thi-ng/umbrella

import { EPS } from "@thi.ng/math/api";
import type { ReadonlyVec } from "@thi.ng/vectors";
import { corner2 } from "@thi.ng/vectors/clockwise";

/**
 * Polygon convexity classifier.
 */
export enum Convexity {
    ILLEGAL = -1,
    COLINEAR = 0,
    CONVEX,
    CONCAVE,
}

export const convexity = (pts: ReadonlyVec[], eps = EPS) => {
    let n = pts.length;
    if (n < 3) {
        return n < 2 ? Convexity.ILLEGAL : Convexity.COLINEAR;
    }
    let type = 0;
    let a = pts[n - 2];
    let b = pts[n - 1];
    let c = pts[0];
    for (let i = 0; i < n && type < 3; a = b, b = c, c = pts[++i]) {
        const t = corner2(a, b, c, eps);
        type |= t < 0 ? 1 : t > 0 ? 2 : 0;
    }
    return type === 3
        ? Convexity.CONCAVE
        : type > 0
        ? Convexity.CONVEX
        : Convexity.COLINEAR;
};