PR#1835: add function Digest.from_hex

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@12082 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02

VERSION

@@ -1,4 +1,4 @@
-3.13.0+dev10 (2012-01-10)
+3.13.0+dev11 (2012-01-26)
 
 # The version string is the first line of this file.
 # It must be in the format described in stdlib/sys.mli

stdlib/digest.ml

@@ -50,3 +50,19 @@ let to_hex d =
     String.blit (Printf.sprintf "%02x" (int_of_char d.[i])) 0 result (2*i) 2;
   done;
   result
+
+let from_hex s =
+  if String.length s <> 32 then raise (Invalid_argument "Digest.from_hex");
+  let digit c =
+    match c with
+    | '0'..'9' -> Char.code c - Char.code '0'
+    | 'A'..'F' -> Char.code c - Char.code 'A' + 10
+    | 'a'..'f' -> Char.code c - Char.code 'a' + 10
+    | _ -> raise (Invalid_argument "Digest.from_hex")
+  in
+  let byte i = digit s.[i] lsl 4 + digit s.[i+1] in
+  let result = String.create 16 in
+  for i = 0 to 15 do
+    result.[i] <- Char.chr (byte (2 * i));
+  done;
+  result

stdlib/digest.mli

@@ -61,3 +61,8 @@ val input : in_channel -> t
 
 val to_hex : t -> string
 (** Return the printable hexadecimal representation of the given digest. *)
+
+val from_hex : string -> t
+(** Convert a hexadecimal representation back into the corresponding digest.
+   Raise [Invalid_argument] if the argument is not exactly 32 hexadecimal
+   characters. *)