------------------------------------------------------------------------
-- The Agda standard library
--
-- Directed acyclic multigraphs
------------------------------------------------------------------------

-- A representation of DAGs, based on the idea underlying Martin
-- Erwig's FGL. Note that this representation does not aim to be
-- efficient.

{-# OPTIONS --cubical-compatible --safe #-}

module Data.Graph.Acyclic where

open import Level using (_⊔_)
open import Data.Nat.Base as ℕ using (ℕ; zero; suc; _<′_)
open import Data.Nat.Induction using (<′-rec; <′-Rec)
import Data.Nat.Properties as ℕ
open import Data.Fin as Fin
  using (Fin; Fin′; zero; suc; #_; toℕ; _≟_; opposite) renaming (_ℕ-ℕ_ to _-_)
import Data.Fin.Properties as Fin
open import Data.Product.Base as Prod using (∃; _×_; _,_)
open import Data.Maybe.Base as Maybe using (Maybe; nothing; just; decToMaybe)
open import Data.Empty
open import Data.Unit.Base using (⊤; tt)
open import Data.Vec.Base as Vec using (Vec; []; _∷_)
open import Data.List.Base as List using (List; []; _∷_)
open import Function.Base using (_$_; _∘′_; _∘_; id)
open import Relation.Nullary
open import Relation.Binary.PropositionalEquality.Core using (_≡_; refl)

------------------------------------------------------------------------
-- A lemma

private

  lemma : ∀ n (i : Fin n) → n - suc i <′ n
  lemma (suc n) i  = ℕ.≤⇒≤′ $ ℕ.s≤s $ Fin.nℕ-ℕi≤n n i

------------------------------------------------------------------------
-- Node contexts

record Context {ℓ e} (Node : Set ℓ) (Edge : Set e) (n : ℕ) : Set (ℓ ⊔ e) where
  constructor context
  field
    label      : Node
    successors : List (Edge × Fin n)

open Context public

-- Map for contexts.

module _ {ℓ₁ e₁} {N₁ : Set ℓ₁} {E₁ : Set e₁}
         {ℓ₂ e₂} {N₂ : Set ℓ₂} {E₂ : Set e₂} where

  cmap : ∀ {n} → (N₁ → N₂) → (List (E₁ × Fin n) → List (E₂ × Fin n)) →
         Context N₁ E₁ n → Context N₂ E₂ n
  cmap f g c = context (f (label c)) (g (successors c))

------------------------------------------------------------------------
-- Graphs

infixr  _&_

-- The DAGs are indexed on the number of nodes.

data Graph {ℓ e} (Node : Set ℓ) (Edge : Set e) : ℕ → Set (ℓ ⊔ e) where
  ∅   : Graph Node Edge 
  _&_ : ∀ {n} (c : Context Node Edge n) (g : Graph Node Edge n) →
        Graph Node Edge (suc n)

private

  example : Graph ℕ ℕ 
  example = context  [] &
            context  (( , # ) ∷ ( , # ) ∷ []) &
            context  (( , # ) ∷ []) &
            context  [] &
            context  [] &
            ∅

------------------------------------------------------------------------
-- Higher-order functions

module _ {ℓ e} {N : Set ℓ} {E : Set e} {t} where

-- "Fold right".

  foldr : (T : ℕ → Set t) →
          (∀ {n} → Context N E n → T n → T (suc n)) →
          T  →
          ∀ {m} → Graph N E m → T m
  foldr T _∙_ x ∅       = x
  foldr T _∙_ x (c & g) = c ∙ foldr T _∙_ x g

-- "Fold left".

  foldl : ∀ {n} (T : ℕ → Set t) →
          ((i : Fin n) → T (toℕ i) → Context N E (n - suc i) →
          T (suc (toℕ i))) →
          T  →
          Graph N E n → T n
  foldl T f x ∅       = x
  foldl T f x (c & g) = foldl (T ∘′ suc) (f ∘ suc) (f zero x c) g


module _ {ℓ₁ e₁} {N₁ : Set ℓ₁} {E₁ : Set e₁}
         {ℓ₂ e₂} {N₂ : Set ℓ₂} {E₂ : Set e₂} where

-- Maps over node contexts.

  map : (∀ {n} → Context N₁ E₁ n → Context N₂ E₂ n) →
        ∀ {n} → Graph N₁ E₁ n → Graph N₂ E₂ n
  map f = foldr _ (λ c → f c &_) ∅

-- Maps over node labels.

nmap : ∀ {ℓ₁ ℓ₂ e} {N₁ : Set ℓ₁} {N₂ : Set ℓ₂} {E : Set e} →
       ∀ {n} → (N₁ → N₂) → Graph N₁ E n → Graph N₂ E n
nmap f = map (cmap f id)

-- Maps over edge labels.

emap : ∀ {ℓ e₁ e₂} {N : Set ℓ} {E₁ : Set e₁} {E₂ : Set e₂} →
       ∀ {n} → (E₁ → E₂) → Graph N E₁ n → Graph N E₂ n
emap f = map (cmap id (List.map (Prod.map f id)))

-- Zips two graphs with the same number of nodes. Note that one of the
-- graphs has a type which restricts it to be completely disconnected.

zipWith : ∀ {ℓ₁ ℓ₂ ℓ e} {N₁ : Set ℓ₁} {N₂ : Set ℓ₂} {N : Set ℓ} {E : Set e} →
          ∀ {n} → (N₁ → N₂ → N) → Graph N₁ ⊥ n → Graph N₂ E n → Graph N E n
zipWith _∙_ ∅         ∅         = ∅
zipWith _∙_ (c₁ & g₁) (c₂ & g₂) =
  context (label c₁ ∙ label c₂) (successors c₂) & zipWith _∙_ g₁ g₂

------------------------------------------------------------------------
-- Specific graphs

-- A completeley disconnected graph.

disconnected : ∀ n → Graph ⊤ ⊥ n
disconnected zero    = ∅
disconnected (suc n) = context tt [] & disconnected n

-- A complete graph.

complete : ∀ n → Graph ⊤ ⊤ n
complete zero    = ∅
complete (suc n) =
  context tt (List.map (tt ,_) $ Vec.toList (Vec.allFin n)) &
  complete n

------------------------------------------------------------------------
-- Queries

module _ {ℓ e} {N : Set ℓ} {E : Set e} where

-- The top-most context.

  head : ∀ {n} → Graph N E (suc n) → Context N E n
  head (c & g) = c

-- The remaining graph.

  tail : ∀ {n} → Graph N E (suc n) → Graph N E n
  tail (c & g) = g

-- Finds the context and remaining graph corresponding to a given node
-- index.

  infix  _[_]

  _[_] : ∀ {n} → Graph N E n → (i : Fin n) → Graph N E (suc (n - suc i))
  (c & g) [ zero ]  = c & g
  (c & g) [ suc i ] = g [ i ]

-- The nodes of the graph (node number relative to "topmost" node ×
-- node label).

  nodes : ∀ {n} → Graph N E n → Vec (Fin n × N) n
  nodes = Vec.zip (Vec.allFin _) ∘
          foldr (Vec N) (λ c → label c ∷_) []

private

  test-nodes : nodes example ≡ (#  , ) ∷ (#  , ) ∷ (#  , ) ∷
                               (#  , ) ∷ (#  , ) ∷ []
  test-nodes = refl


module _ {ℓ e} {N : Set ℓ} {E : Set e} where

-- Topological sort. Gives a vector where earlier nodes are never
-- successors of later nodes.

  topSort : ∀ {n} → Graph N E n → Vec (Fin n × N) n
  topSort = nodes

-- The edges of the graph (predecessor × edge label × successor).
--
-- The predecessor is a node number relative to the "topmost" node in
-- the graph, and the successor is a node number relative to the
-- predecessor.

  edges : ∀ {n} → Graph N E n → List (∃ λ i → E × Fin (n - suc i))
  edges {n} =
    foldl (λ _ → List (∃ λ i → E × Fin (n - suc i)))
          (λ i es c → es List.++ List.map (i ,_) (successors c))
          []

private

  test-edges : edges example ≡ (#  ,  , # ) ∷ (#  ,  , # ) ∷
                               (#  ,  , # ) ∷ []
  test-edges = refl

-- The successors of a given node i (edge label × node number relative
-- to i).

sucs : ∀ {ℓ e} {N : Set ℓ} {E : Set e} →
       ∀ {n} → Graph N E n → (i : Fin n) → List (E × Fin (n - suc i))
sucs g i = successors $ head (g [ i ])

private

  test-sucs : sucs example (# ) ≡ ( , # ) ∷ ( , # ) ∷ []
  test-sucs = refl

-- The predecessors of a given node i (node number relative to i ×
-- edge label).

preds : ∀ {ℓ e} {N : Set ℓ} {E : Set e} →
        ∀ {n} → Graph N E n → (i : Fin n) → List (Fin′ i × E)
preds g       zero    = []
preds (c & g) (suc i) =
  List._++_ (List.mapMaybe (p i) $ successors c)
            (List.map (Prod.map suc id) $ preds g i)
  where
  p : ∀ {e} {E : Set e} {n} (i : Fin n) → E × Fin n → Maybe (Fin′ (suc i) × E)
  p i (e , j) = Maybe.map (λ{ refl → zero , e }) (decToMaybe (i ≟ j))

private

  test-preds : preds example (# ) ≡
               (#  , ) ∷ (#  , ) ∷ (#  , ) ∷ []
  test-preds = refl

------------------------------------------------------------------------
-- Operations

-- Weakens a node label.

weaken : ∀ {n} {i : Fin n} → Fin (n - suc i) → Fin n
weaken {n} {i} j = Fin.inject≤ j (Fin.nℕ-ℕi≤n n (suc i))

-- Labels each node with its node number.

number : ∀ {ℓ e} {N : Set ℓ} {E : Set e} →
         ∀ {n} → Graph N E n → Graph (Fin n × N) E n
number {N = N} {E} =
  foldr (λ n → Graph (Fin n × N) E n)
        (λ c g → cmap (zero ,_) id c & nmap (Prod.map suc id) g)
        ∅

private

  test-number : number example ≡
                (context (#  , ) [] &
                 context (#  , ) (( , # ) ∷ ( , # ) ∷ []) &
                 context (#  , ) (( , # ) ∷ []) &
                 context (#  , ) [] &
                 context (#  , ) [] &
                 ∅)
  test-number = refl

-- Reverses all the edges in the graph.

reverse : ∀ {ℓ e} {N : Set ℓ} {E : Set e} →
          ∀ {n} → Graph N E n → Graph N E n
reverse {N = N} {E} g =
  foldl (Graph N E)
        (λ i g′ c →
           context (label c)
                   (List.map (Prod.swap ∘ Prod.map opposite id) $
                             preds g i)
           & g′)
        ∅ g

private

  test-reverse : reverse (reverse example) ≡ example
  test-reverse = refl

------------------------------------------------------------------------
-- Views

-- Expands the subgraph induced by a given node into a tree (thus
-- losing all sharing).

data Tree {ℓ e} (N : Set ℓ) (E : Set e) : Set (ℓ ⊔ e) where
  node : (label : N) (successors : List (E × Tree N E)) → Tree N E

module _ {ℓ e} {N : Set ℓ} {E : Set e} where

  toTree : ∀ {n} → Graph N E n → Fin n → Tree N E
  toTree g i = <′-rec Pred expand _ (g [ i ])
    where
    Pred = λ n → Graph N E (suc n) → Tree N E

    expand : (n : ℕ) → <′-Rec Pred n → Pred n
    expand n rec (c & g) =
      node (label c)
           (List.map
              (Prod.map id (λ i → rec (lemma n i) (g [ i ])))
              (successors c))

-- Performs the toTree expansion once for each node.

  toForest : ∀ {n} → Graph N E n → Vec (Tree N E) n
  toForest g = Vec.map (toTree g) (Vec.allFin _)

private

  test-toForest : toForest example ≡
                    let n3 = node  [] in
                    node  [] ∷
                    node  (( , n3) ∷ ( , n3) ∷ []) ∷
                    node  (( , n3) ∷ []) ∷
                    node  [] ∷
                    node  [] ∷
                    []
  test-toForest = refl