[fix] keep display stream IDs stable after monitor changes

src/common/display_stream_id.h

@@ -0,0 +1,59 @@
+#ifndef CROSSDESK_DISPLAY_STREAM_ID_H_
+#define CROSSDESK_DISPLAY_STREAM_ID_H_
+
+#include <cstddef>
+#include <string>
+
+namespace crossdesk {
+
+// MiniRTC stream identifiers are protocol-facing logical identifiers. Keep
+// them independent from platform display handles and user-visible names, both
+// of which may change or contain duplicate/Unicode text.
+inline std::string MakeDisplayStreamId(size_t display_index) {
+  return "Display" + std::to_string(display_index + 1);
+}
+
+inline bool IsRegisteredDisplayStreamId(const std::string& stream_id,
+                                        size_t display_count) {
+  if (stream_id.empty()) {
+    return false;
+  }
+
+  for (size_t index = 0; index < display_count; ++index) {
+    if (stream_id == MakeDisplayStreamId(index)) {
+      return true;
+    }
+  }
+  return false;
+}
+
+// Resolves a backend-reported identifier to a stream registered with MiniRTC.
+// Backends should report MakeDisplayStreamId(index); the fallbacks keep older
+// capture plugins and hotplug transitions safe.
+inline std::string ResolveDisplayStreamId(
+    const char* reported_id, size_t display_count, int preferred_index = -1,
+    const std::string& previous_id = {}) {
+  const std::string candidate = reported_id ? reported_id : "";
+  if (IsRegisteredDisplayStreamId(candidate, display_count)) {
+    return candidate;
+  }
+
+  if (preferred_index >= 0 &&
+      preferred_index < static_cast<int>(display_count)) {
+    return MakeDisplayStreamId(static_cast<size_t>(preferred_index));
+  }
+
+  if (IsRegisteredDisplayStreamId(previous_id, display_count)) {
+    return previous_id;
+  }
+
+  if (display_count == 1) {
+    return MakeDisplayStreamId(0);
+  }
+
+  return {};
+}
+
+}  // namespace crossdesk
+
+#endif  // CROSSDESK_DISPLAY_STREAM_ID_H_