md5.rb

#!/usr/bin/env ruby
require 'digest/md5'

def find_match(a, b)
  largest = 0
  current = 0
  (32-largest).times do |aix|
    (32-largest).times do |bix|
      offset  = 0
      current = 0
      while a[aix+offset] == b[bix+offset]
        offset += 1
        current +=1
        break if (aix+offset)>31 or (bix+offset)>31
      end
      if current > largest
        largest = current
      end
    end
  end
  return largest
end


x = "42"
i = 0
largest = 0
loop do
  i += 1
  old = x
  x = Digest::MD5.hexdigest(x)
  curr = find_match(old,x)
  if curr > largest
    puts "#{old} => #{x} : #{curr} (#{i} scanned)"
    largest = curr
  end
end

md5cycle.rb

#!/usr/bin/env ruby

require 'digest/md5'

x = "42"
save = "0"
i = 0
j = 0
loop do
  i += 1
  j += 1
  x = Digest::MD5.hexdigest(x)

  if x == save
    puts "Cycle found at #{x}"
    exit
  end
    
  if j == 100000
    save = x
    print "."
    $stdout.flush
    j = 0
  end
end