090 nbody multistep

// TEST: Multi-step nbody — kernel:step wrapping pairwise + self
//
// Full fusion test with real solar system data.
// 10 steps of gravitational simulation.
// Verifies that step(0..N) wrapping pairwise + self produces correct output.

~import "$std/kernel"
~import "$std/io"

const PI: f64 = 3.141592653589793;
const SOLAR_MASS: f64 = 4.0 * PI * PI;
const DAYS_PER_YEAR: f64 = 365.24;
const DT: f64 = 0.01;

~std.kernel:shape(Body) {
    x: f64,
    y: f64,
    z: f64,
    vx: f64,
    vy: f64,
    vz: f64,
    mass: f64,
}

~std.kernel:init(Body) {
    { x: 0, y: 0, z: 0, vx: 0, vy: 0, vz: 0, mass: SOLAR_MASS },
    { x: 4.84143144246472090e+00, y: -1.16032004402742839e+00, z: -1.03622044471123109e-01, vx: 1.66007664274403694e-03 * DAYS_PER_YEAR, vy: 7.69901118419740425e-03 * DAYS_PER_YEAR, vz: -6.90460016972063023e-05 * DAYS_PER_YEAR, mass: 9.54791938424326609e-04 * SOLAR_MASS },
    { x: 8.34336671824457987e+00, y: 4.12479856412430479e+00, z: -4.03523417114321381e-01, vx: -2.76742510726862411e-03 * DAYS_PER_YEAR, vy: 4.99852801234917238e-03 * DAYS_PER_YEAR, vz: 2.30417297573763929e-05 * DAYS_PER_YEAR, mass: 2.85885980666130812e-04 * SOLAR_MASS },
    { x: 1.28943695621391310e+01, y: -1.51111514016986312e+01, z: -2.23307578892655734e-01, vx: 2.96460137564761618e-03 * DAYS_PER_YEAR, vy: 2.37847173959480950e-03 * DAYS_PER_YEAR, vz: -2.96589568540237556e-05 * DAYS_PER_YEAR, mass: 4.36624404335156298e-05 * SOLAR_MASS },
    { x: 1.53796971148509165e+01, y: -2.59193146099879641e+01, z: 1.79258772950371181e-01, vx: 2.68067772490389322e-03 * DAYS_PER_YEAR, vy: 1.62824170038242295e-03 * DAYS_PER_YEAR, vz: -9.51592254519715870e-05 * DAYS_PER_YEAR, mass: 5.15138902046611451e-05 * SOLAR_MASS },
}
| kernel k |>
    std.kernel:step(0..10)
    | step |>
        std.kernel:pairwise {
        const dx = k.x - k.other.x;
        const dy = k.y - k.other.y;
        const dz = k.z - k.other.z;
        const dsq = dx*dx + dy*dy + dz*dz;
        const mag = DT / (dsq * @sqrt(dsq));
        k.vx -= dx * k.other.mass * mag;
        k.vy -= dy * k.other.mass * mag;
        k.vz -= dz * k.other.mass * mag;
        k.other.vx += dx * k.mass * mag;
        k.other.vy += dy * k.mass * mag;
        k.other.vz += dz * k.mass * mag;
        }
        |> std.kernel:self {
        k.x += DT * k.vx;
        k.y += DT * k.vy;
        k.z += DT * k.vz;
    }
| computed c |>
    std.io:print.blk {
        vx[0]={{ c[0].vx:f }}
    }