each_pair.rb

# Cantor-numbering: maps Integer onto (Integer,Integer)
def cantor(z)
  w = ((Math.sqrt(8*z+1)-1) / 2).floor
  t = ((w**2 + w) / 2).floor
  y = z-t
  x = w-y
  [x,y]
end

# Iterates over the Cartesian product xs*ys in Cantor order
def each_pair(xs,ys)
  i = 0
  nxs, nys = xs.length, ys.length
  loop do
    xi, yi = cantor(i)
    i+=1

    break if xi >= nxs && yi >= nys
    next if xi >= nxs || yi >= nys

    yield(xs[xi], ys[yi])
  end
end

# >> each_pair([1,2,3,4],[9,8,7]){|x, y| puts "x=#{x}, y=#{y}"}
# x=1, y=9
# x=2, y=9
# x=1, y=8
# x=3, y=9
# x=2, y=8
# x=1, y=7
# x=4, y=9
# x=3, y=8
# x=2, y=7
# x=4, y=8
# x=3, y=7
# x=4, y=7