func mdroot(n) {
  var (mdr, persist) = (n, 0)
  while (mdr >= 10) {
    mdr = mdr.digits.prod
    ++persist
  }
  [mdr, persist]
}

say "Number: MDR  MP\n======  ===  =="
[123321, 7739, 893, 899998].each{|n| "%6d: %3d %3d\n" \
                           .printf(n, mdroot(n)...) }

var counter = Hash()

Inf.times { |j|
  counter{mdroot(j).first} := [] << j
  break if counter.values.all {|v| v.len >= 5 }
}
 
say "\nMDR: [n0..n4]\n===  ========"
10.times {|i| "%3d: %s\n".printf(i, counter{i}.first(5)) }