Try implementing keeping old server jars

apps/amethyst/lib/datagen.ex

@@ -43,15 +43,31 @@ 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
+  @spec get_server_jar(:latest | String.t(), path) :: {:ok, path} | {:error, term} when path: String.t()
+  def get_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}
+        # Check if the file already exists and has the right hash
+        Logger.debug("Checking if server jar exists at #{path} with hash #{sha1}")
+        case File.read(path) do
+          {:ok, data} ->
+            if :crypto.hash(:sha, data) == sha1 do
+              Logger.debug("Using cached server jar at #{path}")
+              {:ok, path}
+            else
+              Logger.debug("Mismatched hash, redownloading server jar")
+              download_jar(url, path)
+            end
+          {:error, _} -> download_jar(url, path)
+        end
       {:error, err} -> {:error, err}
     end
+  end
+
+  defp download_jar(url, path) do
+    Logger.debug("Downloading server jar from #{url}")
+    case Req.get(url, into: File.stream!(path)) do
+      {:ok, _} -> {:ok, path}
       {:error, err} -> {:error, err}
     end
   end