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string_ parser: check equality char by char to avoid unecessary IO #224

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string_ parser: check equality char by char to avoid unecessary IO #224

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66 changes: 41 additions & 25 deletions lib/angstrom.ml
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Original file line number Diff line number Diff line change
Expand Up @@ -183,13 +183,6 @@ let unsafe_apply len ~f =
succ input (pos + len) more (Input.apply input pos len ~f)
}

let unsafe_apply_opt len ~f =
{ run = fun input pos more fail succ ->
match Input.apply input pos len ~f with
| Error e -> fail input pos more [] e
| Ok x -> succ input (pos + len) more x
}

let ensure n p =
{ run = fun input pos more fail succ ->
if pos + n <= Input.length input
Expand Down Expand Up @@ -395,24 +388,6 @@ let rec count_while1 ~f ~with_buffer =
prompt input pos fail' succ'
}

let string_ f s =
(* XXX(seliopou): Inefficient. Could check prefix equality to short-circuit
* the io. *)
let len = String.length s in
ensure len (unsafe_apply_opt len ~f:(fun buffer ~off ~len ->
let i = ref 0 in
while !i < len && Char.equal (f (Bigstringaf.unsafe_get buffer (off + !i)))
(f (String.unsafe_get s !i))
do
incr i
done;
if len = !i
then Ok (Bigstringaf.substring buffer ~off ~len)
else Error "string"))

let string s = string_ (fun x -> x) s
let string_ci s = string_ Char.lowercase_ascii s

let skip_while f =
count_while ~init:0 ~f ~with_buffer:(fun _ ~off:_ ~len:_ -> ())

Expand Down Expand Up @@ -451,6 +426,47 @@ let take_while1 f =
let take_till f =
take_while (fun c -> not (f c))

let string_ f s =
{
run =
(fun input pos more fail succ ->
let len = String.length s in
(* Empty string matches trivially *)
if len = 0 then succ input pos more s
else
let curr_index = ref 0 in
let successful_match = ref false in
let p =
take_while (fun c ->
if
!curr_index < len
&& Char.equal (f c) (f (String.unsafe_get s !curr_index))
then (
incr curr_index;
(* Proactively check if there are no more characters left in `s` and set as successful if that is so.
* We need to do this *now* without waiting for the next call to this callback, in case the stream
* is eof (and no more characters will available) in which case this callback will not be called again *)
if !curr_index = len then successful_match := true;
(* We always return `true` here to so that `c` can get incorporated into the matched string *)
true)
else false)
in
let succ' input' pos' more' matched_s =
(* `fail` with original `pos` rather than new `pos'` as pos <> pos' in case of partial prefix match *)
if !successful_match = false then fail input' pos more' [] "string"
else succ input' pos' more' matched_s
in
let fail' _ _ _ _ =
failwith
"string_: the impossible happened! `take_while` called `fail`"
in
(* [take_while] never fails so p should never fail *)
p.run input pos more fail' succ');
}

let string s = string_ (fun x -> x) s
let string_ci s = string_ Char.lowercase_ascii s

let choice ?(failure_msg="no more choices") ps =
List.fold_right (<|>) ps (fail failure_msg)

Expand Down