LL(1) parser

#LL(1) parser as a solution for exercise 4.9 in "Compiler Construction: Principles and Practice"
#String given is (a,(b,(2)),(c))$
#deduce the grammar ;) 

class Parser
  def initialize()
  	@parsing_stack = []
		@parsing_stack.push('$')
		@parsing_stack.push('lexp-seq')
	end

	def launch!
		introduction
		result = nil
		until result == :done
			print "> "
			user_response = gets.chomp
			result = parse(user_response)
		end
		@parsing_stack.inspect
		conclusion
	end

	def parse (input)
		output = ""
		counter = 0
		puts "This is how your stack looks like: \n#{@parsing_stack.inspect}"
		input.each_char do |single_char|
		case single_char
			when '('
				while @parsing_stack[-1] != single_char
					if @parsing_stack[-1] == 'lexp-seq'
						@parsing_stack.pop
						@parsing_stack.push('lexp-seq1','lexp')
						puts @parsing_stack.inspect
					elsif @parsing_stack[-1] == 'lexp'
						@parsing_stack.pop
						@parsing_stack.push('list')
						puts @parsing_stack.inspect
					elsif @parsing_stack[-1] == 'list'
						@parsing_stack.pop
						@parsing_stack.push(')','lexp-seq','(')
						puts @parsing_stack.inspect
					end
				end
				if @parsing_stack[-1] == single_char
					@parsing_stack.pop
					puts @parsing_stack.inspect
					output += single_char
					counter += 1
					puts "read #{counter} characters so far"
				end
			when 'a','b','c'
				while @parsing_stack[-1] != 'identifier'
					if @parsing_stack[-1] == 'atom'
						@parsing_stack.pop
						@parsing_stack.push('identifier')
						puts @parsing_stack.inspect
					elsif @parsing_stack[-1] == 'lexp'
						@parsing_stack.pop
						@parsing_stack.push('atom')
						puts @parsing_stack.inspect
					elsif @parsing_stack[-1] == 'lexp-seq'
						@parsing_stack.pop
						@parsing_stack.push('lexp-seq1', 'lexp')
						puts @parsing_stack.inspect

					end
				end
				if @parsing_stack[-1] == 'identifier'
					@parsing_stack.pop
					puts @parsing_stack.inspect
					output += single_char
					counter += 1
					puts "read #{counter} characters so far"
				end
			when '0'..'9'
				while @parsing_stack[-1] != 'number'
					if @parsing_stack[-1] == 'lexp'
						@parsing_stack.pop
						@parsing_stack.push('atom')
						puts @parsing_stack.inspect
					elsif @parsing_stack[-1] == 'atom'
						@parsing_stack.pop
						@parsing_stack.push('number')
						puts @parsing_stack.inspect
					elsif @parsing_stack[-1] == 'lexp-seq'
						@parsing_stack.pop
						@parsing_stack.push('lexp-seq1', 'lexp')
						puts @parsing_stack.inspect
					end
				end
				if @parsing_stack[-1] == 'number'
					@parsing_stack.pop
					puts @parsing_stack.inspect
					output += single_char
					counter += 1
					puts "read #{counter} characters so far"
				end
			when ')'
				while @parsing_stack[-1] != single_char
					if @parsing_stack[-1] == 'lexp-seq1'
						@parsing_stack.pop
						puts @parsing_stack.inspect
					end
				end
				if @parsing_stack[-1] == ')'
					@parsing_stack.pop
					puts @parsing_stack.inspect
					output += single_char
					counter += 1
					puts "read #{counter} characters so far"
				end
			when ','
				while @parsing_stack[-1] != single_char
					if @parsing_stack[-1] == 'lexp-seq1'
						@parsing_stack.pop
						@parsing_stack.push('lexp-seq')
						@parsing_stack.push(',')
						puts @parsing_stack.inspect
					end
				end
				if @parsing_stack[-1] == ','
					@parsing_stack.pop
					puts @parsing_stack.inspect
					output += single_char
					counter += 1
					puts "read #{counter} characters so far"
				end
			when '$'
				while @parsing_stack[-1] != single_char
					if @parsing_stack[-1] == 'lexp-seq1'
						@parsing_stack.pop
						puts @parsing_stack.inspect
					end
				end
				if @parsing_stack[-1] == '$'
					@parsing_stack.pop
					output += single_char
					counter += 1
					puts "read #{counter} characters so far"
					puts "#{output}"
					return :done
				end
			when "\s"

			else
				puts "\nInput not recognized!"
			end
		end
	end

	def introduction
		puts "\n<<< Welcome to the LL(1) Parser >>>\n"
		puts "This is an interactive LL(1) Parser for you to play with.\n\n"
	end

	def conclusion
		puts "\n<<< Goodbye and happy parsing! >>>"
	end
end

parser = Parser.new()
parser.launch!