Support bounded shared transverse motion and steady pinball demo

docs/steady_pinball_fiv.md

@@ -0,0 +1,69 @@
+# Uncontrolled steady pinball: development protocol
+
+Frozen before results: 1280 x 512 D2Q9/FP32/MRT, unshifted double buffer;
+rho=1, mean U=.01, nu=.004, D=20, Re_D=50. Centres are (600,255.5),
+(626,270.5), (626,240.5), radius10. No upstream cylinder. Passive probes stay
+at (800,295.5), (800,255.5), (800,215.5), radius5. Every omega is zero.
+Parabolic inlet, regularized NEQ damping1; y bounce-back planes .5/510.5;
+NEQ-extrapolated outlet with existing backflow clamp/blending. Initial fluid is
+parabolic equilibrium. Fixed warmup is 4 NX/U=512000 steps.
+
+Geometry and fluid parameters follow DynamisLab 609116ec26f19766f5e9ce2b0df427d4e24faae8,
+`legacy_test/core/legacy_env_builder.py:build_steady_cloak` and the retained
+`config_lbm_pinball_legacy_re50d_u001.json`. This is a modern Celeris reconstruction,
+not a bitwise Legacy reproduction: Legacy uses shifted incompressible MRT, a
+different curved interpolation and prescribed parabolic outlet velocity. Current
+Celeris uses standard MRT, two-branch BFL and extrapolated outlet velocity.
+The fixed-hold control uses the SAME modern settings as the free case.
+
+## Shared translation and ownership
+
+`Translation(..., shared=True)` is explicitly transverse only. Reusing one
+envelope INSTANCE declares common motion; equal but separate envelopes are not
+linked. Before initialization and every positive Simulation.run, group members
+must have identical applied offsets, effective target offsets and interval wall
+velocities. Stage each member with set_body, then call run(1) once. An incomplete
+or inconsistent target group is rejected before device uploads and can be fixed.
+Existing pending-target direct-stepper restrictions remain.
+
+Only members of that group may have overlapping whole swept bands. Their fixed
+surface clearance must exceed4 lattice cells. This conservative bound exceeds
+2 sqrt(2)+MAX_STEP = 3.329: M1 changed cells lie in old/new disks and read one-link
+neighbors; M2 reads at most two links outside its new disk. Old/new relative
+displacement is at most .5. Thus reads cannot reach another member's old/new
+solid or M1 write region. M1 writes are disjoint. M2 writes opposite population
+slots inside its own new disk, also disjoint. Separate first_slot ranges still
+allocate each body's logical band/debug storage. Shared band cells alone do not
+write anything: M1 returns unless membership changes; M2 returns unless a cut
+link exists. Unrelated moving/static body and probe overlaps still reject.
+
+This is a sufficient restricted-group condition, not support for near-contact
+or independent motion. The r4, distance8.3, dy=.5 counterexample remains rejected.
+CPU tests enumerate candidate donors/write footprints over subcell phases;
+GPU tests compare fixed vs hold and member-order permutations through crossings.
+CUDA kernels and arithmetic are unchanged.
+
+## New structural parameters (not inherited from the paper)
+
+One transverse state y,v; total assembly M=10 rho (3 pi D^2/4)=9424.77796,
+fn=U/(5D)=.0001, Tn=10000, zeta=.01, K=M(2 pi fn)^2,
+C=2 zeta M(2 pi fn). Initial y=v=0, no seed or later kick. Force is the sum
+of the three instantaneous CME_y+Wen_y values from the previous fluid step;
+node impulses are logged separately and never added. The damped oscillator is
+integrated exactly for that piecewise-constant force over dt=1. Wall velocity
+is y[n+1]-y[n], not the endpoint oscillator velocity. No filtering or retuning.
+
+Observe600000 steps=60Tn=300 D/U after warmup, in each of free and fixed-hold
+cases. Final20Tn is a descriptive comparison window; ten-Tn blocks expose drift.
+No settled/periodic claim follows merely from reaching the cap. Stop on |y|>D,
+nonfinite values, no_donor, rejected displacement or speed>.05. Report any quiet
+response honestly. No automatic extension. --smoke uses32 warmup+32 steps only.
+
+Native-step rows retain y,v, each body's CME/Wen/node terms and refill counters;
+each600000x15 FP64 file is72MB. No full-field output or policy training. Compile
+and run on one device serially, keep original outputs and dirty trees untouched.
+This stage makes no claim of physical grid convergence or vibration suppression.
+
+Run from checkout with PYTHONPATH=src and the existing Python environment:
+`python research/steady_pinball/run.py --smoke --out <new-external-directory>`
+then separately `--out <new-free-directory>` and `--fixed --out <new-fixed-directory>`.

research/steady_pinball/config.json

@@ -0,0 +1,49 @@
+{
+    "_doc": "Legacy geometry Re_D=50, U0=0.01, SI=800 baseline (kar_legacy_re50d_u001)",
+    "grid": {
+        "lattice_model": "D2Q9",
+        "nx": 1280,
+        "ny": 512,
+        "nz": 1
+    },
+    "physics": {
+        "data_type": "FP32",
+        "viscosity": 0.004,
+        "velocity": 0.01,
+        "rho": 1.0
+    },
+    "method": {
+        "collision": "MRT",
+        "streaming": "double_buffer",
+        "store_precision": "FP32",
+        "ddf_shifting": false,
+        "les": {
+            "enabled": false,
+            "cs": 0.16,
+            "closed_form": true
+        },
+        "trt": {
+            "magic_param": 0.1875
+        },
+        "inlet": {
+            "profile": "parabolic",
+            "scheme": "regularized",
+            "regularized_neq_damp": 1.0
+        },
+        "outlet": {
+            "mode": "neq_extrap",
+            "backflow_clamp": true,
+            "blend_alpha": 0.7,
+            "srt_neq_damp": 0.5
+        },
+        "y_wall_bc": "bounce_back",
+        "omega_guard": {
+            "min": 0.01,
+            "max": 1.99
+        }
+    },
+    "cuda": {
+        "threads_per_block": 256,
+        "compute_capability": "auto"
+    }
+}

research/steady_pinball/run.py

@@ -0,0 +1,157 @@
+"""Uncontrolled paper-geometry steady Re_D=50, one transverse assembly DOF.
+
+One fixed protocol; --smoke performs 32 warmup + 32 observation steps only.
+See docs/steady_pinball_fiv.md for historical solver differences and new structure.
+"""
+from __future__ import annotations
+
+import argparse
+import json
+import math
+from pathlib import Path
+import subprocess
+import time
+
+import numpy as np
+
+D, U, NU = 20., .01, .004
+CENTRES = ((600., 255.5), (626., 270.5), (626., 240.5))
+PROBES = ((800., 295.5), (800., 255.5), (800., 215.5))
+WARMUP, STEPS, PERIOD = 512000, 600000, 10000
+COLUMNS = ('step', 'y', 'v', 'cme_F', 'cme_U', 'cme_L', 'wen_F', 'wen_U', 'wen_L',
+           'node_F', 'node_U', 'node_L', 'eq_only', 'no_donor', 'crossings')
+ROOT = Path(__file__).resolve().parents[2]
+
+
+def config():
+    return json.loads(Path(__file__).with_name('config.json').read_text())
+
+
+class Structure:
+    """Exact damped oscillator update for a force held over dt=1; no force filter."""
+    def __init__(self):
+        self.mass = 10. * 3. * math.pi * D * D / 4.
+        self.omega = 2. * math.pi / PERIOD
+        self.zeta = .01
+        self.stiffness = self.mass * self.omega**2
+        self.damping = 2. * self.zeta * self.mass * self.omega
+        self.y = self.v = 0.
+        a = self.zeta * self.omega
+        wd = self.omega * math.sqrt(1. - self.zeta**2)
+        self.ec = math.exp(-a) * math.cos(wd)
+        self.es = math.exp(-a) * math.sin(wd) / wd
+        self.a = a
+
+    def candidate(self, force):
+        equilibrium = force / self.stiffness
+        z = self.y - equilibrium
+        return (equilibrium + (self.ec + self.a*self.es)*z + self.es*self.v,
+                -self.omega**2*self.es*z + (self.ec - self.a*self.es)*self.v)
+
+
+def summarize(rows):
+    windows = []
+    for start in range(0, len(rows), 10*PERIOD):
+        block = rows[start:start + 10*PERIOD]
+        fy = block[:, 3:9].sum(axis=1)
+        windows.append(dict(start_step=int(block[0, 0]), end_step=int(block[-1, 0]),
+                            y_mean_D=float(block[:, 1].mean()/D),
+                            y_rms_D=float(np.sqrt(np.mean(block[:, 1]**2))/D),
+                            y_peak_D=float(np.abs(block[:, 1]).max()/D),
+                            fy_mean=float(fy.mean()), fy_rms=float(np.sqrt(np.mean(fy**2))),
+                            mean_power=float(np.mean(fy*block[:, 2]))))
+    return windows
+
+
+def main():
+    ap = argparse.ArgumentParser(description=__doc__)
+    ap.add_argument('--out', required=True, type=Path)
+    ap.add_argument('--fixed', action='store_true', help='same modern moving-path geometry held fixed')
+    ap.add_argument('--smoke', action='store_true', help='32+32 steps, no physical result')
+    args = ap.parse_args()
+    out = args.out.resolve()
+    if out == ROOT or ROOT in out.parents:
+        ap.error('output must be outside the checkout')
+    out.mkdir(parents=True, exist_ok=False)
+    cfg = config()
+    (out/'config.json').write_text(json.dumps(cfg, indent=2)+'\n')
+    source = subprocess.check_output(['git', '-C', str(ROOT), 'rev-parse', 'HEAD'], text=True).strip()
+    s = Structure()
+    warmup, count = (32, 32) if args.smoke else (WARMUP, STEPS)
+    rows = np.lib.format.open_memmap(out/'steps.npy', mode='w+', dtype=np.float64,
+                                     shape=(count, len(COLUMNS)))
+    rows[:] = np.nan
+    from CelerisLab import Simulation
+    from CelerisLab.body.motion import MovingTables, Translation
+    names = MovingTables.DIAG
+    col = {k: names.index(k) for k in names}
+    status, completed, f_last = 'running', 0, 0.
+    started = time.monotonic()
+    report = dict(source=source, fixed=args.fixed, smoke=args.smoke, columns=COLUMNS,
+                  structure=dict(mass=s.mass, stiffness=s.stiffness, damping=s.damping,
+                                 zeta=s.zeta, fn=1./PERIOD, y0=0., v0=0.),
+                  warmup_steps=warmup, requested_steps=count, dt=1., D=D, U=U, nu=NU,
+                  duration_Tn=count/PERIOD, duration_tU_D=count*U/D,
+                  rhs='sum of three instantaneous CME_y + Wen_y, one-step lag, no filtering')
+    try:
+        with Simulation(str(out/'config.json')) as sim:
+            # Match Legacy object order; probes remain passive and fixed in the lab.
+            for c in PROBES:
+                sim.add_body('sensor', c, 5.)
+            envelope = Translation((0., 0., -D, D), .05, shared=True)
+            bodies = [sim.add_body('circle', c, D/2, translation=envelope) for c in CENTRES]
+            sim.initialize()
+            # At release obtain the last instantaneous force, not a warmup average.
+            remaining = warmup - 1
+            while remaining:
+                chunk = min(10000, remaining)
+                sim.run(chunk)
+                remaining -= chunk
+            sim.run(1)
+            d = sim.moving_diagnostics()
+            f_last = float(d[:, col['cme_y']].sum(dtype=np.float64) + d[:, col['wen_y']].sum(dtype=np.float64))
+            report['warmup_elapsed_s'] = time.monotonic()-started
+            print(json.dumps(dict(event='release', **report)), flush=True)
+            for n in range(count):
+                y1, v1 = (0., 0.) if args.fixed else s.candidate(f_last)
+                if not all(map(math.isfinite, (y1, v1))) or abs(y1) > D:
+                    status = 'stopped_nonfinite_or_travel_bound'
+                    break
+                if not args.fixed:
+                    for bid in bodies:
+                        sim.set_body(bid, offset=(0., y1), vy=y1-s.y)
+                sim.run(1)
+                d = sim.moving_diagnostics()
+                cme, wen = d[:, col['cme_y']], d[:, col['wen_y']]
+                node = d[:, col['cover_jy']] - d[:, col['uncover_jy']]
+                row = (n+1, y1, v1, *cme, *wen, *node,
+                       d[:, col['eq_only']].sum(), d[:, col['no_donor']].sum(), d[:, col['crossings']].sum())
+                if not np.isfinite(row).all() or row[-2] != 0:
+                    status = 'stopped_nonfinite_or_no_donor'
+                    break
+                rows[n] = row
+                completed = n+1
+                s.y, s.v = y1, v1
+                f_last = float(cme.sum(dtype=np.float64)+wen.sum(dtype=np.float64))
+                if completed % PERIOD == 0:
+                    rows.flush()
+                    print(json.dumps(dict(event='period', step=completed, t_Tn=completed/PERIOD,
+                                          y_D=s.y/D, elapsed_s=time.monotonic()-started)), flush=True)
+            else:
+                status = 'smoke_completed' if args.smoke else 'duration_completed_not_qualified'
+    except BaseException as exc:
+        status = f'error: {type(exc).__name__}: {exc}'
+        raise
+    finally:
+        rows.flush()
+        report.update(status=status, completed_steps=completed, elapsed_s=time.monotonic()-started,
+                      windows=summarize(rows[:completed]),
+                      totals={key: float(rows[:completed, i].sum()) for key, i in
+                              [('eq_only', 12), ('no_donor', 13), ('crossings', 14)]},
+                      limitation='Uncontrolled development response only. No automatic settling, grid convergence, control or historical-solver equivalence claim.')
+        (out/'report.json').write_text(json.dumps(report, indent=2)+'\n')
+        print(json.dumps(report), flush=True)
+
+
+if __name__ == '__main__':
+    main()

src/CelerisLab/body/motion.py

@@ -14,6 +14,7 @@ from typing import Callable, Optional, Tuple
 
 MAX_STEP = 0.5     # Euclidean centre displacement per step (lattice cells)
 BAND_MARGIN = 2    # cells added around the swept circle AABB
+SHARED_CLEARANCE = 4.0  # > two diagonal links + MAX_STEP; docs/steady_pinball_fiv.md
 R_MIN = 4.0        # smallest radius covered by the donor enumeration
 
 Vec = Tuple[float, float]
@@ -31,6 +32,7 @@ class Translation:
     """Declared motion envelope: offset box (x_min, x_max, y_min, y_max) and speed bound."""
     offset_box: Tuple[float, float, float, float]
     max_speed: float
+    shared: bool = False  # same envelope instance: synchronous transverse motion
 
     def __post_init__(self):
         box = tuple(float(v) for v in self.offset_box)
@@ -41,10 +43,29 @@ class Translation:
             raise ValueError("offset_box must contain the zero offset")
         if not 0.0 < speed <= MAX_STEP:
             raise ValueError(f"max_speed must satisfy 0 < max_speed <= {MAX_STEP}")
+        if self.shared and box[:2] != (0.0, 0.0):
+            raise ValueError("shared translation is transverse only (x offsets must be zero)")
         object.__setattr__(self, "offset_box", box)
         object.__setattr__(self, "max_speed", speed)
 
 
+def validate_shared_translations(circles):
+    """Check common old/target offsets and wall velocities before device work.
+
+    Per-body setters may stage members in any order. Envelope identity (not
+    value equality) defines a group; a missing target means hold at applied.
+    """
+    groups = {}
+    for motion in circles:
+        if not motion.translation.shared:
+            continue
+        target = motion.pending if motion.pending is not None else (motion.applied, (0.0, 0.0))
+        state = (motion.applied, target)
+        key = id(motion.translation)
+        if groups.setdefault(key, state) != state:
+            raise ValueError("shared translation needs identical applied/target offsets and velocities")
+
+
 class MovingCircle:
     """Applied offset, pending target and the fail-closed commit of one circle."""
 
@@ -72,6 +93,8 @@ class MovingCircle:
         x0, x1, y0, y1 = self.translation.offset_box
         if not (x0 <= offset[0] <= x1 and y0 <= offset[1] <= y1):
             raise ValueError(f"offset {offset} outside declared box {self.translation.offset_box}")
+        if self.translation.shared and velocity[0] != 0.0:
+            raise ValueError("shared translation needs zero x velocity")
         step = math.hypot(offset[0] - self.applied[0], offset[1] - self.applied[1])
         if step > MAX_STEP:
             raise ValueError(f"per-step displacement {step} exceeds {MAX_STEP}")

src/CelerisLab/simulation.py

@@ -259,6 +259,8 @@ class Simulation:
         moving = self._moving_bodies()
         if not moving:
             return
+        from .body.motion import SHARED_CLEARANCE, validate_shared_translations
+        validate_shared_translations([obj.motion for obj in moving])
         cfg = self.lbm_cfg
         if (not cfg.is_d2q9 or cfg.streaming != "double_buffer" or cfg.ddf_shifting
                 or cfg.store_precision != "FP32"):
@@ -277,6 +279,13 @@ class Simulation:
                 if other is obj:
                     continue
                 (cx, cy), r = other.center[:2], float(other.radius)
+                if (other.motion is not None and obj.motion.translation.shared
+                        and obj.motion.translation is other.motion.translation):
+                    # Whole envelopes may overlap; simultaneous local stencils may not.
+                    gap = np.hypot(obj.center[0] - cx, obj.center[1] - cy) - obj.radius - r
+                    if gap <= SHARED_CLEARANCE:
+                        raise ValueError("shared translation needs surface clearance > 4 cells")
+                    continue
                 if other.motion is not None:
                     ox0, ox1, oy0, oy1 = other.motion.band()
                 else:
@@ -719,6 +728,9 @@ class Simulation:
         if steps < 0 or checkpoint_interval < 0:
             raise ValueError("steps and checkpoint_interval must be nonnegative")
         moving = self._moving_bodies()
+        if steps > 0 and moving:
+            from .body.motion import validate_shared_translations
+            validate_shared_translations([obj.motion for obj in moving])
         if steps > 0 and moving and self.stepper.moving is None:
             raise RuntimeError(MOVING_BLOCKED)
         if moving and checkpoint_interval > 0:

tests/integration/test_shared_translation_gpu.py

@@ -0,0 +1,60 @@
+"""Small serial GPU gates for shared transverse circles; no physical benchmark."""
+import json
+from pathlib import Path
+
+import numpy as np
+
+from CelerisLab import Simulation
+from CelerisLab.body.motion import MovingTables, Translation
+
+CENTRES = ((70., 79.5), (96., 94.5), (96., 64.5))
+
+
+def simulate(tmp_path, *, moving, offsets, reverse=False):
+    config = json.loads((Path(__file__).resolve().parents[2]/'research/steady_pinball/config.json').read_text())
+    config['grid'].update(nx=192, ny=160)
+    p = tmp_path/'config.json'
+    p.write_text(json.dumps(config))
+    with Simulation(str(p)) as sim:
+        envelope = Translation((0., 0., -20., 20.), .5, shared=True)
+        centres = list(reversed(CENTRES)) if reverse else list(CENTRES)
+        ids = [sim.add_body('circle', c, 10., translation=envelope if moving else None) for c in centres]
+        sim.initialize()
+        previous = 0.
+        crossings = 0.
+        for y in offsets:
+            if moving:
+                for bid in ids:
+                    sim.set_body(bid, offset=(0., float(y)), vy=float(y-previous))
+            sim.run(1)
+            if moving:
+                diag = sim.moving_diagnostics()
+                assert np.isfinite(diag).all()
+                assert np.all(diag[:, MovingTables.DIAG.index('no_donor')] == 0)
+                crossings += diag[:, MovingTables.DIAG.index('crossings')].sum()
+            previous = y
+        forces = np.array([sim.read_force(bid) for bid in ids])
+        if reverse:
+            forces = forces[::-1]
+        return sim.get_ddf(), sim.get_flags(), forces, crossings
+
+
+def test_fixed_vs_shared_hold(tmp_path):
+    fixed = simulate(tmp_path, moving=False, offsets=[0.]*32)
+    held = simulate(tmp_path, moving=True, offsets=[0.]*32)
+    assert np.array_equal(fixed[0], held[0], equal_nan=True)
+    assert np.array_equal(fixed[1], held[1])
+    # FP32 atomic force reduction order only; populations remain exactly equal.
+    np.testing.assert_allclose(fixed[2], held[2], rtol=0., atol=1e-4)
+    assert held[3] == 0
+
+
+def test_common_motion_member_order_has_no_population_effect(tmp_path):
+    offsets = np.concatenate((np.arange(.5, 20.5, .5), np.arange(19.5, -20.5, -.5),
+                              np.arange(-19.5, .5, .5)))
+    forward = simulate(tmp_path, moving=True, offsets=offsets)
+    reverse = simulate(tmp_path, moving=True, offsets=offsets, reverse=True)
+    assert forward[3] > 0 and forward[3] == reverse[3]
+    assert np.array_equal(forward[0], reverse[0], equal_nan=True)
+    assert np.array_equal(forward[1], reverse[1])
+    np.testing.assert_allclose(forward[2], reverse[2], rtol=0., atol=1e-4)

tests/unit/test_shared_translation.py

@@ -0,0 +1,122 @@
+"""Host admission and independent lattice-stencil checks; no CUDA context."""
+from types import SimpleNamespace
+
+import numpy as np
+import pytest
+
+from CelerisLab.body.motion import MovingCircle, MovingTables, Translation, validate_shared_translations
+from CelerisLab.lbm.descriptors import FLUID
+from tests.unit.test_moving_bodies import STREAM, body, moving_simulation, routed_simulation
+
+
+def scene(monkeypatch, envelope=None, gap=10.):
+    sim = routed_simulation(monkeypatch)
+    envelope = envelope or Translation((0., 0., -6., 6.), .5, shared=True)
+    for y in (40. - (20. + gap) / 2., 40. + (20. + gap) / 2.):
+        sim.bodies.add(body(60., y, 10., envelope))
+    sim.field = SimpleNamespace(nx=120, ny=80, build_channel_flags=lambda: np.full((80, 120), FLUID))
+    return sim
+
+
+def test_shared_bands_are_admitted_but_other_groups_are_not(monkeypatch):
+    sim = scene(monkeypatch)
+    sim._validate_moving_bodies()
+    a, b = [o.motion for o in sim.bodies.objects]
+    assert a.band()[3] >= b.band()[2]  # whole envelopes genuinely overlap
+    band, geom = MovingTables.layout([(a, 0), (b, 1)])
+    assert band[10] == band[2] * band[3] * 8
+    assert len(geom) == 14
+    b.translation = Translation((0., 0., -6., 6.), .5, shared=True)
+    with pytest.raises(ValueError, match='overlaps body'):
+        sim._validate_moving_bodies()  # equal values do not link independent motion
+    b.translation = a.translation
+    sim.bodies.add(body(60., 40., 2.))
+    with pytest.raises(ValueError, match='overlaps body'):
+        sim._validate_moving_bodies()
+
+
+@pytest.mark.parametrize('gap', (.3, 4.))
+def test_small_surface_gaps_rejected(monkeypatch, gap):
+    with pytest.raises(ValueError, match='clearance'):
+        scene(monkeypatch, gap=gap)._validate_moving_bodies()
+
+
+def test_shared_envelope_is_transverse_only():
+    with pytest.raises(ValueError, match='transverse'):
+        Translation((-.1, .1, -1., 1.), .5, shared=True)
+
+    envelope = Translation((0., 0., -1., 1.), .5, shared=True)
+    with pytest.raises(ValueError, match="zero x velocity"):
+        MovingCircle((0., 0.), 10., envelope).check_target((0., .1), (.1, .1))
+
+def test_group_rejection_is_before_writes_and_can_be_corrected(monkeypatch):
+    sim, writes = moving_simulation(monkeypatch)
+    env = Translation((0., 0., -2., 2.), .5, shared=True)
+    sim.bodies.objects = [body(40., y, 10., env) for y in (20., 50.)]
+    a, b = [o.motion for o in sim.bodies.objects]
+    sim.stepper.moving = object()
+    a.set_target((0., .2), (0., .2))
+    with pytest.raises(ValueError, match='identical'):
+        sim.run(1, stream=STREAM)
+    assert writes == [] and a.applied == b.applied == (0., 0.)
+    b.set_target((0., .2), (0., .1))
+    with pytest.raises(ValueError, match='identical'):
+        sim.run(1, stream=STREAM)
+    assert writes == []
+    b.set_target((0., .2), (0., .2))
+    validate_shared_translations([a, b])
+    b.applied = (0., -.1)
+    with pytest.raises(ValueError, match='identical'):
+        validate_shared_translations([a, b])
+
+
+C = np.array([(x, y) for x in (-1, 0, 1) for y in (-1, 0, 1) if (x, y) != (0, 0)])
+
+
+def footprint_conflicts(centres, radius, dy):
+    """Enumerate geometric write/read supports, not refill/BFL formulas.
+
+    Tests every candidate donor, conservatively including M2's second link.
+    Body-indexed slot numbers are separate from physical (cell,direction) writes.
+    """
+    centres = np.asarray(centres, float)
+    lo = np.floor(centres.min(0) - radius - 4).astype(int)
+    hi = np.ceil(centres.max(0) + radius + 4).astype(int)
+    yy, xx = np.mgrid[lo[1]:hi[1]+1, lo[0]:hi[0]+1]
+    points = np.column_stack((xx.ravel(), yy.ravel()))
+    def inside(p, c):
+        return ((p - c)**2).sum(axis=-1) < radius**2
+    old = np.array([inside(points, c) for c in centres])
+    new_centres = centres + (0., dy)
+    new = np.array([inside(points, c) for c in new_centres])
+    changed = old ^ new
+    conflicts = int(np.count_nonzero(changed.sum(0) > 1))
+    writes = set()
+    for i, (oc, nc) in enumerate(zip(centres, new_centres)):
+        others = [j for j in range(len(centres)) if j != i]
+        fresh = points[old[i] & ~new[i]]
+        for e in C:
+            donor = fresh + e
+            valid = ~inside(donor, oc) & ~inside(donor, nc)
+            for j in others:
+                conflicts += int(np.count_nonzero(valid & (inside(donor, centres[j]) | inside(donor, new_centres[j]))))
+            link = ~new[i] & inside(points + e, nc)
+            fluid = points[link]
+            solid = fluid + e
+            for p in solid:
+                slot = (int(p[0]), int(p[1]), int(e[0]), int(e[1]))
+                conflicts += int(slot in writes)
+                writes.add(slot)
+            for read in (fluid, fluid - e):
+                for j in others:
+                    conflicts += int(np.count_nonzero(inside(read, centres[j]) | inside(read, new_centres[j])))
+    return conflicts
+
+
+def test_counterexample_and_pinball_local_ownership():
+    # Close but disjoint circles are insufficient: parent/Grok independent example.
+    assert footprint_conflicts(((.25, .25), (.25, 8.55)), 4., .5) > 0
+    for phase in (0., .25, .5, .75):
+        for dy in (-.5, -.1, .1, .5):
+            centres = np.array(((0., 0.), (26., 15.), (26., -15.))) + phase
+            assert footprint_conflicts(centres, 10., dy) == 0

tests/unit/test_steady_pinball.py

@@ -0,0 +1,54 @@
+"""Real config loading and pure structural update for the frozen development case."""
+import importlib.util
+import math
+from pathlib import Path
+
+import numpy as np
+import pytest
+
+path = Path(__file__).resolve().parents[2] / 'research/steady_pinball/run.py'
+spec = importlib.util.spec_from_file_location('steady_pinball_run', path)
+runner = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(runner)
+
+
+def test_actual_loader_and_paper_geometry(tmp_path):
+    import json
+    from CelerisLab.config import load_lbm_config
+    p = tmp_path/'config.json'
+    p.write_text(json.dumps(runner.config()))
+    cfg = load_lbm_config(str(p))
+    cfg.validate()
+    assert (cfg.nx, cfg.ny, cfg.viscosity, cfg.velocity) == (1280, 512, .004, .01)
+    assert cfg.collision == 'MRT' and cfg.y_wall_bc == 'bounce_back'
+    assert runner.U * runner.D / runner.NU == 50.
+    assert runner.CENTRES == ((600., 255.5), (626., 270.5), (626., 240.5))
+
+
+def test_structure_rest_equilibrium_and_energy_decay():
+    s = runner.Structure()
+    assert s.candidate(0.) == (0., 0.)
+    assert s.mass == pytest.approx(10*3*math.pi*100)
+    s.y, s.v = .2, 0.
+    # One damped period: independent continuous solution at total elapsed time.
+    for _ in range(runner.PERIOD):
+        s.y, s.v = s.candidate(0.)
+    t = runner.PERIOD
+    a = s.zeta*s.omega
+    w = s.omega*math.sqrt(1-s.zeta**2)
+    expected = .2*math.exp(-a*t)*(math.cos(w*t)+a/w*math.sin(w*t))
+    assert s.y == pytest.approx(expected, abs=2e-12)
+    assert .5*s.mass*s.v*s.v+.5*s.stiffness*s.y*s.y < .5*s.stiffness*.2**2
+    s.y, s.v = .3, 0.
+    assert s.candidate(s.stiffness*.3) == pytest.approx((.3, 0.), abs=1e-15)
+
+
+def test_report_uses_completed_rows_and_net_force():
+    rows = np.zeros((4, 15))
+    rows[:, 0] = np.arange(1, 5)
+    rows[:, 1] = 2.
+    rows[:, 3:9] = 1.
+    got = runner.summarize(rows)
+    assert got[0]['end_step'] == 4
+    assert got[0]['y_rms_D'] == .1
+    assert got[0]['fy_rms'] == 6.