embedded3 PERF-3 P1-5 (j): the step apertures, open flags and step activity patched on the changed cells' faces with the old mask's arrays moved over (a face of no changed cell keeps its trapezoid: its corners are untouched in both builds); the touched-cell map stored per build — slab CSV byte-identical, band check passed, device moving/cg green, host suite 21/21; ny 124 rebuild block 1,578 → 1,348 ms per step (refill + apertures + merging 196 → 64)

Co-Authored-By: Claude Fable 5.1 <[email protected]>

crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/cut.rs

@@ -37,6 +37,8 @@ pub struct CutGeometry {
     /// narrow band) — a cell whose corners are all untouched has unchanged
     /// apertures, volume and activity.
     pub touched: Vec<bool>,
+    /// Per cell: any of its eight corners touched (computed once per build).
+    pub touched_cell: Vec<bool>,
 }
 
 impl CutGeometry {
@@ -191,6 +193,21 @@ impl CutGeometry {
                 let sz = (a_w[g.wface(k + 1, j, i)] - a_w[g.wface(k, j, i)]) * az;
                 *w = [-sx, -sy, -sz];
             });
+        let touched_cell: Vec<bool> = (0..g.cells())
+            .into_par_iter()
+            .map(|idx| {
+                let (k, j, i) = (idx / (ny * nx), (idx / nx) % ny, idx % nx);
+                let mut t = false;
+                for dk in 0..2 {
+                    for dj in 0..2 {
+                        for di in 0..2 {
+                            t |= touched[Self::node(g, k + dk, j + dj, i + di)];
+                        }
+                    }
+                }
+                t
+            })
+            .collect();
         Self {
             grid,
             phi,
@@ -204,12 +221,16 @@ impl CutGeometry {
             d_w,
             bound,
             touched,
+            touched_cell,
         }
     }
 
-    /// The cells with a touched corner (P1-4).
+    /// The cells with a touched corner (P1-4; the stored map).
     #[must_use]
     pub fn touched_cells(&self) -> Vec<bool> {
+        if !self.touched_cell.is_empty() {
+            return self.touched_cell.clone();
+        }
         use rayon::prelude::*;
         let g = self.grid;
         let (nx, ny) = (g.nx, g.ny);

crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/cutwall.rs

@@ -283,52 +283,27 @@ impl Mask {
     /// or with `inner` intermediate geometries the composite trapezoid
     /// over the step (the exact time integral of the space-time cut cell,
     /// arXiv 2512.23358, approached as the sub-sampling refines).
-    pub fn set_step_apertures(&mut self, old: &Mask) {
+    pub fn set_step_apertures(&mut self, old: &mut Mask) {
         self.set_step_apertures_with(old, &[]);
     }
 
     /// As [`Self::set_step_apertures`] with the apertures of the
     /// intermediate geometries `inner` (in time order) inside the step.
-    pub fn set_step_apertures_with(&mut self, old: &Mask, inner: &[&CutGeometry]) {
+    pub fn set_step_apertures_with(&mut self, old: &mut Mask, inner: &[&CutGeometry]) {
         let (Some(cut), Some(old_cut)) = (self.cut.as_ref(), old.cut.as_ref()) else {
             return;
         };
         let n = inner.len() + 1;
         let w_end = 0.5 / n as f64;
         let w_in = 1.0 / n as f64;
-        let avg = |pick: &dyn Fn(&CutGeometry) -> &[f64]| -> Vec<f64> {
-            let a = pick(cut);
-            let b = pick(old_cut);
-            let mut out: Vec<f64> = a.iter().zip(b).map(|(x, y)| w_end * (x + y)).collect();
-            for g in inner {
-                for (o, v) in out.iter_mut().zip(pick(g)) {
-                    *o += w_in * v;
-                }
-            }
-            out
-        };
-        let au = avg(&|g: &CutGeometry| &g.a_u);
-        let av = avg(&|g: &CutGeometry| &g.a_v);
-        let aw = avg(&|g: &CutGeometry| &g.a_w);
-        let open = |a: &[f64]| -> Vec<bool> { a.iter().map(|&x| x > 0.0).collect() };
-        let active = self
-            .cell_fluid
-            .iter()
-            .zip(&old.cell_fluid)
-            .map(|(&n, &o)| n || o)
-            .collect();
-        self.step_open = Some((open(&au), open(&av), open(&aw), active));
-        self.step_apertures = Some((au, av, aw));
-        // P1-4: the changed set — cells touched by either build (this step's
-        // apertures average the two geometries), dilated by one.
-        let t_new = cut.touched_cells();
-        let t_old = old_cut.touched_cells();
-        self.compute_merging(Some(old));
-        {
         use rayon::prelude::*;
         let g = self.grid;
         let (nx, ny, nz) = (g.nx, g.ny, g.nz);
         let nxy = nx * ny;
+        // P1-4: the changed set — cells touched by either build (this step's
+        // apertures average the two geometries), dilated by one.
+        let t_new = &cut.touched_cell;
+        let t_old = &old_cut.touched_cell;
         let periodic = self.periodic_z;
         let changed: Vec<usize> = (0..g.cells())
             .into_par_iter()
@@ -348,8 +323,59 @@ impl Mask {
                     || (periodic && nz > 1 && k == 0 && t(idx + (nz - 1) * nxy))
             })
             .collect();
-            self.changed_cells = Some(changed);
+        // P1-5 (j): with the trapezoid alone and a previous step's arrays, a
+        // face of no changed cell keeps its step aperture (its corners were
+        // untouched in both builds, so αⁿ⁻¹ = αⁿ = αⁿ⁺¹): the old mask's
+        // arrays move over and only the changed cells' faces and the
+        // changed cells' activity are recomputed, with the same expressions.
+        let prev = if inner.is_empty() { old.step_apertures.take().zip(old.step_open.take()) } else { None };
+        if let Some(((mut au, mut av, mut aw), (mut ou, mut ov, mut ow, mut active))) = prev {
+            let mix = |a: &[f64], b: &[f64], f: usize| w_end * (a[f] + b[f]);
+            for &idx in &changed {
+                let (k, j, i) = (idx / nxy, (idx % nxy) / nx, idx % nx);
+                for f in [g.uface(k, j, i), g.uface(k, j, i + 1)] {
+                    au[f] = mix(&cut.a_u, &old_cut.a_u, f);
+                    ou[f] = au[f] > 0.0;
                 }
+                for f in [g.vface(k, j, i), g.vface(k, j + 1, i)] {
+                    av[f] = mix(&cut.a_v, &old_cut.a_v, f);
+                    ov[f] = av[f] > 0.0;
+                }
+                for f in [g.wface(k, j, i), g.wface(k + 1, j, i)] {
+                    aw[f] = mix(&cut.a_w, &old_cut.a_w, f);
+                    ow[f] = aw[f] > 0.0;
+                }
+                active[idx] = self.cell_fluid[idx] || old.cell_fluid[idx];
+            }
+            self.step_open = Some((ou, ov, ow, active));
+            self.step_apertures = Some((au, av, aw));
+        } else {
+            let avg = |pick: &dyn Fn(&CutGeometry) -> &[f64]| -> Vec<f64> {
+                let a = pick(cut);
+                let b = pick(old_cut);
+                let mut out: Vec<f64> = a.iter().zip(b).map(|(x, y)| w_end * (x + y)).collect();
+                for gi in inner {
+                    for (o, v) in out.iter_mut().zip(pick(gi)) {
+                        *o += w_in * v;
+                    }
+                }
+                out
+            };
+            let au = avg(&|gg: &CutGeometry| &gg.a_u);
+            let av = avg(&|gg: &CutGeometry| &gg.a_v);
+            let aw = avg(&|gg: &CutGeometry| &gg.a_w);
+            let open = |a: &[f64]| -> Vec<bool> { a.iter().map(|&x| x > 0.0).collect() };
+            let active = self
+                .cell_fluid
+                .iter()
+                .zip(&old.cell_fluid)
+                .map(|(&n, &o)| n || o)
+                .collect();
+            self.step_open = Some((open(&au), open(&av), open(&aw), active));
+            self.step_apertures = Some((au, av, aw));
+        }
+        self.compute_merging(Some(old));
+        self.changed_cells = Some(changed);
     }
 
     pub(super) fn lattice(&self) -> Lattice {

crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/moving.rs

@@ -21,7 +21,7 @@ impl Solver {
         let lap = std::time::Instant::now();
         let mut new_mask = self.build_mask(body, field.grid, t_new, dt);
         let l_build = lap.elapsed();
-        if let Some(old_mask) = &self.mask {
+        if let Some(old_mask) = self.mask.as_mut() {
             fresh_cells = refill_fresh_cells(old_mask, &new_mask, field);
             let n_in = self.params.aperture_substeps;
             if n_in == 0 {