fix(captions): match preferred language to auto-generated tracks

Preferred caption slots use names such as English, but many videos only
expose English (auto-generated) with language code en. Exact name matching
left those tracks unordered, so the player could auto-select another
auto-generated language.

Match preference names to regional and auto-generated variants, keep human
tracks ahead of auto-generated ones, and share the matcher on watch and
embed.

Fixes #6007

spec/invidious/videos/captions_matching_spec.cr

@@ -0,0 +1,36 @@
+require "../../spec_helper"
+
+Spectator.describe Invidious::Videos::Captions do
+  describe ".matching" do
+    it "matches English to English auto-generated ahead of other auto tracks" do
+      arabic = Invidious::Videos::Captions::Metadata.new("Arabic (auto-generated)", "ar", "http://x", true)
+      english_auto = Invidious::Videos::Captions::Metadata.new("English (auto-generated)", "en", "http://x", true)
+
+      matched = Invidious::Videos::Captions.matching([arabic, english_auto], ["English"])
+      expect(matched.map(&.name)).to eq(["English (auto-generated)"])
+    end
+
+    it "prefers a human English track over auto-generated English" do
+      english_auto = Invidious::Videos::Captions::Metadata.new("English (auto-generated)", "en", "http://x", true)
+      english = Invidious::Videos::Captions::Metadata.new("English", "en", "http://x", false)
+
+      matched = Invidious::Videos::Captions.matching([english_auto, english], ["English"])
+      expect(matched.map(&.name)).to eq(["English", "English (auto-generated)"])
+    end
+
+    it "matches regional English names and language codes" do
+      us = Invidious::Videos::Captions::Metadata.new("English (United States)", "en-US", "http://x", false)
+      uk = Invidious::Videos::Captions::Metadata.new("English (United Kingdom)", "en-GB", "http://x", false)
+      german = Invidious::Videos::Captions::Metadata.new("German (Germany)", "de-DE", "http://x", false)
+
+      matched = Invidious::Videos::Captions.matching([german, us, uk], ["English", "en"])
+      expect(matched.map(&.name)).to eq(["English (United States)", "English (United Kingdom)"])
+    end
+
+    it "ignores blank preference slots" do
+      arabic = Invidious::Videos::Captions::Metadata.new("Arabic (auto-generated)", "ar", "http://x", true)
+      matched = Invidious::Videos::Captions.matching([arabic], ["", "", ""])
+      expect(matched).to be_empty
+    end
+  end
+end

src/invidious/routes/embed.cr

@@ -182,14 +182,7 @@ module Invidious::Routes::Embed
 
     captions = video.captions
 
-    preferred_captions = captions.select { |caption|
-      params.preferred_captions.includes?(caption.name) ||
-        params.preferred_captions.includes?(caption.language_code.split("-")[0])
-    }
-    preferred_captions.sort_by! { |caption|
-      (params.preferred_captions.index(caption.name) ||
-        params.preferred_captions.index(caption.language_code.split("-")[0])).not_nil!
-    }
+    preferred_captions = Invidious::Videos::Captions.matching(captions, params.preferred_captions)
     captions = captions - preferred_captions
 
     aspect_ratio = nil

src/invidious/routes/watch.cr

@@ -149,14 +149,7 @@ module Invidious::Routes::Watch
 
     captions = video.captions
 
-    preferred_captions = captions.select { |caption|
-      params.preferred_captions.includes?(caption.name) ||
-        params.preferred_captions.includes?(caption.language_code.split("-")[0])
-    }
-    preferred_captions.sort_by! { |caption|
-      (params.preferred_captions.index(caption.name) ||
-        params.preferred_captions.index(caption.language_code.split("-")[0])).not_nil!
-    }
+    preferred_captions = Invidious::Videos::Captions.matching(captions, params.preferred_captions)
     captions = captions - preferred_captions
 
     aspect_ratio = "16:9"

src/invidious/videos/caption.cr

@@ -86,6 +86,49 @@ module Invidious::Videos
       end
     end
 
+    # Tracks whose name or language matches the user's caption preferences,
+    # ranked by preference slot then human tracks before auto-generated.
+    def self.matching(captions : Array(Metadata), names : Array(String)) : Array(Metadata)
+      wanted = names.map(&.strip).reject(&.empty?)
+      return [] of Metadata if wanted.empty?
+
+      selected = captions.select { |caption| matches?(caption, wanted) }
+      selected.sort_by! { |caption| rank(caption, wanted) }
+      selected
+    end
+
+    def self.matches?(caption : Metadata, names : Array(String)) : Bool
+      names.any? { |name| name_matches?(caption, name) }
+    end
+
+    private def self.name_matches?(caption : Metadata, name : String) : Bool
+      needle = name.strip.downcase
+      return false if needle.empty?
+
+      caption_name = caption.name.downcase
+      lang = caption.language_code.downcase
+      base_lang = lang.split("-")[0]
+
+      caption_name == needle ||
+        caption_name.starts_with?(needle + " (") ||
+        caption_name.starts_with?(needle + " - ") ||
+        lang == needle ||
+        base_lang == needle
+    end
+
+    private def self.rank(caption : Metadata, names : Array(String)) : Tuple(Int32, Int32)
+      pref_rank = names.size
+      names.each_with_index do |name, index|
+        if name_matches?(caption, name)
+          pref_rank = index
+          break
+        end
+      end
+
+      auto_rank = caption.auto_generated ? 1 : 0
+      {pref_rank, auto_rank}
+    end
+
     # List of all caption languages available on Youtube.
     LANGUAGES = {
       "",