Getting key value in nested Hash by key array values

Question

Sample hash:

  {
    "audio" =>  {
      "audio/aac" => ["aac"],
      "audio/mpeg" => ["mp3", "mp2"],
      "audio/mp4" => ["m4a", "m4b", "m4r", "3gp"],
      "audio/ogg" => ["ogg", "oga"],
      "audio/flac" => ["flac"],
      "audio/speex" => ["spx"],
      "audio/x-ms-wma" => ["wma"],
      "audio/x-pn-realaudio" => ["rm", "ram"],
      "audio/vnd.wave" => ["wav"],
      "audio/x-musepack" => ["mpc", "mp+", "mpp"],
      "audio/x-aiff" => ["aiff", "aif", "aifc"],
      "audio/x-tta" => ["tta"]
    },
    "video" =>  {
      "video/mp4" => ["mp4"],
      "video/mpeg" => ["mpg", "mpeg"],
      "video/x-m4v" => ["m4v"],
      "video/quicktime" => ["mov"],
      "video/x-msvideo" => ["avi"],
      "video/x-flv" => ["flv"],
      "video/webm" => ["webm"]
    }
  }

What's the best way given a file extension to get the associated content type (first match is okay)?

Searching for "flac" should return "audio/flac".

Currently I'm using this:

hsh.each_key do |group|
  hsh[group].each do |k,v|
    return k if v.include?(extension)
  end
end

the Tin Man · Accepted Answer · 2012-04-30 06:08:42Z

4

Unraveling that sort of structure is best done when it's created. But, you can loop through the various levels and get something useful from it. If I assign your initial hash to mime_hash I can unravel it using:

Hash[*mime_hash.map{ |av, types| types.map{ |mime_type, extensions| extensions.product([mime_type]) } }.flatten]

or more verbosely:

Hash[
  *mime_hash.map{ |av, types| 
    types.map{ |mime_type, extensions| 
      extensions.product([mime_type]) 
    } 
  }.flatten
]

Which will return:

{
     "aac" => "audio/aac",
     "mp3" => "audio/mpeg",
     "mp2" => "audio/mpeg",
     "m4a" => "audio/mp4",
     "m4b" => "audio/mp4",
     "m4r" => "audio/mp4",
     "3gp" => "audio/mp4",
     "ogg" => "audio/ogg",
     "oga" => "audio/ogg",
    "flac" => "audio/flac",
     "spx" => "audio/speex",
     "wma" => "audio/x-ms-wma",
      "rm" => "audio/x-pn-realaudio",
     "ram" => "audio/x-pn-realaudio",
     "wav" => "audio/vnd.wave",
     "mpc" => "audio/x-musepack",
     "mp+" => "audio/x-musepack",
     "mpp" => "audio/x-musepack",
    "aiff" => "audio/x-aiff",
     "aif" => "audio/x-aiff",
    "aifc" => "audio/x-aiff",
     "tta" => "audio/x-tta",
     "mp4" => "video/mp4",
     "mpg" => "video/mpeg",
    "mpeg" => "video/mpeg",
     "m4v" => "video/x-m4v",
     "mov" => "video/quicktime",
     "avi" => "video/x-msvideo",
     "flv" => "video/x-flv",
    "webm" => "video/webm"
}

answered Apr 30, 2012 at 6:08

the Tin Man

161k44 gold badges222 silver badges308 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Andrew Marshall Over a year ago

Nice, didn't know about Array#product.

tokland Over a year ago

map + flatten(1) = flat_map. Hash[...] takes pairs, no need to do a total flatten.

David Underwood · Accepted Answer · 2012-04-30 05:38:34Z

0

As you've already realized the data structure you have is horrible to search in the fashion you want. Assuming you're going to be searching the same data over and over what you should do is create an index for it.

There are many ways of doing this but the simplest is probably just to flatten the hash and invert it so that your keys become values and vice-versa. That way you can simply search it by calling content_types['flac']

A section of the example hash might end up like this:

{
  "aac" => "audio/aac",
  "mp3" => "audio/mpeg",
  "mp2" => "audio/mpeg",
  "m4a" => "audio/mp4",
  "m4b" => "audio/mp4",
  "m4r" => "audio/mp4",
  "3gp" => "audio/mp4",
  "flac" => "audio/flac"
}

answered Apr 30, 2012 at 5:38

David Underwood

4,9761 gold badge21 silver badges25 bronze badges

1 Comment

kain Over a year ago

Yes, the structure is not ideal, but it serves mainly for other purposes (hence the audio/video groups) in the app, and was not meant to be asked for mime type resolution by ext.

TheRealKingK · Accepted Answer · 2012-04-30 05:40:37Z

0

Try using rassoc()

Definition:

Searches through the hash comparing obj with the value using ==. Returns the first key-value pair (two-element array) that matches. See also Array#rassoc.

a = {1=> "one", 2 => "two", 3 => "three", "ii" => "two"}
a.rassoc("two")    #=> [2, "two"]
a.rassoc("four")   #=> nil

answered Apr 30, 2012 at 5:40

TheRealKingK

8397 silver badges14 bronze badges

1 Comment

kain Over a year ago

I'm afraid that doesn't quite work as expected on this kind of hash. If the keys are an array it becomes a problem since hash["audio"].rassoc(["m4b"]) won't return anything. Do you have an example?

Collectives™ on Stack Overflow

Getting key value in nested Hash by key array values

3 Answers 3

2 Comments

1 Comment

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

2 Comments

1 Comment

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related