1 files changed, 84 insertions, 0 deletions
diff --git a/lib/pleroma/pagination.ex b/lib/pleroma/pagination.ex
new file mode 100644
index 000000000..f435e5c9c
--- /dev/null
+++ b/lib/pleroma/pagination.ex
@@ -0,0 +1,84 @@
+defmodule Pleroma.Pagination do
+  @moduledoc """
+  Implements Mastodon-compatible pagination.
+  """
+
+  import Ecto.Query
+  import Ecto.Changeset
+
+  alias Pleroma.Repo
+
+  @default_limit 20
+
+  def fetch_paginated(query, params) do
+    options = cast_params(params)
+
+    query
+    |> paginate(options)
+    |> Repo.all()
+    |> enforce_order(options)
+  end
+
+  def paginate(query, options) do
+    query
+    |> restrict(:min_id, options)
+    |> restrict(:since_id, options)
+    |> restrict(:max_id, options)
+    |> restrict(:order, options)
+    |> restrict(:limit, options)
+  end
+
+  defp cast_params(params) do
+    param_types = %{
+      min_id: :string,
+      since_id: :string,
+      max_id: :string,
+      limit: :integer
+    }
+
+    params =
+      Enum.reduce(params, %{}, fn
+        {key, _value}, acc when is_atom(key) -> Map.drop(acc, [key])
+        {key, value}, acc -> Map.put(acc, key, value)
+      end)
+
+    changeset = cast({%{}, param_types}, params, Map.keys(param_types))
+    changeset.changes
+  end
+
+  defp restrict(query, :min_id, %{min_id: min_id}) do
+    where(query, [q], q.id > ^min_id)
+  end
+
+  defp restrict(query, :since_id, %{since_id: since_id}) do
+    where(query, [q], q.id > ^since_id)
+  end
+
+  defp restrict(query, :max_id, %{max_id: max_id}) do
+    where(query, [q], q.id < ^max_id)
+  end
+
+  defp restrict(query, :order, %{min_id: _}) do
+    order_by(query, [u], fragment("? asc nulls last", u.id))
+  end
+
+  defp restrict(query, :order, _options) do
+    order_by(query, [u], fragment("? desc nulls last", u.id))
+  end
+
+  defp restrict(query, :limit, options) do
+    limit = Map.get(options, :limit, @default_limit)
+
+    query
+    |> limit(^limit)
+  end
+
+  defp restrict(query, _, _), do: query
+
+  defp enforce_order(result, %{min_id: _}) do
+    result
+    |> Enum.reverse()
+  end
+
+  defp enforce_order(result, _), do: result
+end