module RegexpExamples
  class Parser
    attr_reader :regexp_string
    def initialize(regexp_string)
      @regexp_string = regexp_string
      @num_groups = 0
      @current_position = 0
    end

    def parse
      repeaters = []
      while @current_position < regexp_string.length
        group = parse_group(repeaters)
        break if group.is_a? MultiGroupEnd
        repeaters = [] if group.is_a? OrGroup
        @current_position += 1
        repeaters << parse_repeater(group)
      end
      repeaters
    end

    private

    def parse_group(repeaters)
      char = regexp_string[@current_position]
      case char
      when '('
        group = parse_multi_group
      when ')'
        group = parse_multi_end_group
      when '['
        group = parse_char_group
      when '.'
        group = parse_dot_group
      when '|'
        group = parse_or_group(repeaters)
      when '\\'
        group = parse_after_backslash_group
      else
        group = parse_single_char_group(char)
      end
      group
    end

    def parse_after_backslash_group
      @current_position += 1
      case
      when rest_of_string =~ /\A(\d+)/
        @current_position += ($1.length - 1) # In case of 10+ backrefs!
        group = parse_backreference_group($1)
      when rest_of_string =~ /\Ak<([^>]+)>/ # Named capture group
        @current_position += ($1.length + 2)
        group = parse_backreference_group($1)
      when BackslashCharMap.keys.include?(regexp_string[@current_position])
        group = CharGroup.new(
          BackslashCharMap[regexp_string[@current_position]])
        # TODO: There are also a bunch of multi-char matches to watch out for:
        # http://en.wikibooks.org/wiki/Ruby_Programming/Syntax/Literals
      else
        group = parse_single_char_group( regexp_string[@current_position] )
        # TODO: What about cases like \A, \z, \Z ?
      end
      group
    end

    def parse_repeater(group)
      char = regexp_string[@current_position]
      case char
      when '*'
        repeater = parse_star_repeater(group)
      when '+'
        repeater = parse_plus_repeater(group)
      when '?'
        repeater = parse_question_mark_repeater(group)
      when '{'
        repeater = parse_range_repeater(group)
      else
        repeater = parse_one_time_repeater(group)
      end
      repeater
    end

    def parse_multi_group
      @current_position += 1
      @num_groups += 1
      group_id = nil # init
      rest_of_string.match(/\A(\?)?(:|!|=|<(!|=|[^!=][^>]*))?/) do |match|
        case
        when match[1].nil? # e.g. /(normal)/
          group_id = @num_groups.to_s
        when match[2] == ':' # e.g. /(?:nocapture)/
          @current_position += 2
          group_id = nil
        when %w(! =).include?(match[2]) # e.g. /(?=lookahead)/, /(?!neglookahead)/
          # TODO: Raise exception
        when %w(! =).include?(match[3]) # e.g. /(?<=lookbehind)/, /(?<!neglookbehind)/
          # TODO: Raise exception
        else # e.g. /(?<name>namedgroup)/
          @current_position += (match[3].length + 3)
          group_id = match[3]
        end
      end
      groups = parse
      MultiGroup.new(groups, group_id)
    end

    def parse_multi_end_group
      MultiGroupEnd.new
    end

    def parse_char_group
      chars = []
      @current_position += 1
      if regexp_string[@current_position] == ']'
        # Beware of the sneaky edge case:
        # /[]]/ (match "]")
        chars << ']'
        @current_position += 1
      end
      until regexp_string[@current_position] == ']' \
        && !regexp_string[0..@current_position-1].match(/[^\\](\\{2})*\\\z/)
        # Beware of having an ODD number of "\" before the "]", e.g.
        # /[\]]/ (match "]")
        # /[\\]/ (match "\")
        # /[\\\]]/ (match "\" or "]")
        chars << regexp_string[@current_position]
        @current_position += 1
      end
      CharGroup.new(chars)
    end

    def parse_dot_group
      DotGroup.new
    end

    def parse_or_group(left_repeaters)
      @current_position += 1
      right_repeaters = parse
      OrGroup.new(left_repeaters, right_repeaters)
    end


    def parse_single_char_group(char)
      SingleCharGroup.new(char)
    end

    def parse_backreference_group(match)
      BackReferenceGroup.new(match)
    end

    def parse_star_repeater(group)
      @current_position += 1
      StarRepeater.new(group)
    end

    def parse_plus_repeater(group)
      @current_position += 1
      PlusRepeater.new(group)
    end

    def parse_question_mark_repeater(group)
      @current_position += 1
      QuestionMarkRepeater.new(group)
    end

    def parse_range_repeater(group)
      match = rest_of_string.match(/\A\{(\d+)(,)?(\d+)?\}/)
      @current_position += match[0].size
      min = match[1].to_i if match[1]
      has_comma = !match[2].nil?
      max = match[3].to_i if match[3]
      RangeRepeater.new(group, min, has_comma, max)
    end

    def parse_one_time_repeater(group)
      OneTimeRepeater.new(group)
    end

    def rest_of_string
      regexp_string[@current_position..-1]
    end
  end
end