gistfile1.rb

class InversionsCalculator
  def inversions_count(arr)
    sort_and_count(arr)[0]
  end

  private

  def sort_and_count(arr)
    return [0, arr] if arr.length == 1
    a, b = split_array(arr)
    x, a = sort_and_count(a)
    y, b = sort_and_count(b)
    z, c = merge_and_count_split(a,b)
    [x + y + z, c]
  end

  def merge_and_count_split(a, b)
    inversions = i = j = 0
    c = []
    (a.length + b.length).times do
      if b[j].nil? || a[i] && a[i] < b[j]
        c << a[i]
        i += 1
      else
        c << b[j]
        j += 1
        inversions += a.length - i
      end
    end
    [inversions, c]
  end

  def split_array(arr)
    n = arr.length / 2
    [arr[0..n-1], arr[n..-1]]
  end
end

calc = InversionsCalculator.new
arr = []
File.open('./IntegerArray.txt') do |f|
  f.each_line{ |line| arr << line.to_i }
end
puts calc.inversions_count(arr)