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);