#302

backend/app/services/theatre_post_selector.rb

@@ -0,0 +1,92 @@
+class TheatrePostSelector
+  Candidate = Struct.new(:post, :weight, :penalty, :tags, keyword_init: true)
+
+  def initialize(theatre:)
+    @theatre = theatre
+  end
+
+  def select
+    candidates = weighted_candidates
+    return nil if candidates.empty?
+
+    total = candidates.sum(&:weight)
+    target = rand * total
+
+    candidates.each do |candidate|
+      target -= candidate.weight
+      return candidate.post if target <= 0
+    end
+
+    candidates.last.post
+  end
+
+  def weight_json(limit: 20)
+    candidates = weighted_candidates
+    sorted = candidates.sort_by { |candidate| [candidate.weight, candidate.post.id] }
+
+    { tag_penalties: tag_penalty_json,
+      lightest_posts: post_weight_json(sorted.first(limit)),
+      heaviest_posts: post_weight_json(sorted.reverse.first(limit)) }
+  end
+
+  private
+
+  attr_reader :theatre
+
+  def weighted_candidates
+    @weighted_candidates ||= begin
+      penalties = tag_penalties
+      posts = eligible_posts.includes(tags: :tag_name).to_a
+
+      posts.map do |post|
+        post_tags = post.tags.to_a
+        penalty = post_tags.sum { |tag| penalties[tag.id].to_i }
+        Candidate.new(post:, penalty:, tags: post_tags, weight: 1.0 / (1.0 + penalty))
+      end
+    end
+  end
+
+  def eligible_posts
+    posts = Post.where("url LIKE '%nicovideo.jp%'")
+    posts = posts.where.not(id: theatre.current_post_id) if theatre.current_post_id
+    posts
+  end
+
+  def active_user_ids
+    @active_user_ids ||= theatre.watching_users.ids
+  end
+
+  def tag_penalties
+    @tag_penalties ||=
+        if active_user_ids.empty?
+          {}
+        else
+          TheatreSkipEventVoter
+          .joins(theatre_skip_event: :event_tags)
+          .where(user_id: active_user_ids)
+          .group('theatre_skip_event_tags.tag_id')
+          .count
+        end
+  end
+
+  def tag_penalty_json
+    return [] if tag_penalties.empty?
+
+    tags = Tag.where(id: tag_penalties.keys).includes(:tag_name).index_by(&:id)
+    tag_penalties.map { |tag_id, penalty|
+      tag = tags[tag_id]
+      next unless tag
+
+      { tag: TagRepr.inline(tag), penalty: }
+    }.compact.sort_by { |row| [-row[:penalty], row[:tag]['name'].to_s] }
+  end
+
+  def post_weight_json(candidates)
+    candidates.map { |candidate|
+      { post: PostRepr.base(candidate.post),
+        weight: candidate.weight,
+        penalty: candidate.penalty,
+        tags: candidate.tags.map { |tag| TagRepr.inline(tag) } }
+    }
+  end
+end