search: make order:random truly random; add random:N metatag.

Make the `order:random` metatag truly randomize the search. Add a
`random:N` metatag that returns up to N random posts, like what
`order:random` did before.

`order:random` now returns the entire search in random order. Before it
just returned a pageful of pseudorandom posts. This will be more
accurate for small searches, but slower for large searches. If
`order:random` times out, try `random:N` instead.

The `random:N` metatag returns up to N pseudorandom posts. This is
faster than `order:random` for large searches, but for small searches,
it may return less than N posts, and the randomness may be biased. Some
posts may be more likely than others to appear. N must be between 0 and
200.

Also, `/posts?tags=touhou&random=1` now redirects to `/posts?tags=touhou+random:N`.
Before the `random=1` param acted like a free `order:random` tag; now it
redirects to a `random:N` search, so it counts against your tag limit.

app/controllers/posts_controller.rb

@@ -8,9 +8,13 @@ class PostsController < ApplicationController
       respond_with(@post) do |format|
         format.html { redirect_to(@post) }
       end
+    elsif params[:random].to_s.truthy?
+      post_set = PostSets::Post.new(params[:tags], params[:page], params[:limit], format: request.format.symbol, view: params[:view])
+      query = "#{post_set.normalized_query.to_s} random:#{post_set.per_page}".strip
+      redirect_to posts_path(tags: query, page: params[:page], limit: params[:limit], format: request.format.symbol, view: params[:view])
     else
       tag_query = params[:tags] || params.dig(:post, :tags)
-      @post_set = PostSets::Post.new(tag_query, params[:page], params[:limit], random: params[:random], format: params[:format], view: params[:view])
+      @post_set = PostSets::Post.new(tag_query, params[:page], params[:limit], format: request.format.symbol, view: params[:view])
       @posts = authorize @post_set.posts, policy_class: PostPolicy
       @post_set.log!
       respond_with(@posts) do |format|

app/logical/post_query_builder.rb

@@ -38,7 +38,7 @@ class PostQueryBuilder
     ordpool note comment commentary id rating source status filetype
     disapproved parent child search embedded md5 width height mpixels ratio
     score upvotes downvotes favcount filesize date age order limit tagcount pixiv_id pixiv
-    unaliased exif duration
+    unaliased exif duration random
   ] + COUNT_METATAGS + COUNT_METATAG_SYNONYMS + CATEGORY_COUNT_METATAGS
 
   ORDER_METATAGS = %w[
@@ -218,6 +218,8 @@ class PostQueryBuilder
       user_subquery_matches(PostVote.active.positive.visible(current_user), value, field: :user)
     when "downvoter", "downvote"
       user_subquery_matches(PostVote.active.negative.visible(current_user), value, field: :user)
+    when "random"
+      Post.all # handled in the `build` method
     when *CATEGORY_COUNT_METATAGS
       short_category = name.delete_suffix("tags")
       category = TagCategory.short_name_mapping[short_category]
@@ -513,6 +515,11 @@ class PostQueryBuilder
       relation = search_order(relation, find_metatag(:order))
     end
 
+    if count = find_metatag(:random)
+      count = Integer(count).clamp(0, PostSets::Post::MAX_PER_PAGE)
+      relation = relation.random(count)
+    end
+
     relation
   end
 
@@ -688,6 +695,9 @@ class PostQueryBuilder
     when /(#{TagCategory.short_name_list.join("|")})tags_asc/
       relation = relation.order("posts.tag_count_#{TagCategory.short_name_mapping[$1]} ASC")
 
+    when "random"
+      relation = relation.order("random()")
+
     when "rank"
       relation = relation.where("posts.score > 0 and posts.created_at >= ?", 2.days.ago)
       relation = relation.order(Arel.sql("log(3, posts.score) + (extract(epoch from posts.created_at) - extract(epoch from timestamp '2005-05-24')) / 35000 DESC"))

app/logical/post_sets/post.rb

@@ -7,16 +7,15 @@ module PostSets
     MAX_PER_PAGE = 200
     MAX_SIDEBAR_TAGS = 25
 
-    attr_reader :page, :random, :format, :tag_string, :query, :normalized_query, :view
+    attr_reader :page, :format, :tag_string, :query, :normalized_query, :view
     delegate :post_count, to: :normalized_query
 
-    def initialize(tags, page = 1, per_page = nil, user: CurrentUser.user, random: false, format: "html", view: "simple")
+    def initialize(tags, page = 1, per_page = nil, user: CurrentUser.user, format: "html", view: "simple")
       @query = PostQueryBuilder.new(tags, user, tag_limit: user.tag_query_limit, safe_mode: CurrentUser.safe_mode?, hide_deleted_posts: user.hide_deleted_posts?)
       @normalized_query = query.normalized_query
       @tag_string = tags
       @page = page
       @per_page = per_page
-      @random = random.to_s.truthy?
       @format = format.to_s
       @view = view.presence || "simple"
     end
@@ -92,25 +91,11 @@ module PostSets
     end
 
     def max_per_page
-      (format == "sitemap") ? 10_000 : MAX_PER_PAGE
-    end
-
-    def is_random?
-      random || query.find_metatag(:order) == "random"
-    end
-
-    def get_random_posts
-      ::Post.user_tag_match(tag_string).random(per_page)
+      (format.to_sym == :sitemap) ? 10_000 : MAX_PER_PAGE
     end
 
     def posts
-      @posts ||= begin
-        if is_random?
-          get_random_posts.paginate(page, search_count: false, limit: per_page, max_limit: max_per_page).load
-        else
-          normalized_query.paginated_posts(page, includes: includes, count: post_count, search_count: !post_count.nil?, limit: per_page, max_limit: max_per_page).load
-        end
-      end
+      @posts ||= normalized_query.paginated_posts(page, includes: includes, count: post_count, search_count: !post_count.nil?, limit: per_page, max_limit: max_per_page).load
     end
 
     def hide_from_crawler?

app/views/posts/partials/index/_posts.html.erb

@@ -36,7 +36,5 @@
     <% end %>
   <% end %>
 
-  <% unless post_set.is_random? %>
-    <%= numbered_paginator(post_set.posts) %>
-  <% end %>
+  <%= numbered_paginator(post_set.posts) %>
 </div>