Specialize std::hash for NodeId/SubmapId

Signed-off-by: Bosco Siu <[email protected]>

cartographer/mapping/id.h

@@ -25,8 +25,10 @@
 #include <memory>
 #include <ostream>
 #include <tuple>
+#include <utility>
 #include <vector>
 
+#include "absl/hash/hash.h"
 #include "absl/memory/memory.h"
 #include "cartographer/common/port.h"
 #include "cartographer/common/time.h"
@@ -420,4 +422,21 @@ class MapById {
 }  // namespace mapping
 }  // namespace cartographer
 
+template <>
+struct std::hash<cartographer::mapping::NodeId> {
+  std::size_t operator()(
+      const cartographer::mapping::NodeId& id) const noexcept {
+    return absl::Hash<std::pair<int, int>>{}({id.trajectory_id, id.node_index});
+  }
+};
+
+template <>
+struct std::hash<cartographer::mapping::SubmapId> {
+  std::size_t operator()(
+      const cartographer::mapping::SubmapId& id) const noexcept {
+    return absl::Hash<std::pair<int, int>>{}(
+        {id.trajectory_id, id.submap_index});
+  }
+};
+
 #endif  // CARTOGRAPHER_MAPPING_ID_H_