1 files changed, 37 insertions, 0 deletions
diff --git a/src/features/plugins/logic/highlight.ts b/src/features/plugins/logic/highlight.ts
new file mode 100644
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+++ b/src/features/plugins/logic/highlight.ts
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+import type { PluginNode } from "./types";
+
+/** The two highlight sets for the §8 bidirectional dependency hover. */
+export interface HighlightSets {
+  /** The hovered row's DIRECT dependencies (highlighted when hovering an item). */
+  readonly deps: ReadonlySet<string>;
+  /** The loaded/unloaded rows that DIRECTLY depend on the hovered row
+   * (highlighted when hovering a dependency). */
+  readonly dependents: ReadonlySet<string>;
+}
+
+const EMPTY: HighlightSets = { deps: new Set(), dependents: new Set() };
+
+/**
+ * Bidirectional hover highlighting (§8): hover any item → its DIRECT
+ * dependencies highlight; hover a dependency → its dependents highlight.
+ * One hop in each direction (the plan's grammar is "its dependencies" /
+ * "its dependents" — the visual neighborhood, not the transitive closure;
+ * the transitive closures live in `computeCascade` / `autoCheckBackendPrereqs`).
+ *
+ * Works across BOTH manager tables (a loaded row can highlight an unloaded
+ * dependency row and vice versa) — the node list spans the whole catalog.
+ * Pure input → output; `hoveredId` null (no hover) → both sets empty.
+ */
+export function dependencyHighlightSets(
+  hoveredId: string | null,
+  nodes: readonly PluginNode[],
+): HighlightSets {
+  if (hoveredId === null) return EMPTY;
+  const hovered = nodes.find((n) => n.id === hoveredId);
+  if (hovered === undefined) return EMPTY;
+  const deps = new Set(hovered.dependsOn);
+  const dependents = new Set(
+    nodes.filter((n) => n.id !== hoveredId && n.dependsOn.includes(hoveredId)).map((n) => n.id),
+  );
+  return { deps, dependents };
+}