/**
 * 이진 탐색 ( binary search )
 * @param {array} list - Sorted Array.
 * @param {*} item - The key for search.
 * @returns {number} idx - Index of key in array. -1 means "can't find the key."
 */
function binary_search(list, item) {
  // init
  var idx = -1;
  var low = 0;
  var high = list.length - 1;

  var mid = 0;
  var guess = 0;

  while (low <= high) {
    mid = Math.ceil((low + high) / 2);
    guess = list[mid];

    if (guess == item) {
      return mid;
    } else if (guess > item) {
      high = mid - 1;
    } else {
      low = mid + 1;
    }
  }
  return -1;  // 못찾음
}