//! A-P2, step S5 (`docs/overset_metal_campaign.md` §5.9): the overset with a
//! MOVING patch.
//!
//! 1. A stationary patch pushed through `set_patch_mesh` every step is the
//!    static overset path to the bit (the overlap rebuild and the fringe
//!    re-stamping reproduce the same numbers).
//! 2. The phantom circle and its patch translating through the steady
//!    manufactured field (the `embedded_moving.rs` pattern: the wall carries
//!    the exact velocity, so the exact solution is unchanged while the
//!    hole, fringe and acceptors sweep the background) keep the time-max
//!    L2 on both meshes within 1.5× the static level; reclassification
//!    counts are printed.
//! 3. Snapshot/restore with a pending patch mesh re-runs bit-identically.

mod overset_common;

use overset_common::{build, errors, p2_patch};
use rtx_cfd::CfdResult;
use rtx_cfd::mesh::PatchMesh;
use rtx_cfd::mesh::patch_gen::annulus_skewed;
use rtx_cfd::solvers::incompressible::OversetField;

fn fields_identical(a: &OversetField, b: &OversetField) -> bool {
    let bits = |v: &[f64], w: &[f64]| v.iter().zip(w).all(|(x, y)| x.to_bits() == y.to_bits());
    bits(a.background.u.as_slice(), b.background.u.as_slice())
        && bits(a.background.v.as_slice(), b.background.v.as_slice())
        && bits(a.background.p.as_slice(), b.background.p.as_slice())
        && bits(&a.patch.u, &b.patch.u)
        && bits(&a.patch.v, &b.patch.v)
        && bits(&a.patch.p, &b.patch.p)
        && bits(&a.patch.flux, &b.patch.flux)
}

fn max_diff(a: &OversetField, b: &OversetField) -> f64 {
    let d = |v: &[f64], w: &[f64]| {
        v.iter()
            .zip(w)
            .map(|(x, y)| (x - y).abs())
            .fold(0.0, f64::max)
    };
    d(a.background.u.as_slice(), b.background.u.as_slice())
        .max(d(a.background.v.as_slice(), b.background.v.as_slice()))
        .max(d(a.background.p.as_slice(), b.background.p.as_slice()))
        .max(d(&a.patch.u, &b.patch.u))
        .max(d(&a.patch.v, &b.patch.v))
        .max(d(&a.patch.p, &b.patch.p))
        .max(d(&a.patch.flux, &b.patch.flux))
}

/// The P2 patch translated to centre `(cx, cy)`.
fn translated_patch(n: usize, cx: f64, cy: f64) -> CfdResult<PatchMesh> {
    annulus_skewed([cx, cy], 0.2, 0.354, 9 * n / 4, n / 4, 0.3, 3.0)
}

fn time_step(solver: &rtx_cfd::solvers::incompressible::OversetPisoSolver, n: usize) -> f64 {
    let h = 1.0 / n as f64;
    let mut hp = f64::INFINITY;
    for c in 0..solver.patch().mesh().cell_count() {
        for (f, _) in solver.patch().mesh().cell_faces(c) {
            let d = solver.patch().mesh().faces()[f].d;
            hp = hp.min((d[0] * d[0] + d[1] * d[1]).sqrt());
        }
    }
    0.4 * (hp * hp / (4.0 * 0.05)).min(h)
}

#[tokio::test]
async fn stationary_patch_through_the_moving_path_is_bit_identical() -> CfdResult<()> {
    let n = 32;
    let (mut plain, mut f_plain) = build(n, p2_patch(n)?, 1e-3)?;
    let (mut moving, mut f_moving) = build(n, p2_patch(n)?, 1e-3)?;
    let dt = time_step(&plain, n);
    for _ in 0..20 {
        plain.advance(&mut f_plain, dt).await?;
        let same = moving.patch().mesh().clone();
        moving.set_patch_mesh(same)?;
        let r = moving.advance(&mut f_moving, dt).await?;
        assert_eq!(r.reclassified_cells, 0);
    }
    assert_eq!(plain.time().to_bits(), moving.time().to_bits());
    assert!(
        fields_identical(&f_plain, &f_moving),
        "moving path with a stationary patch differs from the static path by {:.3e}",
        max_diff(&f_plain, &f_moving)
    );
    println!("  stationary patch through the moving overset path — bit-identical over 20 steps");
    Ok(())
}

#[tokio::test]
async fn translating_phantom_circle_keeps_the_static_error_level() -> CfdResult<()> {
    let n: usize = std::env::var("RTX_OVERSET_N")
        .ok()
        .and_then(|v| v.parse().ok())
        .unwrap_or(32);
    let h = 1.0 / n as f64;
    let (mut solver, mut field) = build(n, p2_patch(n)?, 1e-3)?;
    let dt = time_step(&solver, n);

    // Static phase to the steady state.
    let mut steady = f64::INFINITY;
    for _ in 0..400_000 {
        let before = (field.background.u.clone(), field.patch.u.clone());
        solver.advance(&mut field, dt).await?;
        let change = (&field.background.u - &before.0).abs().max().max(
            field
                .patch
                .u
                .iter()
                .zip(&before.1)
                .map(|(a, b)| (a - b).abs())
                .fold(0.0, f64::max),
        );
        steady = change / dt;
        if steady < 1e-6 {
            break;
        }
    }
    assert!(steady < 1e-6, "no static steady state: {steady:.3e}");
    let (static_bg, static_patch) = errors(&solver, &field, n);
    println!("  static level n = {n}: L2 background {static_bg:.4e}, patch {static_patch:.4e}");

    // Moving phase: the circle and its patch translate at speed 0.3 to the
    // left through ~4.5 background cells (per step ≤ 0.01 h).
    let speed = 0.3;
    let t0 = solver.time();
    let distance = 4.5 * h;
    let steps = (distance / (speed * dt)).ceil() as usize;
    let (mut worst_bg, mut worst_patch) = (0.0_f64, 0.0_f64);
    let (mut reclassified, mut fresh) = (0usize, 0usize);
    let mut max_rounds = 0usize;
    for step in 0..steps {
        let t_new = solver.time() + dt;
        let cx = 0.6 - speed * (t_new - t0);
        solver.set_patch_mesh(translated_patch(n, cx, 0.45)?)?;
        let r = solver.advance(&mut field, dt).await?;
        reclassified += r.reclassified_cells;
        fresh += r.fresh_cells;
        max_rounds = max_rounds.max(r.rounds.iter().copied().max().unwrap_or(0));
        let (eb, ep) = errors(&solver, &field, n);
        worst_bg = worst_bg.max(eb);
        worst_patch = worst_patch.max(ep);
        if std::env::var("RTX_OVERSET_TRACE").is_ok() && step % 200 == 0 {
            println!(
                "    moving step {step}: cx {cx:.4} L2 bg {eb:.4e} patch {ep:.4e} reclassified {} fresh {} rounds {:?} defect bg {:.2e}",
                r.reclassified_cells,
                r.fresh_cells,
                r.rounds,
                r.background_mass_defect / r.overlap_flux_scale.max(1e-300)
            );
        }
    }
    println!(
        "  moving n = {n}: {steps} steps, {distance:.3} travelled; time-max L2 background {worst_bg:.4e} ({:.2}x static), \
         patch {worst_patch:.4e} ({:.2}x static); reclassified {reclassified} cells, fresh {fresh}; max Schwarz rounds {max_rounds}",
        worst_bg / static_bg,
        worst_patch / static_patch
    );
    assert!(
        worst_bg < 1.5 * static_bg,
        "background error rose to {worst_bg:.3e} (static {static_bg:.3e})"
    );
    assert!(
        worst_patch < 1.5 * static_patch,
        "patch error rose to {worst_patch:.3e} (static {static_patch:.3e})"
    );
    Ok(())
}

#[tokio::test]
async fn snapshot_restore_with_a_pending_mesh_is_bit_identical() -> CfdResult<()> {
    let n = 32;
    let (mut solver, mut field) = build(n, p2_patch(n)?, 1e-3)?;
    let dt = time_step(&solver, n);
    let speed = 0.3;
    let mut step = 0usize;
    let cx = |s: usize| 0.6 - speed * s as f64 * dt;
    for _ in 0..5 {
        step += 1;
        solver.set_patch_mesh(translated_patch(n, cx(step), 0.45)?)?;
        solver.advance(&mut field, dt).await?;
    }
    solver.set_patch_mesh(translated_patch(n, cx(step + 1), 0.45)?)?;
    let saved = solver.snapshot();
    let field_saved = field.clone();
    let step_saved = step;
    for _ in 0..10 {
        step += 1;
        if step > step_saved + 1 {
            solver.set_patch_mesh(translated_patch(n, cx(step), 0.45)?)?;
        }
        solver.advance(&mut field, dt).await?;
    }
    let reference = field.clone();
    let t_ref = solver.time();

    solver.restore(&saved);
    field = field_saved;
    step = step_saved;
    for _ in 0..10 {
        step += 1;
        if step > step_saved + 1 {
            solver.set_patch_mesh(translated_patch(n, cx(step), 0.45)?)?;
        }
        solver.advance(&mut field, dt).await?;
    }
    assert_eq!(solver.time().to_bits(), t_ref.to_bits());
    assert!(
        fields_identical(&field, &reference),
        "re-run on the moving overset differs by {:.3e}",
        max_diff(&field, &reference)
    );
    println!("  snapshot/restore with a pending patch mesh — bit-identical re-run over 10 steps");
    Ok(())
}

/// One step with the patch jumped by two background cells must reclassify
/// background cells (the overlap follows the pending mesh).
#[tokio::test]
async fn a_translated_patch_reclassifies_the_background() -> CfdResult<()> {
    let n = 32;
    let (mut solver, mut field) = build(n, p2_patch(n)?, 1e-3)?;
    let dt = time_step(&solver, n);
    solver.advance(&mut field, dt).await?;
    let before = solver.overlap().hole_cells();
    solver.set_patch_mesh(translated_patch(n, 0.6 - 2.0 / n as f64, 0.45)?)?;
    let r = solver.advance(&mut field, dt).await?;
    println!(
        "  2 h jump: reclassified {}, fresh {}, holes {} -> {}",
        r.reclassified_cells,
        r.fresh_cells,
        before,
        solver.overlap().hole_cells()
    );
    assert!(
        r.reclassified_cells > 0,
        "the overlap did not follow the patch"
    );
    Ok(())
}