From 31f70b3da4201e6b343b0d30f783b1617d0112a8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Eric=20Meadows-J=C3=B6nsson?=
 <eric.meadows.jonsson@gmail.com>
Date: Tue, 21 Apr 2026 23:00:22 +0200
Subject: [PATCH] Handle latest version fallback

---
 lib/diff_web/live/diff_live_view.ex | 32 ++++++++++++++++++++++-------
 test/diff_web/integration_test.exs  | 28 +++++++++++++++++++++++++
 2 files changed, 53 insertions(+), 7 deletions(-)

diff --git a/lib/diff_web/live/diff_live_view.ex b/lib/diff_web/live/diff_live_view.ex
index cf2cccb..9ffebe9 100644
--- a/lib/diff_web/live/diff_live_view.ex
+++ b/lib/diff_web/live/diff_live_view.ex
@@ -16,7 +16,7 @@ defmodule DiffWeb.DiffLiveView do
             mount_single_diff(socket, package, resolved_from, resolved_to)
 
           {:error, reason} ->
-            {:ok, assign(socket, error: "Package not found: #{reason}")}
+            {:ok, assign(socket, error: latest_version_error(package, reason))}
         end
 
       :error ->
@@ -117,13 +117,15 @@ defmodule DiffWeb.DiffLiveView do
   defp resolve_latest_version(package, from, to) when to == :latest or to == "latest" do
     case Diff.Package.Store.get_versions(package) do
       {:ok, versions} ->
-        to =
-          versions
-          |> Enum.map(&Version.parse!/1)
-          |> Enum.filter(&(&1.pre == []))
-          |> Enum.max(Version)
+        versions = Enum.map(versions, &Version.parse!/1)
 
-        {:ok, from, to_string(to)}
+        case latest_version(versions) do
+          nil ->
+            {:error, :no_versions}
+
+          to ->
+            {:ok, from, to_string(to)}
+        end
 
       {:error, :not_found} ->
         {:error, :not_found}
@@ -132,6 +134,22 @@ defmodule DiffWeb.DiffLiveView do
 
   defp resolve_latest_version(_package, from, to), do: {:ok, from, to}
 
+  defp latest_version([]), do: nil
+
+  defp latest_version(versions) do
+    stable_versions = Enum.filter(versions, &(&1.pre == []))
+
+    case stable_versions do
+      [] -> Enum.max(versions, Version)
+      stable_versions -> Enum.max(stable_versions, Version)
+    end
+  end
+
+  defp latest_version_error(package, :not_found), do: "Package not found: #{package}"
+
+  defp latest_version_error(package, :no_versions),
+    do: "No versions found for package: #{package}"
+
   def handle_event("load-more", _params, socket) do
     batch_size = 5
     {next_batch, _remaining} = Enum.split(socket.assigns.remaining_diffs, batch_size)
diff --git a/test/diff_web/integration_test.exs b/test/diff_web/integration_test.exs
index 2a36b8c..0bd265d 100644
--- a/test/diff_web/integration_test.exs
+++ b/test/diff_web/integration_test.exs
@@ -40,6 +40,34 @@ defmodule DiffWeb.IntegrationTest do
       assert html =~ "Generating diffs"
     end
 
+    test "/diff with implicit latest handles packages without stable versions", %{conn: conn} do
+      Diff.Package.StoreMock
+      |> stub(:get_versions, fn "prerelease_only" ->
+        {:ok, ["1.0.0-alpha.1", "1.0.0-rc.1", "1.0.0-rc.2"]}
+      end)
+
+      Diff.StorageMock
+      |> stub(:get_metadata, fn "prerelease_only", "0.1.0", "1.0.0-rc.2" ->
+        {:error, :not_found}
+      end)
+
+      Diff.HexMock
+      |> stub(:diff, fn "prerelease_only", "0.1.0", "1.0.0-rc.2" -> :error end)
+
+      {:ok, _view, html} = live(conn, "/diff/prerelease_only/0.1.0..")
+
+      assert html =~ "Generating diffs"
+    end
+
+    test "/diff with implicit latest handles packages with an empty version list", %{conn: conn} do
+      Diff.Package.StoreMock
+      |> stub(:get_versions, fn "empty" -> {:ok, []} end)
+
+      {:ok, _view, html} = live(conn, "/diff/empty/0.1.0..")
+
+      assert html =~ "No versions found for package: empty"
+    end
+
     test "/diff handles package not found", %{conn: conn} do
       Diff.Package.StoreMock
       |> stub(:get_versions, fn "nonexistent" -> {:error, :not_found} end)