thi-ng/umbrella

import type { Vec } from "@thi.ng/vectors";
import type { IBoidBehavior, ScalarOrField } from "../api.js";
import { __ensureFn } from "../internal/ensure.js";

export const cohesion = (
    maxDist: ScalarOrField,
    weight: ScalarOrField = 1
): IBoidBehavior => {
    const $maxDist = __ensureFn(maxDist);
    const centroid: Vec = [];
    return {
        weight: __ensureFn(weight),
        update: (boid) => {
            const { add, mulN, setN } = boid.api;
            const neighbors = boid.neighbors($maxDist(boid), boid.pos.curr);
            const num = neighbors.length;
            setN(centroid, 0);
            for (let i = 0; i < num; i++) {
                const n = neighbors[i];
                if (n !== boid) add(centroid, centroid, n.pos.curr);
            }
            return num > 1
                ? boid.steerTowards(mulN(centroid, centroid, 1 / (num - 1)))
                : centroid;
        },
    };
};