## Re: [Ocaml-lib-devel] ExtString module updated

 Re: [Ocaml-lib-devel] ExtString module updated From: Brian Hurt - 2003-05-22 15:37:40 ```On Thu, 22 May 2003, Michal Moskal wrote: > On Thu, May 22, 2003 at 06:51:04PM +0900, Nicolas Cannasse wrote: > > > > val find : string -> string -> int (* find a substring *) > > > > > > Using KMP or some other technique? (i.e. if it takes O(m * n), where m and > > > n are lengths of strings respectively? or O(log m * n + m)) > > > > Right now only a naive O(m*n) technique. Please check the CVS, and submit me > > some code if you have a better implementation. > > You can have a look at http://caml.inria.fr/Examples/oc/basics/kmp.ml > > I tried rewrite of while loop to tail recursion, but it looks ugly. > > Sanity check this code, if you will: let kmp p = let m = String.length p in let init_next p = let next = Array.create m 0 in let rec loop i j = if i >= (m - 1) then next else if p.(i) = p.(j) then begin next.(i+1) <- j+1; loop (i+1) (j+1) end else if j = 0 then begin next.(i+1) <- 0; loop (i+1) j end else loop i next.(j) in loop 1 0 in let next = init_next p in let n = String.length s in let rec loop2 i j = if (j >= m) || (i >= n) then if j >= m then i - m else raise Not_found else if s.(i) = p.(j) then loop2 (i+1) (j+1) else if j = 0 then loop2 (i+1) j else loop2 i next.(j) in loop2 0 0 ;; I don't think it's that ugly. And it gets rid of all the references- which probably means a speed up (haven't tested it yet though). Although one thing we might want to consider: actually exporting the kmp_init function and kmp function seperately. This way if I wanted to find the same substring in a lot of different strings, I don't have to constantly recreate the next table. Maybe something like: let _kmp_init_next p m = let next = Array.create m 0 in let rec loop i j = if i >= (m - 1) then next else if p.(i) = p.(j) then begin next.(i+1) <- j+1; loop (i+1) (j+1) end else if j = 0 then begin next.(i+1) <- 0; loop (i+1) j end else loop i next.(j) in loop 1 0 let kmp_init_next p = _kmp_init_next p (String.length p) let _kmp next p s m = let n = String.length s in let rec loop2 i j = if (j >= m) || (i >= n) then if j >= m then i - m else raise Not_found else if s.(i) = p.(j) then loop2 (i+1) (j+1) else if j = 0 then loop2 (i+1) j else loop2 i next.(j) in loop2 0 0 let kmp next p s = _kmp next p s (String.length p) let find p s = let m = String.length p in let next = _kmp_init_next p m in _kmp next p s m Where kmp_init_next and kmp would be exported in the .mli file. You could then write code like: let find_all_substring substr strlist = let next = kmp_init_next substr in List.find_all (fun s -> try let _ = kmp next substr s in true with Not_found -> false) strlist ;; Brian ```