let find ~what ?(offset=0) str =
let what_idx = ref 0 in
let str_idx = ref offset in
while !str_idx < String.length str &&
!what_idx < String.length what do
if str.[!str_idx] = what.[!what_idx] then
incr what_idx
else
what_idx := 0;
incr str_idx
done;
if !what_idx <> String.length what then
raise Not_found
else
!str_idx - !what_idx