Convert Unicode escaped \uXXXX sequence to utf-8 encoded string

uXXXX.ml

(*
 * Imported from https://gist.github.com/sagotch/
 *
 * For more informations about unicode to utf-8 converting method used see:
 * http://www.ietf.org/rfc/rfc3629.txt (Page3, section "3. UTF-8 definition")
 * 
 * decimal conversions of binary used:
 * 10000000 -> 128; 11000000 -> 192; 11100000 -> 224
 *)

(** 
 * Convert Unicode escaped \uXXXX sequence to utf-8 encoded string
 * [u] XXXX part of the Unicode sequence (as string)
 *)

let to_utf8 (u : string) : string =

  let dec = int_of_string @@ "0x" ^ u in

  let update_byte s i mask shift =
    s.[i] <- Char.chr (Char.code s.[i]
                       + dec lsr shift land int_of_string mask) in

  if dec > 0xFFFF then
    failwith ("Invalid escaped unicode \\u" ^ u)

  else if dec > 0x7FF then
    let s = String.copy "\224\128\128" in
    update_byte s 2 "0b00111111" 0;
    update_byte s 1 "0b00111111" 6;
    update_byte s 0 "0b00001111" 12;
    s

  else if dec > 0x7F then 
    let s = String.copy "\192\128" in
    update_byte s 1 "0b00111111" 0;
    update_byte s 0 "0b00011111" 6;
    s

  else
    let s = String.copy "\000" in
    update_byte s 0 "0b01111111" 0;
    s