From 9a1ac04b4b797bbd92fb67b99eb949cd19c8b966 Mon Sep 17 00:00:00 2001
From: "Claudius \"keldu\" Holeksa" <mail@keldu.de>
Date: Thu, 30 Apr 2026 23:35:40 +0200
Subject: Dangling

---
 examples/poiseulle_particles_2d_ibm_gpu/sim.cpp |  8 +++++--
 lib/core/c++/collision.hpp                      | 28 ++++++++++++++++++-------
 2 files changed, 26 insertions(+), 10 deletions(-)

diff --git a/examples/poiseulle_particles_2d_ibm_gpu/sim.cpp b/examples/poiseulle_particles_2d_ibm_gpu/sim.cpp
index cc108ce..9905906 100644
--- a/examples/poiseulle_particles_2d_ibm_gpu/sim.cpp
+++ b/examples/poiseulle_particles_2d_ibm_gpu/sim.cpp
@@ -46,7 +46,8 @@ using RhoChunk = Chunk<Scalar<T>, 0u, dim_x, dim_y>;
 template<typename T, typename Desc>
 using MacroStruct = Struct<
 	Member<VelChunk<T,Desc>, "velocity">,
-	Member<RhoChunk<T>, "density">
+	Member<RhoChunk<T>, "density">,
+	Member<VelChunk<T,Desc>, "force">
 >;
 
 //template<typename T, typename Desc>
@@ -171,7 +172,10 @@ saw::error_or<void> step(
 	// auto coll_ev = 
 	q.submit([&](acpp::sycl::handler& h){
 		// Need nicer things to handle the flow. I see improvement here
-		component<T,Desc,cmpt::BGKGuo, encode::Sycl<saw::encode::Native>> collision{0.65};
+			saw::data<sch::Vector<T,Desc::D>> f;
+			f.at({{0u}}) = 0.0;
+			f.at({{1u}}) = -1.0;
+		component<T,Desc,cmpt::BGKGuo, encode::Sycl<saw::encode::Native>> collision{0.65,f};
 		component<T,Desc,cmpt::BounceBack,encode::Sycl<saw::encode::Native>> bb;
 		component<T,Desc,cmpt::AntiBounceBack<0u>,encode::Sycl<saw::encode::Native>> abb;
 
diff --git a/lib/core/c++/collision.hpp b/lib/core/c++/collision.hpp
index df45bbe..cd941bd 100644
--- a/lib/core/c++/collision.hpp
+++ b/lib/core/c++/collision.hpp
@@ -66,7 +66,7 @@ public:
 			dfs_old_f.at(index).at({i}) = dfs_old_f.at(index).at({i}) + frequency_ * (eq.at(i) - dfs_old_f.at(index).at({i}));
 		}
 	}
-	
+
 	template<typename CellFieldSchema, typename MacroFieldSchema>
 	void apply(const saw::data<CellFieldSchema, Encode>& field, const saw::data<MacroFieldSchema,Encode>& macros, saw::data<sch::FixedArray<sch::UInt64,Descriptor::D>> index, saw::data<sch::UInt64> time_step) const {
 		bool is_even = ((time_step.get() % 2) == 0);
@@ -76,12 +76,12 @@ public:
 
 		auto& rho_f = macros.template get<"density">();
 		auto& vel_f = macros.template get<"velocity">();
-	
+
 		saw::data<sch::Scalar<T>>& rho = rho_f.at(index);
 		saw::data<sch::Vector<T,Descriptor::D>>& vel = vel_f.at(index);
 
 		compute_rho_u<T,Descriptor>(dfs_old_f.at(index),rho,vel);
-		
+
 		auto eq = equilibrium<T,Descriptor>(rho,vel);
 
 		for(uint64_t i = 0u; i < Descriptor::Q; ++i){
@@ -138,18 +138,26 @@ public:
 		auto& vel_f = macros.template get<"velocity">();
 	
 		saw::data<sch::Scalar<T>>& rho = rho_f.at(index);
-		saw::data<sch::Vector<T,Descriptor::D>>& vel = vel_f.at(index);
+		
+		auto& force_f = macros.template get<"force">();
+		auto& force = force_f.at(index);
+
+		auto total_force = force + global_force_;
+		saw::data<sch::Scalar<T>> half;
+		half.at({}).set(0.5);
+		saw::data<sch::Vector<T,Descriptor::D>> vel = vel_f.at(index) + total_force * ( half / rho );
 
 		compute_rho_u<T,Descriptor>(dfs_old_f.at(index),rho,vel);
 		auto eq = equilibrium<T,Descriptor>(rho,vel);
 
 		using dfi = df_info<T,Descriptor>;
 
-		auto& force_f = macros.template get<"force">();
-		auto& force = force_f.at(index);
 		
 		saw::data<sch::Scalar<T>> dfi_inv_cs2;
 		dfi_inv_cs2.at({}).set(dfi::inv_cs2);
+			
+
+		// auto vel = vel_f.at(index);
 
 		for(uint64_t i = 0u; i < Descriptor::Q; ++i){
 			// saw::data<T> ci_min_u{0};
@@ -164,7 +172,6 @@ public:
 			saw::data<sch::Scalar<T>> w;
 			w.at({}).set(dfi::weights[i]);
 
-			auto F_i_d = saw::math::dot((force+global_force_) * w, (ci - vel * dfi_inv_cs2 + ci * ci_dot_u * dfi_inv_cs2 * dfi_inv_cs2 ));
 			/*
 			saw::data<sch::Scalar<T>> F_i_sum;
 			for(uint64_t d = 0u; d < Descriptor::D; ++d){
@@ -173,9 +180,14 @@ public:
 				F_i_sum = F_i_sum + F_i_d;
 			}
 			*/
+			auto term1 = (ci-vel) * dfi_inv_cs2;
+			auto term2 = ci * (ci_dot_u * dfi_inv_cs2 * dfi_inv_cs2);
+
+			auto force_projection = saw::math::dot(term1 + term2, total_force);
 
+			auto F_i = w * force_projection;
 
-			dfs.at({i}) = dfs.at({i}) + frequency_ * (eq.at(i) - dfs.at({i}) ) + F_i_d.at({}) * (saw::data<T>{1} - saw::data<T>{0.5f} * frequency_);
+			dfs.at({i}) = dfs.at({i}) + frequency_ * (eq.at(i) - dfs.at({i}) ) + F_i.at({}) * (saw::data<T>{1} - saw::data<T>{0.5f} * frequency_);
 		}
 	}
 };
-- 
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