//! R8-e: the added-mass timing instrument for the coupled 3D FSI's small-dt
//! instability. The R8-a fluid (the embedded3 cut-cell device path, the
//! slab by default) with the flag's centreline PRESCRIBED: rigid to `T0`,
//! then from one snapshot two passes of `N` steps each — the baseline (the
//! flag at rest) and the impulse (the centreline's velocity steps from 0 to
//! `V` × shape at step `N_PRE`: its wall acceleration is a single-step
//! delta V/dt; `MODE=pulse` returns the velocity to 0 one step later, a
//! +V/dt, −V/dt pair). The difference of the two passes' loads, step by
//! step, is the fluid's response to the impulse: an added-mass response
//! lives in the impulse's own step; a lagged part shows one step later.
//!
//! Knobs `RTX_E3IMP_*`: `NY` (62), `NZ` (4), `T0` (0.2 s), `N_PRE` (4),
//! `N` (24), `V` (0.05 m/s at the tip), `SHAPE` (`pitch` about the root,
//! 1 at A; `heave`; `zig`, `tip`), `MODE` (`step`, `pulse`), `CSV` (per-step series).
//! The fluid's own knobs as the R8-a harness (`RTX_E3FSI_DT_SCALE`, …).
#![cfg(feature = "cuda")]

#[path = "fsi2_embedded3/fluid.rs"]
mod fluid;
#[path = "fsi2_embedded3/state.rs"]
mod state;

use std::io::Write as _;

use fluid::{CX, CY, Contribution, E3Fluid, HALF, Line, R_CYL};

pub fn env_f(name: &str, default: f64) -> f64 {
    std::env::var(name)
        .ok()
        .and_then(|v| v.parse().ok())
        .unwrap_or(default)
}

const X0: f64 = 0.25;
const STATIONS: usize = 35;

/// The shared fluid module asks its parent for the tip shape (R8-h); the
/// impulse instrument keeps the capsule on its 35-station centreline.
pub fn flat_tip() -> Option<f64> {
    None
}
const CHORD: f64 = 0.35;

fn line(t: f64, c: &[f64], v: &[f64]) -> Line {
    Line {
        t,
        pts: (0..STATIONS)
            .map(|k| [X0 + 0.01 * k as f64, 0.2 + c[k]])
            .collect(),
        vel: (0..STATIONS).map(|k| [0.0, v[k]]).collect(),
    }
}

/// The flag's loads from the route's contributions (per unit span): fy on
/// the flag, fy by part, and the generalized force Σ fy φ(x) with the
/// prescribed shape's φ.
fn flag_loads(
    contrib: &[Contribution],
    pts: &[[f64; 2]],
    width: f64,
    phi: impl Fn(f64) -> f64,
) -> (f64, f64, [f64; 4]) {
    let (mut fy, mut q, mut parts) = (0.0, 0.0, [0.0; 4]);
    for &(pos, c, part, v) in contrib {
        if c != 1 {
            continue;
        }
        let (x, y) = (pos[0], pos[1]);
        let mut best = f64::INFINITY;
        for m in 0..pts.len() - 1 {
            let (a, b) = (pts[m], pts[m + 1]);
            let (ex, ey) = (b[0] - a[0], b[1] - a[1]);
            let u = (((x - a[0]) * ex + (y - a[1]) * ey) / (ex * ex + ey * ey)).clamp(0.0, 1.0);
            best = best.min(((x - a[0] - u * ex).powi(2) + (y - a[1] - u * ey).powi(2)).sqrt());
        }
        let d_cyl = ((x - CX).powi(2) + (y - CY).powi(2)).sqrt() - R_CYL;
        if d_cyl < best - HALF {
            continue;
        }
        let v = v / width;
        fy += v;
        q += v * phi(x);
        parts[part] += v * phi(x);
    }
    (fy, q, parts)
}

#[test]
#[ignore = "R8-e: the added-mass timing instrument (GPU, a minute)"]
fn impulse_response() {
    let ny = env_f("RTX_E3IMP_NY", 62.0) as usize;
    let nz = env_f("RTX_E3IMP_NZ", 4.0) as usize;
    let t0 = env_f("RTX_E3IMP_T0", 0.2);
    let n_pre = env_f("RTX_E3IMP_N_PRE", 4.0) as usize;
    let n = env_f("RTX_E3IMP_N", 24.0) as usize;
    let vel = env_f("RTX_E3IMP_V", 0.05);
    let shape = std::env::var("RTX_E3IMP_SHAPE").unwrap_or_else(|_| "pitch".into());
    let mode = std::env::var("RTX_E3IMP_MODE").unwrap_or_else(|_| "step".into());
    // The shape per station (`zig`: alternate stations ±, the root clamped;
    // `tip`: the last 5 stations), linear between stations.
    let phis: Vec<f64> = (0..STATIONS)
        .map(|k| {
            let x = X0 + 0.01 * k as f64;
            match shape.as_str() {
                "heave" => 1.0,
                "zig" => {
                    if k == 0 {
                        0.0
                    } else if k % 2 == 0 {
                        1.0
                    } else {
                        -1.0
                    }
                }
                "tip" => {
                    if k + 5 >= STATIONS {
                        1.0
                    } else {
                        0.0
                    }
                }
                _ => ((x - X0) / CHORD).clamp(0.0, 1.0),
            }
        })
        .collect();
    let phi = |x: f64| -> f64 {
        let s = ((x - X0) / 0.01).clamp(0.0, (STATIONS - 1) as f64);
        let m = (s.floor() as usize).min(STATIONS - 2);
        let u = s - m as f64;
        (1.0 - u) * phis[m] + u * phis[m + 1]
    };
    let zero = vec![0.0; STATIONS];
    let rest = line(0.0, &zero, &zero);
    let mut fl = E3Fluid::build(ny, nz, 3.0, rest.clone(), None);
    let dt = fl.dt;
    let rigid = (t0 / dt).round() as usize;
    for s in 0..rigid {
        let r = fl.step();
        assert!(r.final_residual.is_finite(), "rigid death at {s}");
    }
    let t_start = fl.time();
    println!(
        "  R8-e impulse: ny {ny} nz {nz} dt {dt:.4e}, rigid {rigid} steps to t {t_start:.4}; shape {shape}, mode {mode}, V {vel} m/s (acceleration V/dt {:.2} m/s²) at step {n_pre} of {n}",
        vel / dt
    );
    let snap = fl.snapshot();
    // Per pass: per step (t, tip velocity, fy, Q, Q parts, lift total).
    let mut rows: Vec<Vec<[f64; 9]>> = Vec::new();
    for pass in 0..2 {
        fl.restore(&snap);
        let mut c = zero.clone();
        let mut prev = rest.clone();
        prev.t = t_start;
        let mut out = Vec::new();
        for s in 0..n {
            let t_new = t_start + (s + 1) as f64 * dt;
            // The step's centreline velocity (constant over the step).
            let v_tip = if pass == 0 || s < n_pre {
                0.0
            } else if mode == "pulse" {
                if s == n_pre { vel } else { 0.0 }
            } else {
                vel
            };
            let v: Vec<f64> = phis.iter().map(|p| p * v_tip).collect();
            for k in 0..STATIONS {
                c[k] += dt * v[k];
            }
            let next = line(t_new, &c, &v);
            fl.set_lines(prev.clone(), next.clone());
            let r = fl.step();
            assert!(
                r.final_residual.is_finite(),
                "death at pass {pass} step {s}"
            );
            let (tot, contrib) = fl.loads();
            let (fy, q, parts) = flag_loads(&contrib, &next.pts, fl.load_width, &phi);
            out.push([
                t_new, v_tip, fy, q, parts[0], parts[1], parts[2], parts[3], tot[1],
            ]);
            prev = next;
        }
        rows.push(out);
    }
    let mut csv = std::env::var("RTX_E3IMP_CSV")
        .ok()
        .map(|p| std::fs::File::create(p).expect("csv"));
    if let Some(f) = csv.as_mut() {
        writeln!(
            f,
            "k,t,v_tip,fy0,q0,fy,q,dfy,dq,dq_p,dq_shear,dq_xdiff,dq_xconv,dlift"
        )
        .unwrap();
    }
    let q_imp = rows[1][n_pre][3] - rows[0][n_pre][3];
    for s in 0..n {
        let (a, b) = (&rows[0][s], &rows[1][s]);
        let k = s as i64 - n_pre as i64;
        let d = |i: usize| b[i] - a[i];
        if let Some(f) = csv.as_mut() {
            writeln!(
                f,
                "{k},{:.6},{:.4e},{:.6},{:.6},{:.6},{:.6},{:.6e},{:.6e},{:.6e},{:.6e},{:.6e},{:.6e},{:.6e}",
                b[0], b[1], a[2], a[3], b[2], b[3], d(2), d(3), d(4), d(5), d(6), d(7), d(8)
            )
            .unwrap();
        }
        if (-1..=6).contains(&k) {
            println!(
                "  k {k:+}: v_tip {:.3e}  ΔQ {:+.5e} ({:+.4} of the impulse step)  parts p {:+.4e} shear {:+.4e} xdiff {:+.4e} xconv {:+.4e}  Δfy {:+.5e}",
                b[1],
                d(3),
                d(3) / q_imp,
                d(4),
                d(5),
                d(6),
                d(7),
                d(2)
            );
        }
    }
    println!(
        "  IMPULSE ΔQ(k=0) {q_imp:+.5e} N/m → generalized added mass −ΔQ·dt/V {:.4} kg/m",
        -q_imp * dt / vel
    );
}