fix(boids): update cohesion(), use filter predicate

Author: postspectacular

packages/boids/src/behaviors/cohesion.ts

@@ -1,31 +1,39 @@
 // SPDX-License-Identifier: Apache-2.0
-import type { FnU2 } from "@thi.ng/api";
+import type { Predicate2 } from "@thi.ng/api";
 import type { Vec } from "@thi.ng/vectors";
 import type { IBoidBehavior, ScalarOrField } from "../api.js";
 import type { Boid } from "../boid.js";
 import { __ensureFn } from "../internal/ensure.js";
 
+/**
+ * Cohesion behavior. Attempts to move boid towards centroid of neighboring
+ * boids within `maxDist`. The behavior itself can be weighted via `weight`.
+ * Neighbors can be filtered via optional `filter` predicate, which receives
+ * current boid as first arg and current neighbor as second. By default all
+ * neighbors are considered.
+ *
+ * @param maxDist
+ * @param weight
+ * @param pred
+ */
 export const cohesion = (
 	maxDist: ScalarOrField,
 	weight: ScalarOrField = 1,
-	amp?: FnU2<Boid, number>
+	pred: Predicate2<Boid> = () => true
 ): IBoidBehavior => {
 	const $maxDist = __ensureFn(maxDist);
 	const centroid: Vec = [];
 	return {
 		weight: __ensureFn(weight),
 		update: (boid) => {
-			const { add, maddN, mulN, setN } = boid.api;
+			const { add, mulN, setN } = boid.api;
 			const neighbors = boid.neighbors($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) {
-					amp
-						? maddN(centroid, n.pos.curr, amp(boid, n), centroid)
-						: add(centroid, centroid, n.pos.curr);
-				}
+				if (n !== boid && pred(boid, n))
+					add(centroid, centroid, n.pos.curr);
 			}
 			return num > 1
 				? boid.steerTowards(mulN(centroid, centroid, 1 / (num - 1)))