@woosh/meep-engine/src/core/geom/vec2/v2_angle_between.js

Pure JavaScript game engine. Fully featured and production ready.

import { clamp } from "../../math/clamp.js";
import { v2_dot } from "./v2_dot.js";
import { v2_length } from "./v2_length.js";

/**
 *
 * @param {number} x0 first vector X
 * @param {number} y0 first vector Y
 * @param {number} x1 second vector X
 * @param {number} y1 second vector Y
 * @returns {number} in radians
 */
export function v2_angle_between(x0, y0, x1, y1) {
    const d = v2_dot(x0, y0, x1, y1);

    const magnitude_0 = v2_length(x0, y0);
    const magnitude_1 = v2_length(x1, y1);

    const l = magnitude_0 * magnitude_1;

    if (l === 0) {
        // collective magnitude is 0, provide arbitrary result to avoid division by 0
        return 0;
    }

    const theta = clamp(d / l, -1, 1);

    return Math.acos(theta);
}