1 files changed, 45 insertions, 0 deletions
diff --git a/src/features/plugins/logic/prereqs.ts b/src/features/plugins/logic/prereqs.ts
new file mode 100644
index 0000000..9066413
--- /dev/null
+++ b/src/features/plugins/logic/prereqs.ts
@@ -0,0 +1,45 @@
+import type { PluginNode } from "./types";
+
+/**
+ * When CHECKING (loading) `targetId`, the transitive set of its dependencies
+ * that are NOT currently loaded — the plugins that must be auto-checked
+ * (loaded) first (§8: "Checking a frontend plugin auto-checks its required
+ * backend plugins (transitively)").
+ *
+ * One-directional by construction: only `dependsOn` edges FROM the target are
+ * followed (frontend→backend / backend→backend). The reverse direction is
+ * NEVER traversed here — backend plugins never depend on frontend ones (§8),
+ * so a dependency auto-check never pulls the other way.
+ *
+ * Returns the ids in deterministic load order (dependencies before their
+ * dependents — alphabetical within a level) so the caller can issue the load
+ * requests in a safe sequence. Pure input → output.
+ */
+export function autoCheckBackendPrereqs(
+  targetId: string,
+  nodes: readonly PluginNode[],
+): readonly string[] {
+  const byId = new Map(nodes.map((n) => [n.id, n]));
+
+  // BFS over dependency edges (target → deps), collecting UNLOADED deps.
+  const out: string[] = [];
+  const seen = new Set<string>([targetId]);
+  // Queue of [id, depth] — sorted per level so ties resolve alphabetically.
+  let frontier: string[] = [targetId];
+  while (frontier.length > 0) {
+    const next: string[] = [];
+    for (const id of frontier) {
+      const node = byId.get(id);
+      for (const dep of node?.dependsOn ?? []) {
+        if (seen.has(dep)) continue;
+        seen.add(dep);
+        next.push(dep);
+        if (!byId.get(dep)?.loaded) out.push(dep);
+      }
+    }
+    frontier = next.sort((a, b) => a.localeCompare(b));
+  }
+  // Load order: dependencies before dependents (BFS level order already is),
+  // alphabetical within a level.
+  return out;
+}