rustytorch/crates/specialized/rtx-fsi/tests/fsi2_interface_noise.rs

//! The FSI2 interface noise floor, measured directly — the lever the
//! benchmark's resonant cycle waits on.
//!
//! # What is being measured
//!
//! The partitioned coupling iterates one pass: subcycled fluid march on a
//! candidate interface geometry, traction sampling on that geometry, one
//! structure step under the sampled load. The tenth-session study found
//! that this map is not continuous at small scales: a vanishing interface
//! change flips embedded-mask cells, the load jumps by a finite amount,
//! and the jump maps through Newmark's `beta dt_c^2 / m` into an
//! end-of-step displacement jump. That jump size is the **interface noise
//! floor**: no coupling tolerance below it is reachable, and the
//! step-to-step scatter of the accepted interface at the tolerance feeds
//! the fluid wall-velocity noise of tolerance / dt_c (solver_status.md
//! §"C2 — FSI2").
//!
//! This test measures the floor as the modulus of continuity of the real
//! pass map, on the real machinery (`fsi2_harness`), at full inflow: a
//! fine sweep of bending amplitudes (every pass from the same snapshot),
//! whose maximum successive difference is the mask-flip jump — plus a
//! decades ladder around zero for the small-scale behaviour.
//!
//! # What the 2026-08-21 measurements found (ny = 62, t = 4 s)
//!
//! 1. **The floor is NOT in the traction sampling.** Surface smoothing of
//!    the sampled tractions ([`rtx_fsi::smooth_tractions`]) at radii of
//!    1–3 cells leaves the flip-scan floor unchanged to 0.2% (3.05e-5 at
//!    every radius): the flip's load jump is **coherent through the
//!    fluid field itself** — the mask rebuild shifts the pressure
//!    solution around the flipped cell and every nearby sample moves
//!    together — and a surface moving-average preserves exactly such
//!    coherent shifts. Smoothing therefore stays available but OFF by
//!    default; reaching for it against this floor is a measured dead
//!    end. The committed run keeps one smoothed scan alive so this
//!    attribution stays loud.
//! 2. **The flip scan does NOT transfer across subcycles** — a
//!    dt_c^2-scaling hypothesis for the floor (load jump through
//!    Newmark's `beta dt_c^2 / m`) was tried against a subcycle-2 scan
//!    and REFUTED in that operationalization: max jump 5.4e-5 vs 3.0e-5
//!    at subcycle 8, median 15x LARGER. The scan's successive passes
//!    differ by a fixed geometry increment, so their wall-velocity
//!    difference is increment / dt_c, and the smooth velocity-response
//!    trend inflates as dt_c shrinks until it swamps the flip signal.
//!    A cross-subcycle floor claim needs the OPERATIONAL measurement
//!    instead:
//! 3. **The stall measurement** — the real release-step map iterated at
//!    a deep tolerance with the per-pass residuals traced — is the
//!    operational floor, and it came out FAR below every march
//!    tolerance: s8 aitken 3.4e-9 / iqn 1.6e-9 in a 12-pass budget, s2
//!    aitken 6.5e-10 / iqn 6.3e-10 CONVERGED below 1e-9 in 5–6 passes.
//!    The flip jumps are events at specific geometries, not a floor
//!    under every step; a typical step's map is locally smooth. The
//!    tenth session's subcycle-2 blowup (tolerance held at 2e-4 while
//!    dt_c shrank — wall-velocity noise tol / dt_c) was a tolerance
//!    mis-budgeting, not an impassable floor: tighter coupling is open
//!    at a tolerance the map demonstrably supports (~1e-5 leaves three
//!    decades of margin). Occasional flip-straddling steps still stall
//!    at the jump scale — the march's stall-accept handles those.
//!
//! Environment knobs: `RTX_NOISE_NY` (default 62), `RTX_NOISE_T` (rigid
//! march horizon, default 4 s), `RTX_NOISE_SUBCYCLE` (baseline subcycle,
//! default 8), `RTX_NOISE_SCAN` (flip-scan resolution, default 40
//! passes), `RTX_NOISE_EPS` (flip-scan amplitude, default 5e-4 m),
//! `RTX_NOISE_FULL` (nonzero: sweep radii 0–3 h and subcycles {8, 4, 2,
//! 1} instead of the committed set), `RTX_NOISE_CASE` (`fsi2` default or
//! `fsi3` — the pins apply to FSI2 only; FSI3's floors are measured, not
//! borrowed).

mod fsi2_harness;

use fsi2_harness::{FLAG_X0, FLAG_X1, Fsi2Harness, clamp_left, env_or};
use rtx_cfd::solvers::incompressible::{EmbeddedPisoSolver, FlowField};
use rtx_fea::analysis::{AnalysisConfig, ConvergenceCriteria, NonlinearDynamicAnalysis};
use rtx_fea::materials::{LinearElastic, MaterialDatabase};
use rtx_fea::mesh::MaterialId;

fn norm(v: &[f64]) -> f64 {
    v.iter().map(|x| x * x).sum::<f64>().sqrt()
}

fn sub(a: &[f64], b: &[f64]) -> Vec<f64> {
    a.iter().zip(b).map(|(x, y)| x - y).collect()
}

#[test]
#[allow(clippy::too_many_lines)]
fn fsi2_interface_noise_floor() {
    let ny = env_or("RTX_NOISE_NY", 62.0) as usize;
    let t_probe = env_or("RTX_NOISE_T", 4.0);
    let base_subcycle = env_or("RTX_NOISE_SUBCYCLE", 8.0) as usize;
    let scan_passes = env_or("RTX_NOISE_SCAN", 40.0) as usize;
    let eps_max = env_or("RTX_NOISE_EPS", 5e-4);
    let full_sweep = env_or("RTX_NOISE_FULL", 0.0) != 0.0;
    let flag_nx = 35;

    let case = match std::env::var("RTX_NOISE_CASE").as_deref() {
        Ok("fsi3" | "FSI3") => fsi2_harness::FSI3,
        _ => fsi2_harness::FSI2,
    };
    let (mut harness, mut solver, mut field) = Fsi2Harness::build_case(case, ny, flag_nx, 0.0);
    // M1 precision probe (`RTX_FSI2_POISSON_F32=1`): the pressure
    // multigrid's V-cycle in f32 inside the f64 CG. Printed so the arm can
    // never pass vacuously.
    if env_or("RTX_FSI2_POISSON_F32", 0.0) != 0.0 {
        solver.set_poisson_precision(rtx_cfd::solvers::incompressible::MgPrecision::F32);
        println!("  poisson V-cycle precision: F32 (M1 probe)");
    }
    let dt_fluid = harness.dt_fluid;

    // Rigid march to operating loads (the floor rides with the loads —
    // measuring at startup would understate it by orders of magnitude).
    let start = std::time::Instant::now();
    let rigid_steps = (t_probe / dt_fluid).round() as usize;
    for _ in 0..rigid_steps {
        futures::executor::block_on(solver.advance(&mut field, dt_fluid)).unwrap();
    }
    println!(
        "  rigid march: {rigid_steps} steps to t = {t_probe:.1} s in {:.0} s wall",
        start.elapsed().as_secs_f64()
    );

    let zero_d = vec![0.0; 2 * harness.interface.wetted.len()];
    let fluid_saved = solver.snapshot();
    let field_saved = field.clone();

    // A smooth cantilever-bending perturbation pattern, unit tip
    // amplitude: p_y = ((x - x0)/(x1 - x0))^2, p_x = 0 — the shape a
    // subiteration increment actually has.
    let pattern: Vec<f64> = harness
        .interface
        .reference
        .iter()
        .flat_map(|&(x, _)| {
            let s = (x - FLAG_X0) / (FLAG_X1 - FLAG_X0);
            [0.0, s * s]
        })
        .collect();

    // The flag stepper is rebuilt per subcycle: the coupled dt (and with
    // it Newmark's beta dt^2 / m response to a load jump) is exactly
    // what the scaling measurement varies.
    let flag_mesh = harness.mesh.clone();
    let make_analysis = move |subcycle: usize| {
        let mut db = MaterialDatabase::new();
        db.add_material(
            MaterialId(0),
            LinearElastic::new(case.e_s, case.nu_s).with_density(case.rho_s),
            None,
        );
        NonlinearDynamicAnalysis::new(
            flag_mesh.clone(),
            db,
            clamp_left(&flag_mesh),
            dt_fluid * subcycle as f64,
            1,
            AnalysisConfig::default(),
        )
        .with_total_lagrangian()
        .with_convergence_criteria(ConvergenceCriteria {
            max_iterations: 60,
            ..ConvergenceCriteria::default()
        })
    };

    // One config's floor: pass-map continuity at a given subcycle and
    // smoothing radius, all passes from the same saved fluid state.
    let measure = |harness: &mut Fsi2Harness,
                   solver: &mut EmbeddedPisoSolver,
                   subcycle: usize,
                   radius_in_h: f64,
                   ladder: bool|
     -> (f64, f64) {
        harness.smooth_radius = radius_in_h * harness.h;
        // Reset the shared body geometry: a previous measurement's last
        // candidate must not leak into this one's initial load sampling
        // (it did, before this line — a 4.5e-5 phantom first residual).
        harness.set_geometry(&zero_d, &zero_d);
        let analysis = make_analysis(subcycle);
        let mut flag = analysis.stepper().unwrap();
        let wetted_dofs: Vec<[usize; 2]> = harness
            .interface
            .wetted
            .iter()
            .map(|&id| {
                let dofs = flag.node_dofs(id);
                [dofs[0], dofs[1]]
            })
            .collect();
        let (nodal0, _, _) = harness.sample_load(solver, &field_saved, &zero_d);
        flag.set_nodal_forces(&nodal0);
        let flag_state = flag.rest_state().unwrap();

        let mut pass = |d_candidate: &[f64]| -> Vec<f64> {
            solver.restore(&fluid_saved);
            let mut trial_field: FlowField = field_saved.clone();
            harness.advance_subcycled(
                solver,
                &mut trial_field,
                &zero_d,
                d_candidate,
                subcycle,
                None,
            );
            let (nodal, _, _) = harness.sample_load(solver, &trial_field, d_candidate);
            flag.set_nodal_forces(&nodal);
            let (candidate_state, _) = flag.step(&flag_state).unwrap();
            let mut d = vec![0.0; 2 * wetted_dofs.len()];
            for (k, dofs) in wetted_dofs.iter().enumerate() {
                d[2 * k] = candidate_state.displacement[dofs[0]];
                d[2 * k + 1] = candidate_state.displacement[dofs[1]];
            }
            d
        };

        let base = pass(&zero_d);
        if ladder {
            print!("  subcycle {subcycle} smooth {radius_in_h:.1}h  ladder:");
            for exp in [-7.0f64, -6.0, -5.0, -4.0] {
                let eps = 10.0f64.powf(exp);
                let d: Vec<f64> = pattern.iter().map(|p| eps * p).collect();
                let response = norm(&sub(&pass(&d), &base));
                print!("  1e{exp:.0} -> {response:.2e}");
            }
            println!();
        }

        let mut previous = base;
        let mut max_jump = 0.0f64;
        let mut jumps = Vec::with_capacity(scan_passes);
        for k in 1..=scan_passes {
            let eps = eps_max * k as f64 / scan_passes as f64;
            let d: Vec<f64> = pattern.iter().map(|p| eps * p).collect();
            let current = pass(&d);
            let jump = norm(&sub(&current, &previous));
            jumps.push(jump);
            max_jump = max_jump.max(jump);
            previous = current;
        }
        jumps.sort_by(|a, b| a.partial_cmp(b).unwrap());
        let median_jump = jumps[jumps.len() / 2];
        println!(
            "  subcycle {subcycle} smooth {radius_in_h:.1}h  flip scan ({scan_passes} passes to \
             eps = {eps_max:.1e}): max successive jump {max_jump:.3e}, median {median_jump:.3e}"
        );
        (max_jump, median_jump)
    };

    if full_sweep {
        for &s in &[base_subcycle, 4, 2, 1] {
            for r in [0.0, 1.0, 2.0, 3.0] {
                measure(&mut harness, &mut solver, s, r, r == 0.0);
            }
        }
        println!(
            "  full sweep total {:.0} s wall",
            start.elapsed().as_secs_f64()
        );
        return;
    }

    // The stall measurement: iterate the REAL release-step map at an
    // unreachable tolerance and trace every pass's residual. Where the
    // subiteration stalls is the operational noise floor — the number a
    // march's coupling tolerance must sit above — measured per subcycle
    // and per coupler on the same fluid state.
    let stall = |harness: &mut Fsi2Harness,
                 solver: &mut EmbeddedPisoSolver,
                 subcycle: usize,
                 coupler: &str|
     -> (f64, f64, usize) {
        harness.smooth_radius = 0.0;
        harness.set_geometry(&zero_d, &zero_d);
        let analysis = make_analysis(subcycle);
        let flag = std::cell::RefCell::new(analysis.stepper().unwrap());
        let wetted_dofs: Vec<[usize; 2]> = harness
            .interface
            .wetted
            .iter()
            .map(|&id| {
                let dofs = flag.borrow().node_dofs(id);
                [dofs[0], dofs[1]]
            })
            .collect();
        let extract = |state: &rtx_fea::analysis::DynamicState| -> Vec<f64> {
            let mut d = vec![0.0; 2 * wetted_dofs.len()];
            for (k, dofs) in wetted_dofs.iter().enumerate() {
                d[2 * k] = state.displacement[dofs[0]];
                d[2 * k + 1] = state.displacement[dofs[1]];
            }
            d
        };
        let (nodal0, _, _) = harness.sample_load(solver, &field_saved, &zero_d);
        flag.borrow_mut().set_nodal_forces(&nodal0);
        let flag_state = flag.borrow_mut().rest_state().unwrap();
        // The predictor the march uses: the structure alone under the
        // committed load.
        let (predicted, _) = flag.borrow_mut().step(&flag_state).unwrap();
        let d_pred = extract(&predicted);

        let solver = std::cell::RefCell::new(solver);
        let trace = std::cell::RefCell::new(Vec::<f64>::new());
        let harness_ref = &*harness;
        let pass = |d_candidate: &[f64]| -> Vec<f64> {
            let mut solver_ref = solver.borrow_mut();
            solver_ref.restore(&fluid_saved);
            let mut trial_field: FlowField = field_saved.clone();
            harness_ref.advance_subcycled(
                &mut solver_ref,
                &mut trial_field,
                &zero_d,
                d_candidate,
                subcycle,
                None,
            );
            let (nodal, _, _) = harness_ref.sample_load(&solver_ref, &trial_field, d_candidate);
            let mut flag_ref = flag.borrow_mut();
            flag_ref.set_nodal_forces(&nodal);
            let (candidate_state, _) = flag_ref.step(&flag_state).unwrap();
            let d_new = extract(&candidate_state);
            trace.borrow_mut().push(norm(&sub(&d_new, d_candidate)));
            d_new
        };

        // A tolerance at the bottom of what the map could conceivably
        // support: the point is the trace, not the verdict. (It turned
        // out to be REACHABLE at subcycle 2 — that reachability is the
        // finding pinned below.)
        let budget = 12;
        let deep = 1e-9;
        let outcome = if coupler == "iqn" {
            rtx_fsi::IqnIls::new(budget, deep)
                .unwrap()
                .solve(&d_pred, pass)
        } else {
            rtx_fsi::Subiterated::aitken(budget, deep)
                .unwrap()
                .solve(&d_pred, pass)
        };
        drop(outcome); // converged or budget-exhausted — the trace has the data
        let trace = trace.into_inner();
        let min = trace.iter().copied().fold(f64::MAX, f64::min);
        let last = *trace.last().unwrap();
        println!(
            "  stall subcycle {subcycle} {coupler}: {} passes, residual first {:.3e} \
             min {min:.3e} last {last:.3e}",
            trace.len(),
            trace.first().unwrap()
        );
        (min, last, trace.len())
    };

    // The committed set: the baseline floor, the smoothed floor (the
    // attribution guard), the cross-subcycle scan (recorded, unpinned —
    // see the module docs for why it does not transfer), and the stall
    // levels per subcycle and coupler.
    let (floor_base, _) = measure(&mut harness, &mut solver, base_subcycle, 0.0, true);
    let (floor_smoothed, _) = measure(&mut harness, &mut solver, base_subcycle, 2.0, false);
    let (floor_tight_scan, _) = measure(&mut harness, &mut solver, base_subcycle / 4, 0.0, true);
    let stall_8_aitken = stall(&mut harness, &mut solver, base_subcycle, "aitken");
    let stall_8_iqn = stall(&mut harness, &mut solver, base_subcycle, "iqn");
    let stall_2_aitken = stall(&mut harness, &mut solver, base_subcycle / 4, "aitken");
    let stall_2_iqn = stall(&mut harness, &mut solver, base_subcycle / 4, "iqn");
    println!(
        "  floors: scan base {floor_base:.3e}, smoothed(2h) {floor_smoothed:.3e}, \
         subcycle/4 scan {floor_tight_scan:.3e} (trend-contaminated, unpinned); \
         stalls (min): s8 aitken {:.3e} / iqn {:.3e}, s2 aitken {:.3e} / iqn {:.3e}; \
         total {:.0} s wall",
        stall_8_aitken.0,
        stall_8_iqn.0,
        stall_2_aitken.0,
        stall_2_iqn.0,
        start.elapsed().as_secs_f64()
    );

    for value in [
        floor_base,
        floor_smoothed,
        floor_tight_scan,
        stall_8_aitken.0,
        stall_8_iqn.0,
        stall_2_aitken.0,
        stall_2_iqn.0,
    ] {
        assert!(value.is_finite() && value > 0.0);
    }

    // Pins are for the default configuration (FSI2) only.
    if case == fsi2_harness::FSI2
        && ny == 62
        && base_subcycle == 8
        && (t_probe - 4.0).abs() < 1e-9
        && scan_passes == 40
    {
        // The unsmoothed floor at these loads, measured 2026-08-21 as
        // 3.05e-5. The band is generous (the maximum of 40 samples of a
        // jump process moves between platforms); leaving it is a
        // material change to the pass map's continuity either way.
        assert!(
            (1.0e-5..8.0e-5).contains(&floor_base),
            "the subcycle-8 noise floor {floor_base:.3e} left its measured \
             band [1e-5, 8e-5] — the pass map's continuity changed"
        );
        // Attribution: smoothing the sampled tractions does NOT move the
        // floor (measured ratio 1.002) — the flip noise is coherent
        // through the fluid field. If this ratio ever leaves [0.5, 2],
        // the noise has moved into the sampling channel and the
        // smoothing lever is worth revisiting.
        let attribution = floor_smoothed / floor_base;
        assert!(
            (0.5..2.0).contains(&attribution),
            "smoothing changed the floor by x{attribution:.2} — the noise \
             channel attribution (coherent-through-the-fluid) no longer holds"
        );
        // The stall measurement, 2026-08-21: on a clean release step the
        // subiteration converges DEEP at both subcycles — s8 aitken
        // 3.4e-9 / iqn 1.6e-9 (12-pass budget), s2 aitken 6.5e-10 / iqn
        // 6.3e-10 (converged below 1e-9 in 5-6 passes). The flip-scan
        // jumps are events at specific geometries, not a floor under
        // every step: a typical step's map is locally smooth far below
        // any march tolerance, and the tenth session's subcycle-2 blowup
        // (tolerance 2e-4 held fixed as dt_c shrank, wall-velocity noise
        // = tol / dt_c) was a tolerance mis-budgeting, not an
        // impassable floor. If any of these stalls rises above 1e-7,
        // the step map's local smoothness is gone and the tight-coupling
        // tolerance budget must be re-measured before the next ladder.
        for (label, value) in [
            ("s8 aitken", stall_8_aitken.0),
            ("s8 iqn", stall_8_iqn.0),
            ("s2 aitken", stall_2_aitken.0),
            ("s2 iqn", stall_2_iqn.0),
        ] {
            assert!(
                value < 1e-7,
                "{label} stall {value:.3e} rose above 1e-7 — the release \
                 step's local smoothness is gone; re-measure the \
                 tight-coupling tolerance budget"
            );
        }
    }
}