Extend ring (#611)

Extend the ring benchmark to have an optional number of synapses attached to each cell, instead of a fixed count of one synapse per cell.
This doesn't change the behavior of the model: only the first synapse is used for communication. The other synapses only effect is to
increase the per-cell computational overheads, to more effectively mimic real world performance.

example/ring/parameters.hpp

@@ -22,6 +22,9 @@ struct cell_parameters {
     std::array<double,2> branch_probs = {1.0, 0.5}; //  Probability of a branch occuring.
     std::array<unsigned,2> compartments = {20, 2};  //  Compartment count on a branch.
     std::array<double,2> lengths = {200, 20};       //  Length of branch in μm.
+
+    // The number of synapses per cell.
+    unsigned synapses = 1;
 };
 
 struct ring_params {
@@ -65,6 +68,7 @@ ring_params read_options(int argc, char** argv) {
     param_from_json(params.cell.branch_probs, "branch-probs", json);
     param_from_json(params.cell.compartments, "compartments", json);
     param_from_json(params.cell.lengths, "lengths", json);
+    param_from_json(params.cell.synapses, "synapses", json);
 
     if (!json.empty()) {
         for (auto it=json.begin(); it!=json.end(); ++it) {

example/ring/ring.cpp

@@ -86,7 +86,7 @@ public:
     std::vector<arb::event_generator> event_generators(cell_gid_type gid) const override {
         std::vector<arb::event_generator> gens;
         if (!gid) {
-            gens.push_back(arb::explicit_generator(arb::pse_vector{{{0, 0}, 0.1, 1.0}}));
+            gens.push_back(arb::explicit_generator(arb::pse_vector{{{0, 0}, event_weight_, 1.0}}));
         }
         return gens;
     }
@@ -109,7 +109,7 @@ private:
     cell_size_type num_cells_;
     cell_parameters cell_params_;
     double min_delay_;
-    float event_weight_ = 0.01;
+    float event_weight_ = 0.05;
 };
 
 struct cell_stats {
@@ -341,6 +341,11 @@ arb::mc_cell branch_cell(arb::cell_gid_type gid, const cell_parameters& params)
     // Add a synapse to the mid point of the first dendrite.
     cell.add_synapse({1, 0.5}, "expsyn");
 
+    // Add additional synapses that will not be connected to anything.
+    for (unsigned i=1u; i<params.synapses; ++i) {
+        cell.add_synapse({1, 0.5}, "expsyn");
+    }
+
     return cell;
 }