Scala binary search for exact match or neighbour on floating point numbers

NeighbourBinarySearch.scala

  /**
    * A functional binary tree search for floating point numbers which finds
    * a neighbour to the search target in the given list.
    * Returns upper or lower index if target is outside min and max of list.
    * @param start Lower index of the array to search
    * @param end Upper index of the array to search
    * @param target Value being searched for in list
    * @param list Sorted list of values to search
    * @return Index of a neighbour to the target within list
    *         (not guaranteed to be nearest neighbour)
    */
  def binarySearch(start: Int = 0,
                   end: Int,
                   target:Double,
                   list: Array[Double]): Int = {

    // midpoint with overflow protection
    val mid = start + (end-start+1)/2
    // if mid is the same as start or finish then we have found a neighbour
    if ( mid == start || mid == end) mid
    else if ( target > list(mid) ) binarySearch(mid, end, target, list)
    else if ( target < list(mid) ) binarySearch(start, mid, target, list)
    else if ( target == list(mid) ) mid
    else -1 // Your list has NaNs in it and you have bigger problems
  }