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

    def current_char
      @source[@position]
    end

    def advance(step = 1)
      @position += step
    end

    def get_next_token
      while @source.slice(@position..-1).match(/^\s/)
        advance
      end

      case current_char
      when nil
        Token.new(:eof, "eof")
      when '('
        advance
        Token.new(:lparen, "(")
      when ')'
        advance
        Token.new(:rparen, ")")
      when '#'
        advance
        if current_char == 't'
          advance
          Token.new(:boolean, true)
        elsif current_char == 'f'
          advance
          Token.new(:boolean, false)
        else
          raise "Unexpected character #{current_char}"
        end
      when '\''
        advance
        Token.new(:quote, '\'')
      when '"'
        delimiter = current_char
        advance
        string = String.new
        until current_char == delimiter || current_char.nil?
          string << current_char
          advance
        end

        if current_char.nil?
          return Error.new("Unterminated string")
        end

        advance

        Token.new(:string, string)
      else
        source = @source.slice(@position..-1)
        if match = source.match(/^[0-9]+(\.[0-9]+)?/)
          advance(match[0].size)
          Token.new(:number, match[0])
        elsif match = source.match(/^[a-z!$%&*\/:<=>?~_^+\-][a-z0-9@!$%&*\/:<=>?~_^+\-]*/)
          advance(match[0].size)
          Token.new(:identifier, match[0])
        else
          raise "Unrecognized character #{current_char}"
        end
      end
    end

    def tokenize
      tokens = Array.new
      loop do
        token = get_next_token
        tokens << token unless token.nil?
        break if token.type == :eof
      end
      tokens
    end
  end
end