module Data.Fin.Permutation.Components where
open import Data.Fin
open import Data.Fin.Properties
open import Data.Nat as ℕ using (zero; suc; _∸_)
import Data.Nat.Properties as ℕₚ
open import Data.Product using (proj₂)
open import Relation.Nullary using (yes; no)
open import Relation.Binary.PropositionalEquality
open import Algebra.FunctionProperties using (Involutive)
open ≡-Reasoning
transpose : ∀ {n} → Fin n → Fin n → Fin n → Fin n
transpose i j k with k ≟ i
... | yes _ = j
... | no _ with k ≟ j
... | yes _ = i
... | no _ = k
reverse : ∀ {n} → Fin n → Fin n
reverse {zero} ()
reverse {suc n} i = inject≤ (n ℕ- i) (ℕₚ.n∸m≤n (toℕ i) (suc n))
transpose-inverse : ∀ {n} (i j : Fin n) {k} →
transpose i j (transpose j i k) ≡ k
transpose-inverse i j {k} with k ≟ j
... | yes p rewrite ≡-≟-identity _≟_ {a = i} refl = sym p
... | no ¬p with k ≟ i
transpose-inverse i j {k} | no ¬p | yes q with j ≟ i
... | yes r = trans r (sym q)
... | no ¬r rewrite ≡-≟-identity _≟_ {a = j} refl = sym q
transpose-inverse i j {k} | no ¬p | no ¬q
rewrite proj₂ (≢-≟-identity _≟_ ¬q)
| proj₂ (≢-≟-identity _≟_ ¬p) = refl
reverse-prop : ∀ {n} → (i : Fin n) → toℕ (reverse i) ≡ n ∸ suc (toℕ i)
reverse-prop {zero} ()
reverse-prop {suc n} i = begin
toℕ (inject≤ (n ℕ- i) _) ≡⟨ toℕ-inject≤ _ _ ⟩
toℕ (n ℕ- i) ≡⟨ toℕ‿ℕ- n i ⟩
n ∸ toℕ i ∎
reverse-involutive : ∀ {n} → Involutive _≡_ (reverse {n})
reverse-involutive {zero} ()
reverse-involutive {suc n} i = toℕ-injective (begin
toℕ (reverse (reverse i)) ≡⟨ reverse-prop (reverse i) ⟩
n ∸ (toℕ (reverse i)) ≡⟨ cong (n ∸_) (reverse-prop i) ⟩
n ∸ (n ∸ (toℕ i)) ≡⟨ ℕₚ.m∸[m∸n]≡n (ℕₚ.≤-pred (toℕ<n i)) ⟩
toℕ i ∎)
reverse-suc : ∀ {n}{i : Fin n} → toℕ (reverse (suc i)) ≡ toℕ (reverse i)
reverse-suc {n} {i} = begin
toℕ (reverse (suc i)) ≡⟨ reverse-prop (suc i) ⟩
suc n ∸ suc (toℕ (suc i)) ≡⟨⟩
n ∸ toℕ (suc i) ≡⟨⟩
n ∸ suc (toℕ i) ≡⟨ sym (reverse-prop i) ⟩
toℕ (reverse i) ∎