------------------------------------------------------------------------
-- The Agda standard library
--
-- Vectors
------------------------------------------------------------------------

module Data.Vec where

open import Category.Applicative
open import Data.Nat
open import Data.Fin using (Fin; zero; suc)
open import Data.List as List using (List)
open import Data.Product as Prod using (; ∃₂; _×_; _,_)
open import Function
open import Relation.Binary.PropositionalEquality using (_≡_; refl)

------------------------------------------------------------------------
-- Types

infixr 5 _∷_

data Vec {a} (A : Set a) :   Set a where
  []  : Vec A zero
  _∷_ :  {n} (x : A) (xs : Vec A n)  Vec A (suc n)

infix 4 _∈_

data _∈_ {a} {A : Set a} : A  {n : }  Vec A n  Set a where
  here  :  {n} {x}   {xs : Vec A n}  x  x  xs
  there :  {n} {x y} {xs : Vec A n} (x∈xs : x  xs)  x  y  xs

infix 4 _[_]=_

data _[_]=_ {a} {A : Set a} :
            {n : }  Vec A n  Fin n  A  Set a where
  here  :  {n}     {x}   {xs : Vec A n}  x  xs [ zero ]= x
  there :  {n} {i} {x y} {xs : Vec A n}
          (xs[i]=x : xs [ i ]= x)  y  xs [ suc i ]= x

------------------------------------------------------------------------
-- Some operations

head :  {a n} {A : Set a}  Vec A (1 + n)  A
head (x  xs) = x

tail :  {a n} {A : Set a}  Vec A (1 + n)  Vec A n
tail (x  xs) = xs

[_] :  {a} {A : Set a}  A  Vec A 1
[ x ] = x  []

infixr 5 _++_

_++_ :  {a m n} {A : Set a}  Vec A m  Vec A n  Vec A (m + n)
[]       ++ ys = ys
(x  xs) ++ ys = x  (xs ++ ys)

infixl 4 _⊛_

_⊛_ :  {a b n} {A : Set a} {B : Set b} 
      Vec (A  B) n  Vec A n  Vec B n
[]        _        = []
(f  fs)  (x  xs) = f x  (fs  xs)

replicate :  {a n} {A : Set a}  A  Vec A n
replicate {n = zero}  x = []
replicate {n = suc n} x = x  replicate x

applicative :  {a n}  RawApplicative  (A : Set a)  Vec A n)
applicative = record
  { pure = replicate
  ; _⊛_  = _⊛_
  }

map :  {a b n} {A : Set a} {B : Set b} 
      (A  B)  Vec A n  Vec B n
map f xs = replicate f  xs

zipWith :  {a b c n} {A : Set a} {B : Set b} {C : Set c} 
          (A  B  C)  Vec A n  Vec B n  Vec C n
zipWith _⊕_ xs ys = replicate _⊕_  xs  ys

zip :  {a b n} {A : Set a} {B : Set b} 
      Vec A n  Vec B n  Vec (A × B) n
zip = zipWith _,_

unzip :  {a b n} {A : Set a} {B : Set b} 
        Vec (A × B) n  Vec A n × Vec B n
unzip []              = [] , []
unzip ((x , y)  xys) = Prod.map (_∷_ x) (_∷_ y) (unzip xys)

foldr :  {a b} {A : Set a} (B :   Set b) {m} 
        (∀ {n}  A  B n  B (suc n)) 
        B zero 
        Vec A m  B m
foldr b _⊕_ n []       = n
foldr b _⊕_ n (x  xs) = x  foldr b _⊕_ n xs

foldr₁ :  {a} {A : Set a} {m} 
         (A  A  A)  Vec A (suc m)  A
foldr₁ _⊕_ (x  [])     = x
foldr₁ _⊕_ (x  y  ys) = x  foldr₁ _⊕_ (y  ys)

foldl :  {a b} {A : Set a} (B :   Set b) {m} 
        (∀ {n}  B n  A  B (suc n)) 
        B zero 
        Vec A m  B m
foldl b _⊕_ n []       = n
foldl b _⊕_ n (x  xs) = foldl  n  b (suc n)) _⊕_ (n  x) xs

foldl₁ :  {a} {A : Set a} {m} 
         (A  A  A)  Vec A (suc m)  A
foldl₁ _⊕_ (x  xs) = foldl _ _⊕_ x xs

concat :  {a m n} {A : Set a} 
         Vec (Vec A m) n  Vec A (n * m)
concat []         = []
concat (xs  xss) = xs ++ concat xss

splitAt :  {a} {A : Set a} m {n} (xs : Vec A (m + n)) 
          ∃₂ λ (ys : Vec A m) (zs : Vec A n)  xs  ys ++ zs
splitAt zero    xs                = ([] , xs , refl)
splitAt (suc m) (x  xs)          with splitAt m xs
splitAt (suc m) (x  .(ys ++ zs)) | (ys , zs , refl) =
  ((x  ys) , zs , refl)

take :  {a} {A : Set a} m {n}  Vec A (m + n)  Vec A m
take m xs          with splitAt m xs
take m .(ys ++ zs) | (ys , zs , refl) = ys

drop :  {a} {A : Set a} m {n}  Vec A (m + n)  Vec A n
drop m xs          with splitAt m xs
drop m .(ys ++ zs) | (ys , zs , refl) = zs

group :  {a} {A : Set a} n k (xs : Vec A (n * k)) 
         λ (xss : Vec (Vec A k) n)  xs  concat xss
group zero    k []                  = ([] , refl)
group (suc n) k xs                  with splitAt k xs
group (suc n) k .(ys ++ zs)         | (ys , zs , refl) with group n k zs
group (suc n) k .(ys ++ concat zss) | (ys , ._ , refl) | (zss , refl) =
  ((ys  zss) , refl)

-- Splits a vector into two "halves".

split :  {a n} {A : Set a}  Vec A n  Vec A  n /2⌉ × Vec A  n /2⌋
split []           = ([]     , [])
split (x  [])     = (x  [] , [])
split (x  y  xs) = Prod.map (_∷_ x) (_∷_ y) (split xs)

reverse :  {a n} {A : Set a}  Vec A n  Vec A n
reverse {A = A} = foldl (Vec A)  rev x  x  rev) []

sum :  {n}  Vec  n  
sum = foldr _ _+_ 0

toList :  {a n} {A : Set a}  Vec A n  List A
toList []       = List.[]
toList (x  xs) = List._∷_ x (toList xs)

fromList :  {a} {A : Set a}  (xs : List A)  Vec A (List.length xs)
fromList List.[]         = []
fromList (List._∷_ x xs) = x  fromList xs

-- Snoc.

infixl 5 _∷ʳ_

_∷ʳ_ :  {a n} {A : Set a}  Vec A n  A  Vec A (1 + n)
[]       ∷ʳ y = [ y ]
(x  xs) ∷ʳ y = x  (xs ∷ʳ y)

initLast :  {a n} {A : Set a} (xs : Vec A (1 + n)) 
           ∃₂ λ (ys : Vec A n) (y : A)  xs  ys ∷ʳ y
initLast {n = zero}  (x  [])         = ([] , x , refl)
initLast {n = suc n} (x  xs)         with initLast xs
initLast {n = suc n} (x  .(ys ∷ʳ y)) | (ys , y , refl) =
  ((x  ys) , y , refl)

init :  {a n} {A : Set a}  Vec A (1 + n)  Vec A n
init xs         with initLast xs
init .(ys ∷ʳ y) | (ys , y , refl) = ys

last :  {a n} {A : Set a}  Vec A (1 + n)  A
last xs         with initLast xs
last .(ys ∷ʳ y) | (ys , y , refl) = y

infixl 1 _>>=_

_>>=_ :  {a b m n} {A : Set a} {B : Set b} 
        Vec A m  (A  Vec B n)  Vec B (m * n)
xs >>= f = concat (map f xs)

infixl 4 _⊛*_

_⊛*_ :  {a b m n} {A : Set a} {B : Set b} 
       Vec (A  B) m  Vec A n  Vec B (m * n)
fs ⊛* xs = fs >>= λ f  map f xs

-- Interleaves the two vectors.

infixr 5 _⋎_

_⋎_ :  {a m n} {A : Set a} 
      Vec A m  Vec A n  Vec A (m +⋎ n)
[]        ys = ys
(x  xs)  ys = x  (ys  xs)

-- A lookup function.

lookup :  {a n} {A : Set a}  Fin n  Vec A n  A
lookup zero    (x  xs) = x
lookup (suc i) (x  xs) = lookup i xs

-- An inverse of flip lookup.

tabulate :  {n a} {A : Set a}  (Fin n  A)  Vec A n
tabulate {zero}  f = []
tabulate {suc n} f = f zero  tabulate (f  suc)

-- Update.

infixl 6 _[_]≔_

_[_]≔_ :  {a n} {A : Set a}  Vec A n  Fin n  A  Vec A n
[]       [ ()    ]≔ y
(x  xs) [ zero  ]≔ y = y  xs
(x  xs) [ suc i ]≔ y = x  xs [ i ]≔ y

-- Generates a vector containing all elements in Fin n. This function
-- is not placed in Data.Fin because Data.Vec depends on Data.Fin.
--
-- The implementation was suggested by Conor McBride ("Fwd: how to
-- count 0..n-1", http://thread.gmane.org/gmane.comp.lang.agda/2554).

allFin :  n  Vec (Fin n) n
allFin _ = tabulate id