#!/usr/bin/ruby
#
# Download lecture videos of courses from Coursera (http://www.coursera.org).
#
# Install requirements:
# 
#   $ gem install curb trollop nokogiri capybara ruby-progressbar
#
# Example -- Download all video lectures of courses "Calculus: Single Variable" 
# and "Introduction to Astronomy":
#
#   $ ruby coursera-download-videos.rb |
#       --email=user@server.org --password=123456 \
#       "Calculus: Single Variable" "Introduction to Astronomy"
#
# Contact: tokland@gmail.com

require 'rubygems'
require 'fileutils'
require 'logger'
require 'set'

require 'progressbar'
require 'capybara'
require 'nokogiri'
require 'trollop'
require 'curl'

### Extensions

class Hash
  # Keep only given keys in hash
  def slice(*keys)
    keys_set = keys.to_set
    select { |k, v| keys_set.include?(k) }
  end

  # Reverse update hash with a given default hash (performs key checking)
  def defaults(default_hash)
    unknown_options = self.keys - default_hash.keys
    if unknown_options.empty? 
      replace(default_hash.merge(self))
    else
      raise ArgumentError.new("unknown key(s): #{unknown_options.join(', ')}")
    end
  end
end

class MaybeWrapper
  instance_methods.each { |m| undef_method(m) unless m == :object_id || m =~ /^__/ }

  def method_missing(*args, &block)
    nil
  end
end

class Object
  def maybe(&block)
    if block_given?
      nil? ? nil : yield(self)  
    else
      nil? ? MaybeWrapper.new : self
    end
  end
end

module Curl
  def self.download_to_file(url, destination, options = {})
    options.defaults(:progressbar => false, :cookies => [])
    
    open(destination, "wb") do |fd|
      curl = Curl::Easy.new(url).tap do |c|
        c.follow_location = true
        c.enable_cookies = true
        if options[:cookies]
          c.headers["Cookie"] = options[:cookies].join("; ")
        end
        c.on_body { |data| fd.write(data) }
      end
      
      if options[:progressbar]
        title = File.basename(destination, File.extname(destination))
        state = {:pbar => nil, :dl_total => nil}
        
        curl.on_progress do |dl_total, dl_now, ul_total, ul_now|
          if dl_total > 0 
            if !state[:pbar] || (!state[:dl_total] || dl_total > state[:pbar]) 
              state[:dl_total] = dl_total
              state[:pbar] = ProgressBar.new(title, dl_total)
              state[:pbar].format_arguments =
                [:title, :percentage, :bar, :stat_for_file_transfer]
            end
            state[:pbar].set(dl_now)
          end
          true
        end
        curl.perform
        state[:pbar].finish if state[:pbar]
      else
        curl.perform
      end
    end
  end
end

class CapybaraBrowser
  include Capybara::DSL
  
  def initialize(options = {})
    options.defaults(:driver => :selenium, :default_wait_time => 60)
    Capybara.current_driver = options.fetch(:driver)
    Capybara.default_wait_time = options.fetch(:default_wait_time)
    Capybara.run_server = false
  end

  def cookies
    driver = Capybara.current_session.driver 
    case driver.class.name
    when "Capybara::Webkit::Driver" 
      driver.browser.get_cookies.map { |s| s.split(";").first }
    when "Capybara::Selenium::Driver"
      driver.browser.manage.all_cookies.map { |c| [c[:name], c[:value]].join("=") }
    else
      raise ArgumentError.new("Unsupported driver: #{driver.class.name}") 
    end
  end
end

### Application

class Coursera
  attr_reader :browser, :logger
  SessionError = Class.new(StandardError) 
  ParserError = Class.new(StandardError)
  
  def initialize(options = {})
    @browser = CapybaraBrowser.new
    @logger = Logger.new(STDERR)
    @logger.formatter = proc { |severity, datetime, progname, msg| "#{msg}\n" }
  end
  
  def login(email, password)
    logger.debug("Login: email='#{email}' password='#{'*' * password.size}'")
    browser.visit("https://www.coursera.org/account/signin")
    browser.fill_in("signin-email", :with => email)
    browser.fill_in("signin-password", :with => password)
    browser.click_button("Sign In")
    browser.find(".coursera-header-account-name")
  end
  
  def download_videos(course_name, options = {})
    options.defaults(:destination_directory => nil)
    browser.visit("https://www.coursera.org/")
    
    if !browser.find(".coursera-header-account-name")
      raise SessionError.new("Not logged in")
    else
      browser.click_link(course_name)
      browser.find(:xpath, 
        '//*[@class="course-navbar-item"]/a[contains(@href, "lecture/index")]').click
      browser.find("#spark")
      get_videos_from_course_page(browser.html).map do |info|
        course, section, index, lecture, url = info
        directory = File.join([options[:destination_directory], course, section].compact)
        path = File.join(directory, "%02d - %s.mp4" % [index+1, lecture])
        logger.debug("Download video: #{path}")
        safe_download(url, path, browser.cookies)
        path
      end
    end     
  end

  private

  def get_videos_from_course_page(html)
    doc = Nokogiri::HTML(html)
    course_name = doc.at_css("h1").maybe.text or
      raise ParserError.new("Cannot find course name")
    logger.debug("Course '#{course_name}'")
    
    # Some courses have the videos in reverse order, detect this case 
    # with a simple heuristics (check if the first integers in the section 
    # titles are in descending order).
    lis = doc.css(".course-item-list-header")
    ns = lis.map { |li| li.text.match(/\d+/).maybe[0].maybe.to_i }.compact
    is_reversed = !ns.empty? && ns.each_cons(2).all? { |x, y| x > y }
    ordered_lis = is_reversed ? lis.reverse : lis
    
    ordered_lis.flat_map.with_index do |section, section_index|
      h3 = section.at_css("h3") or
        raise ParserError.new("Cannot find h3")
      section_title = h3.text.gsub(/[[:space:]]+/, ' ').strip
      section_name = "%02d - %s" % [section_index+1, section_title]
      lecture_video_list = section.next or
        raise ParserError.new("Cannt find lecture video list")
        
      lecture_video_list.css("li").map.with_index do |lecture, index|
        link = lecture.at_css("a.lecture-link") or
          raise ParserError.new("Cannot find lecture link")
        lecture_title = link.text.gsub("/", "-").strip
        url = lecture.at_css("a[title='Video (MP4)']").maybe["href"] or
          raise ParserError.new("Cannot find video link")
        [course_name, section_name, index, lecture_title, url]
      end
    end
  end

  def safe_download(url, path, cookies)
    unless File.exists?(path)
      FileUtils.mkpath(File.dirname(path))
      temp_path = path + ".partial"
      begin
        Curl.download_to_file(url, temp_path, :progressbar => true, :cookies => cookies)
        FileUtils.copy(temp_path, path)
      ensure
        FileUtils.safe_unlink(temp_path)
      end
    end
  end
end

if __FILE__ == $0
  options = Trollop.options do
    banner "Usage: download-coursera-videos [OPTIONS] COURSENAME [COURSENAME2 ...]"
    
    opt :destination_directory, "Directory destination", :type => :string
    opt :email, "Email", :type => :string, :required => true
    opt :password, "Password", :type => :string, :required => true
  end
  
  if ARGV.empty?
    Trollop.die("Specify at least one course to download")
  else
    coursera = Coursera.new
    coursera.login(options[:email], options[:password])
    download_options = options.slice(:destination_directory)
    ARGV.each do |course_name|
      coursera.download_videos(course_name, download_options)
    end
  end
end