Add function to get similar artists

lib/last_fm/api_behaviour.ex

@@ -7,4 +7,6 @@ defmodule LastFm.APIBehaviour do
   @callback get_recent_tracks(config) :: {:ok, [Track.t()]} | {:error, String.t()}
   @callback get_artist_info({:musicbrainz_id, musicbrainz_id} | {:name, name}, config) ::
               {:ok, Artist.t()} | {:error, String.t()}
+  @callback get_similar_artists({:musicbrainz_id, musicbrainz_id} | {:name, name}, config) ::
+              {:ok, [Artist.t()]} | {:error, String.t()}
 end

lib/last_fm/api_impl.ex

@@ -55,7 +55,7 @@ defmodule LastFm.APIImpl do
     do_get_artist_info([artist: artist_name], config)
   end
 
-  def do_get_artist_info(options, config) do
+  defp do_get_artist_info(options, config) do
     base_options = [
       method: "artist.getInfo",
       api_key: config.api_key,
@@ -86,6 +86,45 @@ defmodule LastFm.APIImpl do
     end
   end
 
+  @impl true
+  def get_similar_artists({:name, artist_name}, config) do
+    do_get_similar_artists([artist: artist_name], config)
+  end
+
+  def get_similar_artists({:musicbrainz_id, artist_mbid}, config) do
+    do_get_similar_artists([mbid: artist_mbid], config)
+  end
+
+  defp do_get_similar_artists(options, config) do
+    base_options = [
+      method: "artist.getSimilar",
+      api_key: config.api_key,
+      format: "json",
+      limit: 50
+    ]
+
+    options = Keyword.merge(base_options, options)
+
+    url = @base_url <> "?" <> URI.encode_query(options)
+
+    Logger.debug("Fetching data from #{sanitize_url(url, config.api_key)}")
+
+    case json_get(url, config.user_agent) do
+      {:ok, response} ->
+        {:ok,
+         response
+         |> get_in(["similarartists", "artist"])
+         |> Enum.map(&Artist.from_api_response/1)}
+
+      error ->
+        msg =
+          "Failed to fetch data from #{sanitize_url(url, config.api_key)}, reason: #{inspect(error)}"
+
+        Logger.error(msg)
+        error
+    end
+  end
+
   defp json_get(url, user_agent) do
     req =
       Finch.build(:get, url, [

lib/music_library/records.ex

@@ -136,6 +136,26 @@ defmodule MusicLibrary.Records do
     end
   end
 
+  def get_similar_artists(artist) do
+    last_fm_config = last_fm_config()
+    # Sometimes the artist info cannot be identified with the MusicBrainz ID,
+    # because Last.fm doesn't have that information. In that case, we try again with the artist name.
+    case last_fm_config.api.get_similar_artists(
+           {:musicbrainz_id, artist.musicbrainz_id},
+           last_fm_config
+         ) do
+      {:ok, info} ->
+        {:ok, info}
+
+      # TODO: remap error codes
+      {:error, %{"error" => 6}} ->
+        last_fm_config.api.get_similar_artists({:name, artist.name}, last_fm_config)
+
+      error ->
+        error
+    end
+  end
+
   def get_cover(id) do
     q =
       from r in Record,