Hackerrank: Xoring Ninja Solution in Ocaml

Problem Statement
https://www.hackerrank.com/challenges/xoring-ninja

Given a list containing N integers, calculate the XOR_SUM of all the non-empty subsets of the list and print the value of sum % (109 + 7).

XOR operation on a list (or a subset of the list) is defined as the XOR of all the elements present in it.
E.g. XOR of list containing elements {A,B,C} = ((A^B)^C), where ^ represents XOR.

E.g. XOR_SUM of list A having three elements {X1, X2, X3} can be given as follows.
All non-empty subsets will be {X1, X2, X3, (X1,X2), (X2,X3), (X1,X3), (X1,X2,X3)}

XOR_SUM(A) = X1 + X2 + X3 + X1^X2 + X2^X3 + X1^X3 + ((X1^X2)^X3)

Input Format

An integer T, denoting the number of testcases. 2T lines follow.
Each testcase contains two lines, first line will contains an integer N followed by second line containing N integers separated by a single space.

Output Format

T lines, ith line containing the output of the ith testcase.

Constraints

1 <= T <= 5
1 <= N <= 105
0 <= A[i] <= 109

let large_number = 1000000007 ;;

let compute n k =
  if k = 0 then
    0
  else
    let result = ref 1 in
    for i = 1 to (n - 1) do
      result := (!result lsl 1) mod large_number
    done;
    !result
;;

let process arr length =
  let max_elem = ref (-1) in
  for i = 0 to (length - 1 ) do
    if (arr.(i) > !max_elem) then
      max_elem := arr.(i)
  done;
  let level = ref 0
  and result = ref 0 in
  while !max_elem > 0 do

    let count = ref 0 in
    for i = 0 to (length - 1) do
      count := !count + (1 land arr.(i));
      arr.(i) <- (arr.(i) lsr 1)
    done;
    (* Printf.printf "max_elem: %d, level: %d, result: %d, count: %d\n" !max_elem !level !result !count; *)
    let res = compute length !count in
    result := (!result + (res lsl !level) ) mod large_number;
    level := !level + 1 ;
    max_elem := !max_elem lsr 1 ;
  done;
  !result
;;


(* main *)

let num_case = read_int () ;;

for i = 1 to num_case do
  let n = read_int () in
  let line = read_line () in
  let lst_string = Str.split (Str.regexp " ") line in
  let lst_int = List.map int_of_string lst_string in
  let arr = Array.of_list lst_int in
  let result = process arr n in
  Printf.printf "%d\n" result
done ;;