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Utils: fix list_rev complexity.

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Using fold_left here iterates over the entire list, whereas you only
need to iterate over the first `n` elements.
This commit is contained in:
Alex Coventry 2017-08-19 04:58:19 +00:00 committed by Grégoire Henry
parent 4c7d4a8348
commit 59b5b2df49
1 changed files with 9 additions and 3 deletions
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12
src/minutils/utils.ml
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@ -80,9 +80,15 @@ let filter_map f l =
List.rev @@ List.fold_left (fun acc x -> may_cons acc (f x)) [] l
let list_rev_sub l n =
ListLabels.fold_left l ~init:(n, []) ~f:begin fun (n, l) elt ->
if n <= 0 then (n, l) else (n - 1, elt :: l)
end |> snd
if n < 0 then
invalid_arg "Utils.list_rev_sub: `n` must be non-negative.";
let rec append_rev_sub acc l = function
| 0 -> acc
| n ->
match l with
| [] -> acc
| hd :: tl -> append_rev_sub (hd :: acc) tl (n - 1) in
append_rev_sub [] l n
let list_sub l n = list_rev_sub l n |> List.rev