Sorting quickly

quicksort.rb

module QuickSort
  def sort(array, options = {})
    start_index = options[:start_index] || 0
    end_index = options[:end_index] || array.length - 1
    unless start_index >= end_index
      # Randomly choose a pivot and swap with the end index.
      pivot_index = rand(end_index - start_index + 1) + start_index
      temp = array[end_index]
      array[end_index] = array[pivot_index]
      array[pivot_index] = temp
      # Separate the array into elements less than or equal to the pivot, the pivot, and
      # elements greater than the pivot.
      # Note that this means the pivot will be in the correct place.
      last_less_index = start_index - 1
      (start_index...end_index).each do |i|
        if array[i] <= array[end_index]
          last_less_index += 1
          temp = array[i]
          array[i] = array[last_less_index]
          array[last_less_index] = temp
        end
      end
      last_less_index += 1
      temp = array[end_index]
      array[end_index] = array[last_less_index]
      array[last_less_index] = temp
      # Sort the elements less than or equal to the pivot.
      sort(array, start_index: start_index, end_index: last_less_index - 1)
      # Sort the elements greater than the pivot.
      sort(array, start_index: last_less_index + 1, end_index: end_index)
    end
  end
end

quicksort_test.rb

require './quicksort'
require 'test/unit'

class QuickSortTest < Test::Unit::TestCase
  include QuickSort

  def test_sort
    # Empty array.
    array = []
    sort(array)
    assert(array.empty?)

    # Single array.
    array = [1]
    sort(array)
    assert_equal(1, array[0])

    # Two elements.
    array = [1, 2]
    sort(array)
    assert_equal(1, array[0])
    assert_equal(2, array[1])

    # Sorted array.
    array = [1, 2, 3, 4, 5, 6]
    sort(array)
    assert_equal(1, array[0])
    assert_equal(2, array[1])
    assert_equal(3, array[2])
    assert_equal(4, array[3])
    assert_equal(5, array[4])
    assert_equal(6, array[5])

    # Reverse sorted array.
    array = [6, 5, 4, 3, 2, 1]
    sort(array)
    assert_equal(1, array[0])
    assert_equal(2, array[1])
    assert_equal(3, array[2])
    assert_equal(4, array[3])
    assert_equal(5, array[4])
    assert_equal(6, array[5])

    # Arbitrary array.
    array = [5, 2, 4, 1, 6, 3]
    sort(array)
    assert_equal(1, array[0])
    assert_equal(2, array[1])
    assert_equal(3, array[2])
    assert_equal(4, array[3])
    assert_equal(5, array[4])
    assert_equal(6, array[5])
  end
end