Quicksort in ruby

quicksort.rb

# Quicksort is a ruby implementation of the well-known algorithm
# https://en.wikipedia.org/wiki/Quicksort
class Quicksort
  # sort will recursively run the partition function
  # lo and hi identify the index range to be sorted
  def sort(arr, lo, hi)
    return unless lo < hi
    pidx = partition(arr, lo, hi)
    sort(arr, lo, pidx - 1)
    sort(arr, pidx + 1, hi)
  end

  private

  # partition rearranges the array so that all elements less than a pivot
  # are on the left of it, and elements greater than the pivot are on the
  # right. The pivot choice is arbitrary. This implementation uses the
  # high index for a pivot.
  def partition(arr, lo, hi)
    pivot = arr[hi]
    pidx = lo
    (lo...hi).to_a.each do |i|
      if arr[i] < pivot
        arr[i], arr[pidx] = arr[pidx], arr[i]
        pidx += 1
      end
    end
    arr[pidx], arr[hi] = arr[hi], arr[pidx]
    pidx
  end
end

require "minitest/autorun"

class TestQuicksort < MiniTest::Test
  def setup
    @sort = Quicksort.new
  end

  def test_sorts_correctly
    nums = [10, 9, 5, 9, 2.5, 2.6, 6]
    sorted_nums = nums.sort
    @sort.sort(nums, 0, nums.length - 1)
    assert_equal sorted_nums, nums
  end
  
  def test_raises_when_array_is_frozen
    nums = [10, 9, 5, 9, 2.5, 2.6, 6].freeze
    assert_raises FrozenError do
      @sort.sort(nums, 0, nums.length - 1)
    end
  end

end