Implement downloading server jar

apps/amethyst/lib/datagen.ex

@@ -15,6 +15,8 @@
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 defmodule Amethyst.DataGenerator do
+  require Logger
+
   @moduledoc """
   This module allows Amethyst to download the vanilla Minecraft server and generate
   necessary data files.
@@ -28,7 +30,12 @@ defmodule Amethyst.DataGenerator do
           {:ok, %{body: %{"versions" => versions}}} ->
             case Enum.find(versions, fn %{"id" => id} -> id == version end) do
               nil -> {:error, :version_not_found}
-              version_meta -> {:ok, version_meta}
+              version_header ->
+                # Get the version manifest
+                case Req.get(version_header["url"]) do
+                  {:ok, %{body: version_meta}} -> {:ok, version_meta}
+                  {:error, err} -> {:error, err}
+                end
             end
           {:error, err} -> {:error, err}
         end
@@ -36,6 +43,22 @@ defmodule Amethyst.DataGenerator do
     end
   end
 
+  @spec download_server_jar(:latest | String.t(), path) :: {:ok, path} | {:error, term} when path: String.t()
+  def download_server_jar(version, path) do
+    case get_version_meta(version) do
+      {:ok, %{"downloads" => %{"server" => %{"url" => url, "sha1" => sha1}}}} ->
+        Logger.debug("Downloading server jar from #{url}, sha1: #{sha1}")
+        case Req.get(url, into: File.stream!(path)) do
+          {:ok, _} -> {:ok, path}
+          {:error, err} -> {:error, err}
+        end
+      {:error, err} -> {:error, err}
+    end
+  end
+
+  @spec get_latest_version() ::
+          {:error, %{:__exception__ => true, :__struct__ => atom(), optional(atom()) => any()}}
+          | {:ok, any()}
   def get_latest_version() do
     case Req.get(Application.fetch_env!(:amethyst, :mojang_game_meta)) do
       {:ok, %{body: %{"latest" => %{"release" => release}}}} -> {:ok, release}