Use direct momentum in opt-in shifted blocked equilibrium

tests/unit/cpp/shifted_block_operators.cpp

@@ -13,6 +13,8 @@
 #define WALL_SLOT_WIDTH 0
 #define NX 16
 #include "core/descriptors.cuh"
+// Host round-to-nearest shim; CUDA intrinsic behavior is tested on-device.
+inline float __fdiv_rn(float a, float b) { return a / b; }
 #include "operators/macro.cuh"
 #include "operators/collision_srt.cuh"
 #include "boundary/planar_velocity.cuh"
@@ -23,6 +25,13 @@ int main() {
         float eq[9]; compute_feq(1.f+delta,u,-.2f*u,eq,delta);
         printf("E %.9g %.9g %.9g\n",delta,u,-.2f*u);emit(eq);
     }
+    for(float delta : {0.f, 0x1p-30f, -0x1p-30f, .04f, -.01f})
+    for(float jx : {0.f,.005f,-.02f}) {
+        const float jy = -.2f*jx, rho = 1.f+delta;
+        const float ux = jx/rho, uy = jy/rho;
+        float eq[9];compute_feq_from_momentum(delta,jx,jy,ux,uy,eq);
+        printf("J %.9g %.9g %.9g %.9g %.9g\n",delta,jx,jy,ux,uy);emit(eq);
+    }
     for(int mode : {0,1,2}) for(float rho : {1.f,1.04f}) {
         float f[9];const float u=.007f,v=.003f;
         compute_feq(rho,u,v,f,rho-1.f);