Add support for reversable filename format,filename -> url

[wenerme]

Consider youtube-dl have a predefined registry file

{
  "registries": [
    {
      "format": "www.youtube.com/watch?v=%s"
    },
    {
      "format": "www.bilibili.com/video/av%d/"
    },
    {
      "format": "www.bilibili.com/video/av%d/index_%d.html"
    }
  ]
}

For https://www.youtube.com/watch?v=zKN_pnzIM7k match the registry id #0
Save

World's Most Emotional & Powerful Music _ 2-Hours Epic Music Mix - Vol.1-zKN_pnzIM7k.f137.mp4

to

World's Most Emotional & Powerful Music _ 2-Hours Epic Music Mix - Vol.1-0zKN_pnzIM7k.f137.mp4

Notice there is one more char 0,use base64-like,one char for 64bit,it's enough to handle registry id(maybe can use some bits for other use).

For http://www.bilibili.com/video/av2403515/ match the registry id #1
Save to

Don't care the name-12403515.mp4

Consider there is a command

> youtube-dl --url 'Don't care the name-12403515.mp4'
http://www.bilibili.com/video/av2403515/

I thinks this is very useful.

The registry order should never change even the url format was changed,by add more meta info to registry will give more useful infos.

[yan12125]

I think currently it's impossible. For most sites, it's impossible to infer the full URL from ID, as ID is only part of URL. For example, the following URL:

http://www.discovery.com/tv-shows/mythbusters/videos/mission-impossible-outtakes.htm

has ID mission-impossible-outtakes

For sites that URL is directly related to ID (for example youtube and bilibili as you've mentioned), I think write a simple script that maps IDs to URLs and feeds the URL to youtube-dl is enough. A new feature is added to youtube-dl only if it's impossible or too complicated to implement it outside youtube-dl.