Simplify hash function

include/richdem/depressions/depression_hierarchy.hpp

@@ -147,12 +147,16 @@ struct Outlet {
 template <class elev_t>
 struct OutletLinkHash {
   std::size_t operator()(const OutletLink& out) const {
+    // This hash function depends on the data type of `dh_label_t` being
+    // `uint32_t` so that we can bit-shift without having to convert and without
+    // losing bits
+    static_assert(std::is_same_v<dh_label_t, uint32_t>);
+
     // Since depa and depb are sorted on construction, we don't have to worry
     // about which order the invoking code called them in and our hash function
-    // doesn't need to be symmetric with respect to depa and depb.
-
-    // Boost hash combine function: https://stackoverflow.com/a/27952689/752843
-    return out.depa ^ (out.depb + 0x9e3779b9 + (out.depa << 6) + (out.depa >> 2));
+    // doesn't need to be symmetric with respect to depa and depb. A simple
+    // bit-shift is sufficient.
+    return (static_cast<uint64_t>(out.depa) << 32) | static_cast<uint64_t>(out.depb);
   }
 };