class Lexer
  def initialize(source)
    @source = source
    @position = 0
  end

  def get_token
    skip_whitespace
    skip_comment

    return Token.new(TokenKinds::EOF) if at_end

    source = @source.slice(@position..-1)

    if source.match(/\Anull/)
      @position += 4
      Token.new(TokenKinds::NULL)
    elsif source.match(/\Atrue/)
      @position += 4
      Token.new(TokenKinds::BOOLEAN, true)
    elsif source.match(/\Afalse/)
      @position += 5
      Token.new(TokenKinds::BOOLEAN, false)
    elsif source.match(/\Aif/)
      @position += 2
      Token.new(TokenKinds::IF)
    elsif source.match(/\Aelseif/)
      @position += 6
      Token.new(TokenKinds::ELSEIF)
    elsif source.match(/\Aelse/)
      @position += 4
      Token.new(TokenKinds::ELSE)
    elsif source.match(/\A\d+(\.\d+)?/)
      number = source.match(/\A\d+(\.\d+)?/)[0]
      @position += number.size
      Token.new(TokenKinds::NUMBER, number.to_f)
    elsif source[0] == '"'
      @position += 1
      string = String.new

      while !at_end && @source[@position] != '"'
        if @source[@position] == '\\'
          if @source[@position + 1] == '"'
            @position += 1
          elsif @source[@position + 1] == 'n'
            @position += 2
            string << "\n"
            next
          elsif @source[@position + 1] == 't'
            @position += 2
            string << "\t"
            next
          end
        end

        string << @source[@position]
        @position += 1
      end

      if at_end
        raise 'Unterminated string'
      elsif @source[@position] == '"'
        @position += 1
      end

      Token.new(TokenKinds::STRING, string)
    elsif source.match(/\A\:([a-z][a-zA-Z0-9_]*)/)
      atom = source.match(/\A\:([a-z][a-zA-Z0-9_]*)/)[1]
      @position += atom.size + 1
      Token.new(TokenKinds::ATOM, atom.to_sym)
    elsif source.match(/\A\+/)
      @position += 1
      Token.new(TokenKinds::OPERATOR, :+)
    elsif source.match(/\A\-/)
      @position += 1
      Token.new(TokenKinds::OPERATOR, :-)
    elsif source.match(/\A\*/)
      @position += 1
      Token.new(TokenKinds::OPERATOR, :*)
    elsif source.match(%r{\A\/})
      @position += 1
      Token.new(TokenKinds::OPERATOR, :/)
    elsif source.match(/\A\{/)
      @position += 1
      Token.new(TokenKinds::LBRACE)
    elsif source.match(/\A\}/)
      @position += 1
      Token.new(TokenKinds::RBRACE)
    elsif source.match(/\A\(/)
      @position += 1
      Token.new(TokenKinds::LPAREN)
    elsif source.match(/\A\)/)
      @position += 1
      Token.new(TokenKinds::RPAREN)
    elsif source.match(/\A\[/)
      @position += 1
      Token.new(TokenKinds::LBRACKET)
    elsif source.match(/\A\]/)
      @position += 1
      Token.new(TokenKinds::RBRACKET)
    elsif source.match(/\A\;/)
      @position += 1
      Token.new(TokenKinds::SEMICOLON)
    elsif source.match(/\A,/)
      @position += 1
      Token.new(TokenKinds::COMMA)
    elsif source.match(/\A\./)
      @position += 1
      Token.new(TokenKinds::DOT)
    elsif source.match(/\A=>/)
      @position += 2
      Token.new(TokenKinds::ROCKET)
    elsif source.match(/\A\#/)
      @position += 1
      Token.new(TokenKinds::HASH)
    elsif source.match(/\A!/)
      @position += 1
      Token.new(TokenKinds::OPERATOR, :!)
    elsif source.match(/\A==/)
      @position += 2
      Token.new(TokenKinds::OPERATOR, :==)
    elsif source.match(/\A\<=/)
      @position += 2
      Token.new(TokenKinds::OPERATOR, :<=)
    elsif source.match(/\A\>\=/)
      @position += 2
      Token.new(TokenKinds::OPERATOR, :>=)
    elsif source.match(/\A\</)
      @position += 1
      Token.new(TokenKinds::OPERATOR, :<)
    elsif source.match(/\A\>/)
      @position += 1
      Token.new(TokenKinds::OPERATOR, :>)
    elsif source.match(/\Aand/)
      @position += 3
      Token.new(TokenKinds::OPERATOR, :and)
    elsif source.match(/\Aor/)
      @position += 2
      Token.new(TokenKinds::OPERATOR, :or)
    elsif source.match(/\Anot/)
      @position += 3
      Token.new(TokenKinds::OPERATOR, :not)
    elsif source.match(/\A\=/)
      @position += 1
      Token.new(TokenKinds::EQUALS)
    elsif source.match(/\Alet/)
      @position += 3
      Token.new(TokenKinds::LET)
    elsif source.match(/\Afunction/)
      @position += 8
      Token.new(TokenKinds::FUNCTION)
    elsif source.match(/\Amethod/)
      @position += 6
      Token.new(TokenKinds::METHOD)
    elsif source.match(/\Aclassmethod/)
      @position += 11
      Token.new(TokenKinds::CLASSMETHOD)
    elsif source.match(/\Aclass/)
      @position += 5
      Token.new(TokenKinds::CLASS)
    elsif source.match(/\Apublic/)
      @position += 6
      Token.new(TokenKinds::PUBLIC)
    elsif source.match(/\Aprivate/)
      @position += 7
      Token.new(TokenKinds::PRIVATE)
    elsif source.match(/\Afor/)
      @position += 3
      Token.new(TokenKinds::FOR)
    elsif source.match(/\Ain/)
      @position += 2
      Token.new(TokenKinds::IN)
    elsif source.match(/\A[a-z][a-zA-Z0-9_]*/)
      identifier = source.match(/\A[a-z][a-zA-Z0-9_]*/)[0]
      @position += identifier.size
      Token.new(TokenKinds::IDENTIFIER, identifier)
    elsif source.match(/\A[A-Z][a-zA-Z0-9_]*/)
      class_name = source.match(/\A[A-Z][a-zA-Z0-9_]*/)[0]
      @position += class_name.size
      Token.new(TokenKinds::CLASS_NAME, class_name)
    else
      raise "Unrecognized character #{source[0]}"
    end
  end

  def peek
    position = @position
    token = get_token
    @position = position
    token
  end

  def scan_all
    tokens = Array.new
    tokens << get_token until at_end
    tokens << Token.new(TokenKinds::EOF)
  end

  private

  def skip_whitespace
    @position += 1 while !at_end && @source[@position].match(/\A\s/)
  end

  def skip_comment
    if @source.slice(@position..@position + 1) == '//'
      @position += 2
      @position += 1 until @source[@position] == "\n" || at_end
      @position += 1 unless at_end
    end
  end

  def at_end
    @position == @source.size
  end
end