------------------------------------------------------------------------
-- The Agda standard library
--
-- Lists with fast append
------------------------------------------------------------------------

{-# OPTIONS --without-K --safe #-}

module Data.DifferenceList where

open import Level using (Level)
open import Data.List.Base as List using (List)
open import Function
open import Data.Nat.Base

private
  variable
    a b : Level
    A : Set a
    B : Set b

------------------------------------------------------------------------
-- Type definition and list function lifting

DiffList : Set a  Set a
DiffList A = List A  List A

lift : (List A  List A)  (DiffList A  DiffList A)
lift f xs = λ k  f (xs k)

------------------------------------------------------------------------
-- Building difference lists

infixr 5 _∷_ _++_

[] : DiffList A
[] = λ k  k

_∷_ : A  DiffList A  DiffList A
_∷_ x = lift (x List.∷_)

[_] : A  DiffList A
[ x ] = x  []

_++_ : DiffList A  DiffList A  DiffList A
xs ++ ys = λ k  xs (ys k)

infixl 6 _∷ʳ_
_∷ʳ_ : DiffList A  A  DiffList A
xs ∷ʳ x = λ k  xs (x List.∷ k)

------------------------------------------------------------------------
-- Conversion back and forth with List

toList : DiffList A  List A
toList xs = xs List.[]

-- fromList xs is linear in the length of xs.

fromList : List A  DiffList A
fromList xs = λ k  xs  List._++_  k

------------------------------------------------------------------------
-- Transforming difference lists

-- It is OK to use List._++_ here, since map is linear in the length of
-- the list anyway.

map : (A  B)  DiffList A  DiffList B
map f xs = λ k  List.map f (toList xs)  List._++_  k

-- concat is linear in the length of the outer list.

concat : DiffList (DiffList A)  DiffList A
concat xs = concat′ (toList xs)
  where
  concat′ : List (DiffList A)  DiffList A
  concat′ = List.foldr _++_ []

take :   DiffList A  DiffList A
take n = lift (List.take n)

drop :   DiffList A  DiffList A
drop n = lift (List.drop n)