Cache Haven road material discovery

mods/src/patches/haven_road_cache.h

@@ -0,0 +1,62 @@
+#pragma once
+
+#include <chrono>
+#include <cstddef>
+#include <utility>
+#include <vector>
+
+namespace haven_roads
+{
+inline constexpr std::size_t kRendererLimit        = 4096;
+inline constexpr std::size_t kMaterialsPerRenderer = 16;
+inline constexpr std::size_t kMaterialLimit        = kRendererLimit * kMaterialsPerRenderer;
+
+// The backend owns native liveness/rooting. Retained owner and material entries
+// are weak; a refresh may hold a temporary rooted owner without retaining it.
+template <typename Backend> class DiscoveryCache
+{
+public:
+  using Clock = std::chrono::steady_clock;
+  using Weak  = typename Backend::Weak;
+
+  void Refresh(Clock::time_point now)
+  {
+    if (!Backend::GetOwner(owner_)) {
+      owner_ = {};
+      materials_.clear();
+    }
+    if (now < next_discovery_)
+      return;
+    next_discovery_ = now + std::chrono::seconds(5);
+    // A replacement can occupy the canonical path while the old wrapper stays
+    // alive and active. Recheck identity at discovery cadence, never per frame.
+    auto owner = Backend::FindOwner();
+    if (!owner) {
+      owner_ = {};
+      materials_.clear();
+      return;
+    }
+    owner_ = Backend::Retain(owner);
+    if (!owner_) {
+      materials_.clear();
+      return;
+    }
+    std::vector<Weak> next;
+    if (!Backend::ScanOwner(owner, next)) {
+      materials_.clear();
+      return;
+    }
+    if (next.size() > kMaterialLimit)
+      next.resize(kMaterialLimit);
+    materials_ = std::move(next);
+  }
+
+  const std::vector<Weak>& Materials() const
+  { return materials_; }
+
+private:
+  Weak              owner_;
+  std::vector<Weak> materials_;
+  Clock::time_point next_discovery_{};
+};
+} // namespace haven_roads