class Stack                                                         
   def initialize(n = 8)                                            
      @s = Array.new(n, nil)                                        
      @top = -1                                                     
   end                                                              
                                                                    
   def empty?                                                       
      @top == -1                                                    
   end                                                              
                                                                    
   def push(x)                                                      
      raise 'overflow' if @top == @s.size-1                         
      @top += 1                                                     
      @s[@top] = x                                                  
   end                                                              
end                                                                 
                                                                    
require 'test/unit'                                                 
                                                                    
class StackTest < Test::Unit::TestCase                              
   def setup                                                        
      @stack = Stack.new                                            
   end                                                              
                                                                    
   def test_empty                                                   
      assert(@stack.empty?)                                         
                                                                    
      @stack.push 3                                                 
                                                                    
      assert(! @stack.empty?)                                       
   end                                                              
                                                                    
   def test_push                                                    
      8.times {|x| @stack.push x}                                   
      assert_raise(RuntimeError) {                                  
         @stack.push 'illegal'                                      
      }                                                             
   end                                                              
end