getting_to_philosophy.rb

require 'open-uri'
require 'rexml/document'

module GettingToPhilosophy
  module Crawler
    def self.crawl(*trail)
      until trail.last == Wikipedia::PHILOSOPHY || trail.uniq.size != trail.size
        puts trail.last

        doc = Wikipedia.article(trail.last)
        links = doc.elements.to_a('//div[@id="mw-content-text"]/p//a')
        lowercase_links = links.select {|link|
          link.text =~ /^[[:lower:]]+$/
        }
        return trail if lowercase_links.empty?

        href = lowercase_links.first.attribute(:href)
        trail << URI.join(trail.last, href.value).to_s
      end

      trail
    end
  end

  module Wikipedia
    BASE = 'https://en.wikipedia.org/wiki'
    SPECIAL_RANDOM = "#{BASE}/Special:Random"
    PHILOSOPHY = "#{BASE}/Philosophy"

    def self.random_article
      article(SPECIAL_RANDOM)
        .elements['//link[@rel="canonical"]']
        .attribute(:href).value
    end

    def self.article(article)
      html = open(article.to_s).read
      REXML::Document.new(html)
    end
  end
end

if __FILE__ == $0
  include GettingToPhilosophy

  article = ARGV.shift
  article = article ? "#{Wikipedia::BASE}/#{article}" : Wikipedia.random_article

  p Crawler.crawl(article)
end