class Lexer

  KEYWORDS = ["def", "class", "if", "true", "false", "nil"]
  
  def tokenize(code)
    code.chomp! 
    tokens = [] 

    current_indent = 0 
    indent_stack = []
    
    i = 0 
    while i < code.size
      chunk = code[i..-1]
      
      if identifier = chunk[/\A([a-z]\w*)/, 1]
        if KEYWORDS.include?(identifier) 
          tokens << [identifier.upcase.to_sym, identifier]
        else
          tokens << [:IDENTIFIER, identifier]
        end
        i += identifier.size 

      elsif constant = chunk[/\A([A-Z]\w*)/, 1]
        tokens << [:CONSTANT, constant]
        i += constant.size

      elsif number = chunk[/\A([0-9]+)/, 1]
        tokens << [:NUMBER, number.to_i]
        i += number.size
        
      elsif string = chunk[/\A"([^"]*)"/, 1]
        tokens << [:STRING, string]
        i += string.size + 2 
      
      elsif indent = chunk[/\A\:\n( +)/m, 1] 
        if indent.size <= current_indent 
          raise "Bad indent level, got #{indent.size} indents, " +
                "expected > #{current_indent}"
        end
        current_indent = indent.size
        indent_stack.push(current_indent)
        tokens << [:INDENT, indent.size]
        i += indent.size + 2

      elsif indent = chunk[/\A\n( *)/m, 1] 
        if indent.size == current_indent 
          tokens << [:NEWLINE, "\n"] 
        elsif indent.size < current_indent 
          while indent.size < current_indent
            indent_stack.pop
            current_indent = indent_stack.last || 0
            tokens << [:DEDENT, indent.size]
          end
          tokens << [:NEWLINE, "\n"]
        else
          raise "Missing ':'" 
        end
        i += indent.size + 1

      elsif operator = chunk[/\A(\|\||&&|==|!=|<=|>=)/, 1]
        tokens << [operator, operator]
        i += operator.size

      elsif chunk.match(/\A /)
        i += 1

      else
        value = chunk[0,1]
        tokens << [value, value]
        i += 1
        
      end
      
    end

    while indent = indent_stack.pop
      tokens << [:DEDENT, indent_stack.first || 0]
    end
    
    tokens
  end
end