by proving that Elem is finite. empty : ∀ {n} → Injection D.setoid (PropEq.setoid (Fin n)) → [] ⊕ n empty inj = record { kind = inj₂ ∘ _⟨$⟩_ to ; injective = λ {x} {y} {i} eq₁ eq₂ → injective (begin to ⟨$⟩ x ≡⟨ drop-inj₂ eq₁ ⟩ i ≡⟨ PropEq.sym $ drop-inj₂ eq₂ ⟩ to ⟨$⟩ y ∎) } where open Injection inj -- A countdown can also be initialised by proving that Elem is finite. emptyFromList : (counted : List Elem) → (∀ x → x ∈ counted) → [] ⊕ length counted emptyFromList counted complete = empty record { to = record { _⟨$⟩_ = λ x → first-index x (complete x) ; cong = first-index-cong (complete _) (complete _) } ; injective = first-index-injective (complete _) (complete _) } -- Finds out if an element has been counted yet. lookup : ∀ {counted n} → counted ⊕ n → ∀ x → Dec (x ∈ counted) lookup {counted} _ x = Any.any (_≟_ x) counted -- When no element remains to be counted all elements have been -- counted. lookup! : ∀ {counted} → counted ⊕ zero → ∀ x → x ∈ counted lookup! counted⊕0 x with _⊕_.kind counted⊕0 x ... | inj₁ x∈counted = x∈counted ... | inj₂ () private -- A variant of lookup!. lookup‼ : ∀ {m counted} → counted ⊕ m → ∀ x → x ∉ counted → ∃ λ n → m ≡ suc n lookup‼ {suc m} counted⊕n x x∉counted = (m , refl) lookup‼ {zero} counted⊕n x x∉counted = ⊥-elim (x∉counted $ lookup! counted⊕n x) -- Counts a previously uncounted element. insert : ∀ {counted n} → counted ⊕ suc n → ∀ x → x ∉ counted → x ∷ counted ⊕ n insert {counted} {n} counted⊕1+n x x∉counted = record { kind = kind′; injective = inj } where open _⊕_ counted⊕1+n helper : ∀ x y i {j} → kind x ≡ inj₂ i → kind y ≡ inj₂ j → i ≡ j → x ≈ y helper _ _ _ eq₁ eq₂ refl = injective eq₁ eq₂ kind′ : ∀ y → y ∈ x ∷ counted ⊎ Fin n kind′ y with y ≟ x | kind x | kind y | helper x y kind′ y | yes y≈x | _ | _ | _ = inj₁ (here y≈x) kind′ y | _ | inj₁ x∈counted | _ | _ = ⊥-elim (x∉counted x∈counted) kind′ y | _ | _ | inj₁ y∈counted | _ = inj₁ (there y∈counted) kind′ y | no y≉x | inj₂ i | inj₂ j | hlp = inj₂ (thick i j (y≉x ∘ sym ∘ hlp _ refl refl)) inj : ∀ {y z i} → kind′ y ≡ inj₂ i → kind′ z ≡ inj₂ i → y ≈ z inj {y} {z} eq₁ eq₂ with y ≟ x | z ≟ x | kind x | kind y | kind z | helper x y | helper x z | helper y z inj () _ | yes _ | _ | _ | _ | _ | _ | _ | _ inj _ () | _ | yes _ | _ | _ | _ | _ | _ | _ inj _ _ | no _ | no _ | inj₁ x∈counted | _ | _ | _ | _ | _ = ⊥-elim (x∉counted x∈counted) inj () _ | no _ | no _ | inj₂ _ | inj₁ _ | _ | _ | _ | _ inj _ () | no _ | no _ | inj₂ _ | _ | inj₁ _ | _ | _ | _ inj eq₁ eq₂ | no _ | no _ | inj₂ i | inj₂ _ | inj₂ _ | _ | _ | hlp = hlp _ refl refl $ thick-injective i _ _ $ PropEq.trans (drop-inj₂ eq₁) (PropEq.sym (drop-inj₂ eq₂)) -- Counts an element if it has not already been counted. lookupOrInsert : ∀ {counted m} → counted ⊕ m → ∀ x → x ∈ counted ⊎ ∃ λ n → m ≡ suc n × x ∷ counted ⊕ n lookupOrInsert counted⊕n x with lookup counted⊕n x ... | yes x∈counted = inj₁ x∈counted ... | no x∉counted with lookup‼ counted⊕n x x∉counted ... | (n , refl) = inj₂ (n , refl , insert counted⊕n x x∉counted)