From 59b5b2df49521f88855222f9802a5f2c55c885a0 Mon Sep 17 00:00:00 2001
From: Alex Coventry <alx@mit.edu>
Date: Sat, 19 Aug 2017 04:58:19 +0000
Subject: [PATCH] Utils: fix `list_rev` complexity.

Using fold_left here iterates over the entire list, whereas you only
need to iterate over the first `n` elements.
---
 src/minutils/utils.ml | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/src/minutils/utils.ml b/src/minutils/utils.ml
index 189aede0f..54d24e972 100644
--- a/src/minutils/utils.ml
+++ b/src/minutils/utils.ml
@@ -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