Generating all possible strings of characters with a given character set and length

bruteforce.rb

class BruteForce
  extend Enumerable
  
  def initialize(size, alphabet)
    @alphabet = alphabet
    
    if size.respond_to?(:min) and size.respond_to?(:max)
      @start_size = size.min
      @end_size   = size.max
    else
      @start_size = @end_size = size
    end
  end
  
  def each &block
    recursive = Proc.new do |ary|
      if ary.size >= @start_size
        block.call ary # yield to caller
      end
      
      if ary.size < @end_size
        @alphabet.each do |z|
          recursive.call(ary + [z]) # Recurse deeper
        end
      end
    end
    
    recursive.call []
  end
end

alphabet = ('A'..'Z').to_a + ('0'..'9').to_a + ['_']

BruteForce.new(1..4, alphabet).each { |list| puts list.join }