module Data.String.Iterator

import public Data.List.Lazy

%default total

-- Backend-dependent string iteration type,
-- parameterised by the string that it iterates over.
--
-- Beware: the index is checked only up to definitional equality.
-- In theory, you could run `decEq` on two strings
-- with the same content but allocated in different memory locations
-- and use the obtained Refl to coerce iterators between them.
--
-- The strictly correct solution is to make the iterators independent
-- from the exact memory location of the string given to `uncons`.
-- (For example, byte offsets satisfy this requirement.)
export
data StringIterator : String -> Type where [external]

-- This function is private
-- to avoid subverting the linearity guarantees of withString.
%foreign
  "scheme:blodwen-string-iterator-new"
private
fromString : (str : String) -> StringIterator str

-- This function uses a linear string iterator
-- so that backends can use mutating iterators.
export
withString : (str : String) -> ((1 it : StringIterator str) -> a) -> a
withString str f = f (fromString str)

-- We use a custom data type instead of Maybe (Char, StringIterator)
-- to remove one level of pointer indirection
-- in every iteration of something that's likely to be a hot loop,
-- and to avoid one allocation per character.
--
-- The Char field of Character is unrestricted for flexibility.
public export
data UnconsResult : String -> Type where
  EOF : UnconsResult str
  Character : (c : Char) -> (1 it : StringIterator str) -> UnconsResult str

-- We pass the whole string to the uncons function
-- to avoid yet another allocation per character
-- because for many backends, StringIterator can be simply an integer
-- (e.g. byte offset into an UTF-8 string).
%foreign
  "scheme:blodwen-string-iterator-next"
export
uncons : (str : String) -> (1 it : StringIterator str) -> UnconsResult str

export
foldl : (accTy -> Char -> accTy) -> accTy -> String -> accTy
foldl op acc str = withString str (loop acc)
  where
    loop : accTy -> (1 it : StringIterator str) -> accTy
    loop acc it =
      case uncons str it of
        EOF => acc
        Character c it' => loop (acc `op` c) (assert_smaller it it')

export
unpack : String -> LazyList Char
unpack str = withString str unpack'
  where
    unpack' : (1 it : StringIterator str) -> LazyList Char
    unpack' it =
      case uncons str it of
        EOF => []
        Character c it' => c :: Delay (unpack' $ assert_smaller it it')