getConvexHull.js
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// Math Functions
// https://www.geeksforgeeks.org/convex-hull-set-2-graham-scan/
export const getConvexHull = (points) => {
function orientation(p, q, r) {
const val = (q.y - p.y) * (r.x - q.x) - (q.x - p.x) * (r.y - q.y);
if (val == 0) {
return 0; // colinear
}
// clock or counterclock wise
return val > 0 ? 1 : 2;
}
function distSq(p1, p2) {
return (p1.x - p2.x) * (p1.x - p2.x) + (p1.y - p2.y) * (p1.y - p2.y);
}
function compare(p1, p2) {
// Find orientation
const o = orientation(p0, p1, p2);
if (o == 0) return distSq(p0, p2) >= distSq(p0, p1) ? -1 : 1;
return o == 2 ? -1 : 1;
}
// find the lowest point in 2d
let lowestY = Infinity;
let lowestIndex = -1;
for (let i = 0, l = points.length; i < l; i++) {
const p = points[i];
if (p.y < lowestY) {
lowestIndex = i;
lowestY = p.y;
}
}
// sort the points
const p0 = points[lowestIndex];
points[lowestIndex] = points[0];
points[0] = p0;
points = points.sort(compare);
// filter the points
let m = 1;
const n = points.length;
for (let i = 1; i < n; i++) {
while (i < n - 1 && orientation(p0, points[i], points[i + 1]) == 0) {
i++;
}
points[m] = points[i];
m++;
}
// early out if we don't have enough points for a hull
// if (m < 3) console.log(m);
if (m < 3) return null;
// generate the hull
const hull = [points[0], points[1], points[2]];
for (let i = 3; i < m; i++) {
while (orientation(hull[hull.length - 2], hull[hull.length - 1], points[i]) !== 2) {
hull.pop();
}
hull.push(points[i]);
}
return hull;
};