def find_array_start_position(big_array, small_array):
    """
        Find the starting index of a sub_array inside of a larger array

        Returns -1 if the small_array is not contrained within the larger array

        Arguments:
            big_array   (arr) : the larger array to search through
            small_array (arr) : the smaller array to find
        
        Returns:
            int : start position of small_array if small_array is within big_array, else -1
    """
    small_array_len = len(small_array)
    cut_off = len(big_array) - small_array_len
    for i, elem in enumerate(big_array):
        if i <= cut_off:
            if elem == small_array[0]:
                if big_array[i:i+small_array_len] == small_array:
                    return i
        else:
            break

    return -1