timeline shuffling: don't fail for empty networks

include/reticula/microcanonical_reference_models.hpp

@@ -75,13 +75,38 @@ namespace reticula {
     network<EdgeT> topology_constrained_link_shuffling(
         const network<EdgeT>& temp, Gen& generator);
 
+    /**
+      Produces a random shuffling of the temporal network where the events are
+      shuffled by assigning new, uniformly random timetamps and moving it to a
+      randomly selected link with a non-empty timeline. Equivalent to
+      micocanonical reference model with the canonical name $P[\mathcal{L}, E]$.
+
+      The observation window, the window where the original measurement of the
+      temporal network was made, is passed through parameters `t_start` and
+      `t_end`.
+
+      The set of vertices, total number of events and the static projection of
+      the temporal network are conserved.
+    */
+    template <
+      temporal_network_edge EdgeT,
+      std::uniform_random_bit_generator Gen>
+    requires is_dyadic_v<EdgeT>
+    network<EdgeT> timeline_shuffling(
+        const network<EdgeT>& temp, Gen& generator,
+        typename EdgeT::TimeType t_start, typename EdgeT::TimeType t_end);
+
 
     /**
       Produces a random shuffling of the temporal network where the events are
       shuffled by assigning new, uniformly random timetamps and moving it to a
       randomly selected link with a non-empty timeline. Equivalent to
       micocanonical reference model with the canonical name $P[\mathcal{L}, E]$.
 
+      The observation window, the window where the original measurement of the
+      temporal network was made, is derived by minimum and maximum cause time of
+      the events.
+
       The set of vertices, total number of events and the static projection of
       the temporal network are conserved.
     */

src/microcanonical_reference_models.tpp

@@ -286,6 +286,9 @@ namespace reticula {
     network<EdgeT> timeline_shuffling(
         const network<EdgeT>& temp, Gen& generator,
         typename EdgeT::TimeType t_start, typename EdgeT::TimeType t_end) {
+      if (temp.vertices().empty() || temp.edges().empty())
+        return temp;
+
       auto [t0, t1] = cause_time_window(temp);
       if (t0 < t_start || t1 > t_end)
         throw std::invalid_argument("observation window does not cover "
@@ -331,6 +334,9 @@ namespace reticula {
     requires is_dyadic_v<EdgeT>
     network<EdgeT> timeline_shuffling(
         const network<EdgeT>& temp, Gen& generator) {
+      if (temp.vertices().empty() || temp.edges().empty())
+        return temp;
+
       auto [t0, t1] = cause_time_window(temp);
       return timeline_shuffling(temp, generator, t0, t1);
     }
@@ -377,6 +383,9 @@ namespace reticula {
     requires is_dyadic_v<EdgeT>
     network<EdgeT> weight_constrained_timeline_shuffling(
         const network<EdgeT>& temp, Gen& generator) {
+      if (temp.vertices().empty() || temp.edges().empty())
+        return temp;
+
       auto [t0, t1] = cause_time_window(temp);
       return weight_constrained_timeline_shuffling(temp, generator, t0, t1);
     }