class Token
  def initialize(content = nil)
    @content = content
  end

  attr_reader :content

  alias token class

  def to_s
    case self
    when T_End
      "END"
    when T_LParen
      "LEFT_PAREN"
    when T_RParen
      "RIGHT_PAREN"
    when T_Whitespace
      "WHITESPACE"
    when T_Atom
      "ATOM:\"#{content}\""
    end
  end

  def self.end; T_End.new; end
  def self.lparen; T_LParen.new; end
  def self.rparen; T_RParen.new; end
  def self.whitespace; T_Whitespace.new; end
  def self.atom(content); T_Atom.new(content); end
end

class T_End < Token; end
class T_LParen < Token; end
class T_RParen < Token; end
class T_Whitespace < Token; end
class T_Atom < Token; end

def peek_char(input)
  input.first
end

def consume_char(input)
  input.shift
end

def consume_whitespace(input)
  while [?\s, ?\t, ?\r, ?\n].include?(peek_char(input))
    consume_char(input)
  end
end

def get_atom(input)
  val = ""
  until [?(, ?), ?\s, ?\t, ?\r, ?\n].include?(peek_char(input))
    val += consume_char(input)
  end
  val
end

def lex_token(input)
  case peek_char(input)
  when nil
    Token.end
  when ?(
    consume_char(input)
    Token.lparen
  when ?)
    consume_char(input)
    Token.rparen
  when ?\s, ?\t, ?\r, ?\n
    consume_whitespace(input)
    Token.whitespace
  else
    val = get_atom(input)
    Token.atom(val)
  end
end

def lex_tokens(input)
  input = input.chars

  toks = []

  loop do
    tok = lex_token(input)
    break if tok.token == T_End
    toks << tok if [T_LParen, T_RParen, T_Atom].include?(tok.token)
  end

  toks
end