020 nbody benchmark

// TEST: N-body gravitational simulation using kernel:pairwise
//
// This is the classic benchmark for data-parallel computation.
// Each body exerts gravitational force on every other body.
//
// Algorithm (pairwise):
//   For each unique pair (i, j) where i < j:
//     dx = x[j] - x[i], dy = y[j] - y[i]
//     dist_sq = dx*dx + dy*dy + softening
//     force = mass[i] * mass[j] / dist_sq
//     vx[i] += force * dx,  vy[i] += force * dy   (body i)
//     vx[j] -= force * dx,  vy[j] -= force * dy   (body j, symmetric)

import std/kernel
import std/io

// Body shape: position, velocity, mass
std/kernel:shape(Body) {
    x: f64,
    y: f64,
    vx: f64,
    vy: f64,
    mass: f64,
}

// Initialize 4 bodies in a square pattern
std/kernel:init(Body) {
    { x: 0.0, y: 0.0, vx: 0.0, vy: 0.0, mass: 1.0 },
    { x: 1.0, y: 0.0, vx: 0.0, vy: 0.0, mass: 1.0 },
    { x: 0.0, y: 1.0, vx: 0.0, vy: 0.0, mass: 1.0 },
    { x: 1.0, y: 1.0, vx: 0.0, vy: 0.0, mass: 1.0 },
}
| kernel k |> std/kernel:pairwise { const dx = k.other.x - k.x; const dy = k.other.y - k.y; const dist_sq = dx*dx + dy*dy + 0.01; const f = k.mass * k.other.mass / dist_sq; k.vx += f * dx; k.vy += f * dy; k.other.vx -= f * dx; k.other.vy -= f * dy }
| computed c |> std/io:print.blk {
        nbody simulation complete, mass[0]={{ c[0].mass:f }}
    }