perms.rb

require 'pp' 

class Array

  def perms
    return to_enum(:perms) unless block_given?

    if size <= 1
      yield self
    else
      for i in 0...size
        e     = self[i]
        left  = self[0...i]
        right = self[i+1..-1]

        p i: i, e: e, self: self, left: left, right: right

        left.perms.each do |lperm|
          right.perms.each do |rperm|
            yield [e, *lperm, *rperm]
          end
        end
      end
    end

  end

end

a = [1,2,3]

p1 = a.perms.sort
p2 = a.permutation(a.size).sort

pp p1size: p1.size, p2size: p2.size
pp both: p1 & p2
pp mine: p1 - p2
pp ruby: p2 - p1