プレゼント交換のペアを決める #hmrb

xshuffle.rb

class Array
    def xshuffle
        a = (0...size).to_a
        a.shuffle!
        ret = Array.new(size)
        (0...size).each do |ii| 
          ret[a[ii]] = self[a[(ii + 1) % size]]
        end
        ret
    end
end

# %w{alice brian crossby david}.xshuffle