Use FLAC tag names in FLAC metadata

In #183 @graue pointed out that FLAC has its own tag names [1,2]. This
changes the ffmpeg transcoding for FLAC tracks to use those names.
However, it's unclear to me whether we also need to change the encoding
of the metadata - currently I've left the `-write_id3v2 1` &
`-id3v2_version 3` ffmpeg options in place on the basis that @graue is
already able to view the tags on a FLAC track; it was just that the tag
names were incorrect.

An initial search of the internet suggests that it might be better to
use the `metaflac` tool [3] to add metadata to a track, but that would
be a moderately significant change and it would probably be worth
changing MP3 tagging to work in a similar way, i.e. with a specialist
command line tool rather than just using ffmpeg.

Fixes #183.

[1]: https://xiph.org/flac/faq.html#general__tagging
[2]: https://xiph.org/vorbis/doc/v-comment.html
[3]: https://xiph.org/flac/documentation_tools_metaflac.html

app/models/transcode_command.rb

@@ -8,6 +8,13 @@ class TranscodeCommand
     artist_name: 'TPE1'
   }.freeze
 
+  METADATA_KEYS_VS_FLAC_TAGS = {
+    track_title: 'TITLE',
+    track_number: 'TRACKNUMBER',
+    album_title: 'ALBUM',
+    artist_name: 'ARTIST'
+  }.freeze
+
   def initialize(input, output, format, metadata = {}, image = nil)
     @input = input
     @output = output
@@ -69,10 +76,21 @@ def transcoding_options(format)
   end
 
   def id3v23_keys
-    METADATA_KEYS_VS_ID3V23_TAGS.keys
+    metadata_lookup.keys
   end
 
   def id3v23_key_for(key)
-    METADATA_KEYS_VS_ID3V23_TAGS[key]
+    metadata_lookup[key]
+  end
+
+  def metadata_lookup
+    case @format.to_s
+    when /^mp3/
+      METADATA_KEYS_VS_ID3V23_TAGS
+    when /^flac/
+      METADATA_KEYS_VS_FLAC_TAGS
+    else
+      raise ArgumentError, "unsupported format: #{@format}"
+    end
   end
 end

test/models/transcode_command_test.rb

@@ -42,7 +42,7 @@ class TranscodeCommandTest < ActiveSupport::TestCase
     assert_contains_pair(command_string, ['-c', 'copy'])
   end
 
-  test 'adds metadata using ID3v2.3 format' do
+  test 'adds ID3 tags using ID3v2.3 format for MP3 formats' do
     metadata = {
       track_title: 'track-title',
       track_number: 'track-number',
@@ -58,6 +58,22 @@ class TranscodeCommandTest < ActiveSupport::TestCase
     assert_contains_pair(command_string, ['-metadata', 'TPE1="artist-name"'])
   end
 
+  test 'adds FLAC tags using ID3v2.3 format for FLAC formats' do
+    metadata = {
+      track_title: 'track-title',
+      track_number: 'track-number',
+      album_title: 'album-title',
+      artist_name: 'artist-name'
+    }
+    command_string = TranscodeCommand.new(@input, @output, :flac, metadata).generate
+    assert_contains_pair(command_string, ['-write_id3v2', '1'])
+    assert_contains_pair(command_string, ['-id3v2_version', '3'])
+    assert_contains_pair(command_string, ['-metadata', 'TITLE="track-title"'])
+    assert_contains_pair(command_string, ['-metadata', 'TRACKNUMBER="track-number"'])
+    assert_contains_pair(command_string, ['-metadata', 'ALBUM="album-title"'])
+    assert_contains_pair(command_string, ['-metadata', 'ARTIST="artist-name"'])
+  end
+
   test 'transcodes audio to mp3 using libmp3lame codec highest audio quality' do
     command_string = TranscodeCommand.new(@input, @output, :mp3v0).generate
     assert_contains_pair(command_string, ['-codec:a', 'libmp3lame'])