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faster List.take_drop thanks to a trick by nojb

Browse source 
https://discuss.ocaml.org/t/efficient-implementation-of-list-split/17616/2
pretty cool.
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Simon Cruanes 2025-12-20 11:08:30 -05:00
parent c1b13f1c7f
commit 15b421c54e
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1 changed files with 25 additions and 8 deletions
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33
src/core/CCList.ml
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@ -778,6 +778,12 @@ let sorted_diff_uniq ~cmp l1 l2 =
in in
recurse ~cmp [] l1 l2 recurse ~cmp [] l1 l2
let rec drop n l =
match l with
| [] -> []
| _ when n = 0 -> l
| _ :: l' -> drop (n - 1) l'
[@@@iflt 4.14] [@@@iflt 4.14]
let take n l = let take n l =
@ -798,6 +804,8 @@ let take n l =
in in
direct direct_depth_default_ n l direct direct_depth_default_ n l
let take_drop n l = take n l, drop n l
[@@@else_] [@@@else_]
let[@tail_mod_cons] rec take n l = let[@tail_mod_cons] rec take n l =
@ -809,20 +817,29 @@ let[@tail_mod_cons] rec take n l =
else else
[] []
[@@@endif] let take_drop n l =
(* fun idea from nojb in
https://discuss.ocaml.org/t/efficient-implementation-of-list-split/17616/2 *)
let[@tail_mod_cons] rec loop (res_drop : _ ref) n l =
match n, l with
| 0, _ ->
res_drop := l;
[]
| _, [] ->
res_drop := [];
[]
| _, x :: tl -> x :: loop res_drop (n - 1) tl
in
let res_drop = ref [] in
let res_take = loop res_drop n l in
res_take, !res_drop
let rec drop n l = [@@@endif]
match l with
| [] -> []
| _ when n = 0 -> l
| _ :: l' -> drop (n - 1) l'
let hd_tl = function let hd_tl = function
| [] -> failwith "hd_tl" | [] -> failwith "hd_tl"
| x :: l -> x, l | x :: l -> x, l
let take_drop n l = take n l, drop n l
let sublists_of_len ?(last = fun _ -> None) ?offset n l = let sublists_of_len ?(last = fun _ -> None) ?offset n l =
if n < 1 then invalid_arg "sublists_of_len: n must be > 0"; if n < 1 then invalid_arg "sublists_of_len: n must be > 0";
let offset = let offset =