sysprep: Move skip_dashes function into Utils module.

This is mostly code motion, but I also changed the function to use
String.unsafe_get and raise Invalid_argument on failure.

sysprep/sysprep_operation.ml

@@ -16,6 +16,8 @@
  * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  *)
 
+open Utils
+
 open Printf
 
 type flag = [ `Created_files ]
@@ -106,18 +108,6 @@ let dump_pod () =
       printf "%s\n\n" op.pod_description
   ) !ops
 
-(* Skip any leading '-' characters when comparing command line args. *)
-let skip_dashes str =
-  let n = String.length str in
-  let rec loop i =
-    if i >= n then assert false
-    else if str.[i] = '-' then loop (i+1)
-    else i
-  in
-  let i = loop 0 in
-  if i = 0 then str
-  else String.sub str i (n-i)
-
 let dump_pod_options () =
   assert !baked;
 

sysprep/utils.ml

@@ -69,3 +69,15 @@ let string_random8 =
           String.make 1 c
       ) [1;2;3;4;5;6;7;8]
     )
+
+(* Skip any leading '-' characters when comparing command line args. *)
+let skip_dashes str =
+  let n = String.length str in
+  let rec loop i =
+    if i >= n then invalid_arg "skip_dashes"
+    else if String.unsafe_get str i = '-' then loop (i+1)
+    else i
+  in
+  let i = loop 0 in
+  if i = 0 then str
+  else String.sub str i (n-i)

sysprep/utils.mli

@@ -37,3 +37,10 @@ val string_random8 : unit -> string
 (** Return a random 8 character string, suitable as a temporary
     filename since every filesystem supports at least 8 character
     filenames. *)
+
+val skip_dashes : string -> string
+(** Take a string like ["--str"] and return ["str"], that is, skip
+    any leading dash characters.
+
+    If the string contains only dash characters, this raises
+    [Invalid_argument "skip_dashes"]. *)