Generate short URLs using Ruby (See related posts)

This Ruby code generates shortened URLs. It has the added feature that it will not allow words to be created accidentally eg. swear words.

#!/usr/bin/ruby

class ShortUrl
  def initialize()
    @chars = ('a'..'z').to_a + ('1'..'9').to_a + ('A'..'Z').to_a
    @array_size = @chars.size
    @h = Hash.new
    @chars.each {|c| @h[c] = '0'}
    vowels = %w(a e i o u A E I O U 4 3 1 0)
    vowels.each {|v| @h[v] = '1'}
    nums = %w(2 5 6 7 8 9)
    nums.each {|n| @h[n] = '2'}
    @count = 0
    @a = Array.new(7, -1)
    @k = 0
  end 

  def iterate_chars(array_size)
    (0..array_size).each {|i| 
      increment_index(@k)
      convert_to_chars()
    }
  end
  
  def convert_to_chars()
    buffer = ''
    @a.each {|i|
      buffer << @chars[i] if i >= 0
    }
    a = buffer.reverse.scan(/./)
    k = a.length  
    if  (k > 1)  
      if ((@h[a[k-2]] + @h[a[k-1]]) != '10')
        puts buffer.reverse 
      end
    else
      puts buffer.reverse
    end
  end

  def get_short_url(count)
    if count >  @array_size
      new_count = count - @array_size
      iterate_chars(@array_size)
      get_short_url(new_count)
    else
      iterate_chars(count)
    end
  end
  
  def increment_a(i)
    if @a[i] < @array_size - 1
      @a[i] = @a[i] + 1
      return i 
    else
      @a[i] = 0
      return i += 1
    end 
  end

  def increment_index(k)
    old_k = k
    k = increment_a(k)
    
    if k != old_k
      increment_index(k)
    else
      k = 0
    end
    k
  end

end

if __FILE__ == $0
  su = ShortUrl.new  
  su.get_short_url(222761)
end

Note: You will most likely see the " `convert_to_chars': stack level too deep (SystemStackError)" error message after around 253950 iterations, I'm presuming this is expected due to the memory constraints of the language. However 253k short urls is enough for linking my website to itself.