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diff --git a/lib/pleroma/activity/ir/topics.ex b/lib/pleroma/activity/ir/topics.ex
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+# Pleroma: A lightweight social networking server
+# Copyright © 2017-2022 Pleroma Authors <https://pleroma.social/>
+# SPDX-License-Identifier: AGPL-3.0-only
+
+defmodule Pleroma.Activity.Ir.Topics do
+  alias Pleroma.Object
+  alias Pleroma.Web.ActivityPub.Visibility
+
+  def get_activity_topics(activity) do
+    activity
+    |> Object.normalize(fetch: false)
+    |> generate_topics(activity)
+    |> List.flatten()
+  end
+
+  defp generate_topics(%{data: %{"type" => "ChatMessage"}}, %{data: %{"type" => "Delete"}}) do
+    ["user", "user:pleroma_chat"]
+  end
+
+  defp generate_topics(%{data: %{"type" => "ChatMessage"}}, %{data: %{"type" => "Create"}}) do
+    []
+  end
+
+  defp generate_topics(%{data: %{"type" => "Answer"}}, _) do
+    []
+  end
+
+  defp generate_topics(object, activity) do
+    ["user", "list"] ++ visibility_tags(object, activity)
+  end
+
+  defp visibility_tags(object, %{data: %{"type" => type}} = activity) when type != "Announce" do
+    case Visibility.get_visibility(activity) do
+      "public" ->
+        if activity.local do
+          ["public", "public:local"]
+        else
+          ["public"]
+        end
+        |> item_creation_tags(object, activity)
+
+      "local" ->
+        ["public:local"]
+        |> item_creation_tags(object, activity)
+
+      "direct" ->
+        ["direct"]
+
+      _ ->
+        []
+    end
+  end
+
+  defp visibility_tags(_object, _activity) do
+    []
+  end
+
+  defp item_creation_tags(tags, object, %{data: %{"type" => "Create"}} = activity) do
+    tags ++
+      remote_topics(activity) ++ hashtags_to_topics(object) ++ attachment_topics(object, activity)
+  end
+
+  defp item_creation_tags(tags, _, _) do
+    tags
+  end
+
+  defp hashtags_to_topics(object) do
+    object
+    |> Object.hashtags()
+    |> Enum.map(fn hashtag -> "hashtag:" <> hashtag end)
+  end
+
+  defp remote_topics(%{local: true}), do: []
+
+  defp remote_topics(%{actor: actor}) when is_binary(actor),
+    do: ["public:remote:" <> URI.parse(actor).host]
+
+  defp remote_topics(_), do: []
+
+  defp attachment_topics(%{data: %{"attachment" => []}}, _act), do: []
+
+  defp attachment_topics(_object, %{local: true} = activity) do
+    case Visibility.get_visibility(activity) do
+      "public" ->
+        ["public:media", "public:local:media"]
+
+      "local" ->
+        ["public:local:media"]
+
+      _ ->
+        []
+    end
+  end
+
+  defp attachment_topics(_object, %{actor: actor}) when is_binary(actor),
+    do: ["public:media", "public:remote:media:" <> URI.parse(actor).host]
+
+  defp attachment_topics(_object, _act), do: ["public:media"]
+end