feat: add tanh transition curve particle parameter change algorithm

include/swarm.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <cmath>
 #include <cstddef>
 
 #include <functional>
@@ -45,15 +46,38 @@ namespace ParamChange {
 	    };
 	}
     };
+
+    template<auto steepness = 3>
+    struct MTanhAlg {
+	vec<3> operator()(
+	    float visc, float nostal, float peerp, int curr, int max) {
+	    float pct_done = (float)curr / (float)max;
+	    float scale_factor = (std::tanh(steepness * (pct_done - 0.5)) + 1) / 2;
+	    return {
+		visc   - (0.9f - 0.4f) * scale_factor,
+		nostal - (2.5f - 0.5f) * scale_factor,
+		peerp  + (2.5f - 0.5f) * scale_factor,
+	    };
+	}
+    };
 };
 
-template<std::size_t N, ParameterChangeAlgorithm A = ParamChange::MLinAlg>
+template<std::size_t N, ParameterChangeAlgorithm A = ParamChange::MTanhAlg<>>
 struct Particle : public Agent<vec<N>> {
     std::pair<float, vec<N>> best() const override { return {pb, pb_pos}; };
 
     void move(float dt=1) override { position = position + velocity * dt; }
 
     void step() override {
+	vec<3> params = alg(kviscosity, knostalgia, kpeer_pressure,
+			    curr_iter, max_iter);
+
+	viscosity = params.x;
+	nostalgia = params.y;
+	peer_pressure = params.z;
+
+	curr_iter++;
+
 	velocity = viscosity * velocity
 	    + nostalgia * (pb_pos - position)
 	    + peer_pressure * (peer.best().second - position);
@@ -63,15 +87,6 @@ struct Particle : public Agent<vec<N>> {
 	    pb = y;
 	    pb_pos = position;
 	}
-
-	vec<3> params = alg(kviscosity, knostalgia, kpeer_pressure,
-			    curr_iter, max_iter);
-
-	viscosity = params.x;
-	nostalgia = params.y;
-	peer_pressure = params.z;
-
-	curr_iter++;
     };
 
     const float kviscosity = 0.9f;
@@ -146,7 +161,7 @@ struct Swarm : public Agent<vec<N>> {
 
     const std::vector<Particle<N>>& get_particles() { return particles; };
 
-    Swarm(Agent<vec<N>>::F f, unsigned max_iter = 150)
+    Swarm(Agent<vec<N>>::F f, unsigned max_iter)
 	: Agent<vec<N>>(f), max_iter(max_iter) {}
 private:
     unsigned max_iter;