all of the changes

include/FluidSim/Junction.hpp

@@ -0,0 +1,34 @@
+#pragma once
+#include "FluidSim/Fluid.hpp"
+#include <span>
+
+namespace fluidsim::port {
+struct JunctionEnd { int pipe; bool left; float discharge{1.f}; };
+// Finite-volume N-port fitting: its specified physical volume stores gas and
+// energy. All ports share pressure and mixture, with bidirectional nozzle flow.
+// T, Y, cross and collectors share this model; angles require calibrated losses.
+class Junction : public Volume0D {
+public:
+    using Volume0D::Volume0D;
+    void connect(BoundarySystem& boundaries, const std::vector<Pipe1D>& pipes,
+                 std::span<const JunctionEnd> ends) {
+        if(connected_ || ends.size()<2) throw std::invalid_argument("junction needs at least two distinct ends and one connection call");
+        for(std::size_t i=0;i<ends.size();++i) {
+            const auto& e=ends[i];
+            if(e.pipe<0 || std::size_t(e.pipe)>=pipes.size() || !std::isfinite(e.discharge) || e.discharge<=0 || e.discharge>1)
+                throw std::invalid_argument("invalid junction endpoint");
+            for(std::size_t j=0;j<i;++j) if(e.pipe==ends[j].pipe && e.left==ends[j].left)
+                throw std::invalid_argument("duplicate junction endpoint");
+        }
+        for(const auto& e:ends) {
+            const auto& pipe=pipes[e.pipe];
+            areas_.push_back(pipe.cell_area(e.left?0:int(pipe.cells.size())-1));
+            boundaries.add_orifice(create_port(),e.pipe,e.left,areas_.back(),e.discharge);
+        }
+        connected_=true;
+    }
+private:
+    std::deque<float> areas_; // stable addresses for boundary area references
+    bool connected_{};
+};
+}