From e3958fc9c57c913d15be5d0f44bb5f03ceb3ccec Mon Sep 17 00:00:00 2001
From: Omar Sobh <om.sobh@gmail.com>
Date: Sun, 20 Sep 2026 17:55:41 -0500
Subject: [PATCH] =?UTF-8?q?embedded3=20PERF-3=20P1-5=20(a)(b):=20the=20pre?=
 =?UTF-8?q?vious-step=20aperture/volume=20records=20move=20out=20of=20the?=
 =?UTF-8?q?=20old=20mask=20instead=20of=20being=20cloned;=20the=20GCL=20ta?=
 =?UTF-8?q?ble=20over=20the=20changed=20cells=20and=20the=20wall=20cells?=
 =?UTF-8?q?=20(the=20serial=20sums=20with=20their=20exact-zero=20terms=20l?=
 =?UTF-8?q?eft=20out,=20same=20order)=20=E2=80=94=20slab=20flag=20ny=2062?=
 =?UTF-8?q?=20CSV=20byte-identical,=20device=20moving/cg=20green;=20ny=201?=
 =?UTF-8?q?24=20rebuild=20block=201,995=20=E2=86=92=201,895=20ms=20per=20s?=
 =?UTF-8?q?tep?=
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Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
---
 .../incompressible/embedded3/cutwall.rs       | 35 ++++++++++++++---
 .../incompressible/embedded3/step/moving.rs   | 38 ++++++++++++-------
 2 files changed, 54 insertions(+), 19 deletions(-)

diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/cutwall.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/cutwall.rs
index b167cfa..38d627e 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/cutwall.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/cutwall.rs
@@ -679,12 +679,31 @@ impl Mask {
         let g = self.grid;
         let dv = g.dx * g.dy * g.dz;
         let (mut net, mut area) = (0.0, 0.0);
-        for (idx, entry) in table.iter_mut().enumerate() {
+        // P1-5 (b): a cell untouched by this build has the old volume (its
+        // entry is exactly 0 and adds nothing to `net`); a cell without a
+        // wall adds nothing to `area`. The sums run over the changed cells
+        // and the wall cells in ascending order — the serial loop's order
+        // with its zero terms left out, bit for bit.
+        let wall_cells: Vec<usize> = {
+            use rayon::prelude::*;
+            (0..table.len())
+                .into_par_iter()
+                .filter(|&idx| self.cell_active(idx) && cut.wall[idx] != [0.0; 3])
+                .collect()
+        };
+        let cells: Vec<usize> = match self.changed_cells.as_deref() {
+            Some(changed) => changed.to_vec(),
+            None => (0..table.len()).collect(),
+        };
+        for &idx in &cells {
             if !self.cell_active(idx) {
                 continue;
             }
-            *entry = (cut.vol[idx] - old_cut.vol[idx]) * dv / dt;
-            net += *entry;
+            let entry = (cut.vol[idx] - old_cut.vol[idx]) * dv / dt;
+            table[idx] = entry;
+            net += entry;
+        }
+        for &idx in &wall_cells {
             let w = cut.wall[idx];
             area += (w[0] * w[0] + w[1] * w[1] + w[2] * w[2]).sqrt();
         }
@@ -703,9 +722,13 @@ impl Mask {
         }
         let correction = if area > 0.0 { net / area } else { 0.0 };
         if correction != 0.0 {
-            for (idx, w) in cut.wall.iter().enumerate() {
-                let a = (w[0] * w[0] + w[1] * w[1] + w[2] * w[2]).sqrt();
-                table[idx] -= correction * a;
+            // Every cell with a wall (the others subtract exactly 0).
+            for idx in 0..table.len() {
+                let w = cut.wall[idx];
+                if w != [0.0; 3] {
+                    let a = (w[0] * w[0] + w[1] * w[1] + w[2] * w[2]).sqrt();
+                    table[idx] -= correction * a;
+                }
             }
         }
         (table, correction)
diff --git a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/moving.rs b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/moving.rs
index 9b7b739..43931c5 100644
--- a/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/moving.rs
+++ b/crates/specialized/rtx-cfd/src/solvers/incompressible/embedded3/step/moving.rs
@@ -72,19 +72,31 @@ impl Solver {
             self.last_ghost_correction = correction;
         }
         let s_gcl = sub.elapsed();
-        if let Some(old) = &self.mask {
-            self.apertures_old = old
-                .cut()
-                .map(|c| [c.a_u.clone(), c.a_v.clone(), c.a_w.clone()]);
-            self.vol_old = (0..field.grid.cells())
-                .map(|i| {
-                    if old.is_fluid_cell(i) {
-                        old.vol(i)
-                    } else {
-                        0.0
-                    }
-                })
-                .collect();
+        // P1-5 (a): the old mask is replaced below — its arrays MOVE into
+        // the previous-step records instead of being cloned (1 GB per step
+        // at ny 124); the values are the same.
+        if let Some(mut old) = self.mask.take() {
+            let fluid = std::mem::take(&mut old.cell_fluid);
+            if let Some(c) = old.cut.as_mut() {
+                self.apertures_old = Some([
+                    std::mem::take(&mut c.a_u),
+                    std::mem::take(&mut c.a_v),
+                    std::mem::take(&mut c.a_w),
+                ]);
+                let mut vol = std::mem::take(&mut c.vol);
+                use rayon::prelude::*;
+                vol.par_iter_mut()
+                    .zip(fluid.par_iter())
+                    .for_each(|(v, &f)| {
+                        if !f {
+                            *v = 0.0;
+                        }
+                    });
+                self.vol_old = vol;
+            } else {
+                self.apertures_old = None;
+                self.vol_old = vec![0.0; field.grid.cells()];
+            }
         }
         if std::env::var("RTX_E3_MOVING_PROFILE").is_ok() {
             let ms = |d: std::time::Duration| d.as_secs_f64() * 1e3;