An in-place implementation of quicksort in Ruby.

In-Place Ruby Quicksort

def my_quicksort(array)
  left = 0
  right = array.length - 1

  quicksort(array, left, right)
end

def quicksort(array, left, right)
  if left < right
    pivot_index = ((right + left).to_f / 2).ceil

    pivot_new_index = partition(array, left, right, pivot_index)

    quicksort(array, left, pivot_new_index - 1)
    quicksort(array, pivot_new_index + 1, right)
  end

  array
end

def partition(array, left, right, pivot_index)
  pivot_value = array[pivot_index]

  array[pivot_index], array[right] = array[right], array[pivot_index]

  store_index = left

  (left..(right - 1)).each do |i|
    if array[i] < pivot_value
      array[i], array[store_index] = array[store_index], array[i]
      store_index += 1
    end
  end

  array[store_index], array[right] = array[right], array[store_index]

  store_index
end