add CCString.pad for more webscale

src/core/CCString.cppo.ml

@@ -433,6 +433,15 @@ let fold f acc s =
     else fold_rec f (f acc s.[i]) s (i+1)
   in fold_rec f acc s 0
 
+let pad ?(side=`Left) ?(c=' ') n s =
+  let len_s = String.length s in
+  if len_s >= n then s
+  else
+    let pad_len = n - len_s in
+    match side with
+      | `Left -> init n (fun i -> if i < pad_len then c else s.[i-pad_len])
+      | `Right -> init n (fun i -> if i < len_s then s.[i] else c)
+
 let _to_gen s i0 len =
   let i = ref i0 in
   fun () ->

src/core/CCString.mli

@@ -78,6 +78,22 @@ val rev : string -> string
   " " (rev " ")
 *)
 
+val pad : ?side:[`Left|`Right] -> ?c:char -> int -> string -> string
+(** [pad n str] ensures that [str] is at least [n] bytes long,
+    and pads it on the [side] with [c] if it's not the case.
+    @param side determines where padding occurs (default: [`Left])
+    @param c the char used to pad (default: ' ')
+    @since NEXT_RELEASE *)
+
+(*$= & ~printer:Q.Print.string
+  "  42" (pad 4 "42")
+  "0042" (pad ~c:'0' 4 "42")
+  "4200" (pad ~side:`Right ~c:'0' 4 "42")
+  "hello" (pad 4 "hello")
+  "aaa" (pad ~c:'a' 3 "")
+  "aaa" (pad ~side:`Right ~c:'a' 3 "")
+*)
+
 val of_gen : char gen -> string
 val of_seq : char sequence -> string
 val of_klist : char klist -> string