Update

tests/fluid_validation.cpp

@@ -48,6 +48,23 @@ int main() {
             std::cout << "FAIL " << n << ": " << e.what() << '\n';
         }
     };
+    test("tracer underflow preserves bulk gas and normal concentrations", [] {
+        for (float fraction : {1e-40f, 1e-30f, .5f}) {
+            Pipe1D p(.001f, 1, 8);
+            for (int i=0; i<8; ++i) set(p, i, 101325, 1.2f, 0, fraction);
+            p.left_ghost = p.state(0);
+            p.right_ghost = p.state(7);
+            const double before_mass = mass(p), before_energy = energy(p);
+            p.advance(1e-6f);
+            near(mass(p), before_mass, 0, "tracer truncation changed total mass");
+            near(energy(p), before_energy, 0, "tracer truncation changed energy");
+            for (auto cell : p.cells) {
+                check(std::fpclassify(cell.air) != FP_SUBNORMAL, "subnormal tracer survived");
+                near(cell.air, fraction < std::numeric_limits<float>::min() ? 0 : fraction,
+                     fraction * 1e-6, "normal tracer concentration changed");
+            }
+        }
+    });
     test("invalid uniform geometry", [] {
         bool caught = false;
         try {