Focus on elements in an Array

array_focus.rb

class Array
  # Public: slice Array while focusing on the given item in the array
  # and padding around it
  #
  #   item - element in the Array to focus on
  #   padding - Integer of the number of elements to pad by (default: 2)
  #
  # Examples:
  #   items = ["a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t"]
  #
  #   items.focus("g", 3)
  #   => ["d", "e", "f", "g", "h", "i", "j"]
  #
  #   items.focus("b")
  #   => ["a", "b", "c", "d", "e"]
  #
  # Returns a new sliced, focused, padded Array
  def focus(item, padding = 2)
    length = (padding * 2) + 1

    # index of item to focus on
    index = self.rindex(item)
    raise ArgumentError, "item must be an element in the Array" if index.nil?

    # find start point of slice, but limit to starting
    # at 0 or higher
    start = (index - padding)
    start = (start < 0) ? 0 : start

    self.slice(start, length)
  end
end