410 rss cell bug (#411)

Fix bug in `rss_cell_group`s with more than one cell with different `dt` or `start_time` values.
Fixes #410.

src/rss_cell_group.hpp

@@ -29,22 +29,27 @@ public:
 
     void reset() override {
         clear_spikes();
-        time_ = 0;
+        for (auto& cell : cells_) {
+            cell.step = 0;
+        }
     }
 
     void set_binning_policy(binning_kind policy, time_type bin_interval) override {}
 
     void advance(epoch ep, time_type dt, const event_lane_subrange& events) override {
-        for (const auto& cell: cells_) {
-            auto t = std::max(cell.start_time, time_);
+        for (auto& cell: cells_) {
+
             auto t_end = std::min(cell.stop_time, ep.tfinal);
+            auto t = cell.start_time + cell.step*cell.period;
 
             while (t < t_end) {
                 spikes_.push_back({{cell.gid, 0}, t});
-                t += cell.period;
+
+                // Increasing time with period time step has float issues
+                ++cell.step;
+                t = cell.start_time + cell.step*cell.period;
             }
         }
-        time_ = ep.tfinal;
     }
 
     const std::vector<spike>& spikes() const override {
@@ -71,13 +76,15 @@ private:
         {}
 
         cell_gid_type gid;
+
+        // We do not store the time but a count of the number of step since start
+        // of cell. This prevents float problems at high number.
+        std::size_t step;
     };
 
     // RSS cell descriptions.
     std::vector<rss_info> cells_;
 
-    // Simulation time for all RSS cells in the group.
-    time_type time_;
 
     // Spikes that are generated.
     std::vector<spike> spikes_;

tests/unit/test_rss_cell.cpp

@@ -62,6 +62,22 @@ TEST(rss_cell, poll_time_after_end_time)
     EXPECT_EQ(12u, sut.spikes().size());
 }
 
+TEST(rss_cell, rate_bigger_then_epoch)
+{
+    constexpr time_type dt = 0.01; // dt is ignored by rss_cell_group::advance().
+
+    rss_cell desc{ 0.0, 100.0, 1000.0 };
+    rss_cell_group sut({ 0 }, rss_recipe(1u, desc));
+
+    // take time steps of 10 ms
+    for (time_type start = 0.0; start < 1000.0; start += 10) {
+        sut.advance(epoch(start, start + 10.0), dt, {});
+    }
+    // We spike once every 100 ms so in 1000.0 ms we should have 10
+    EXPECT_EQ(10u, sut.spikes().size());
+}
+
+
 TEST(rss_cell, cell_kind_correct)
 {
     rss_cell desc{0.1, 0.01, 0.2};