let find_reverse ?from ?length s ~f =
let length_of_s = String.length s in
if length_of_s = 0 then None
else begin
let from =
match from with
| None -> length_of_s - 1
| Some s when s < 0 -> -1
| Some s when s >= length_of_s - 1 -> length_of_s - 1
| Some s -> s
in
let length =
match length with
| None -> from + 1
| Some l when l <= 0 -> 0
| Some l when l >= from + 1 -> from + 1
| Some l -> l
in
let found = ref None in
let i = ref from in
while !found = None && !i >= from - length + 1 do
if f (get_exn s !i)
then found := Some (!i)
else decr i
done;
!found
end