excalidraw/packages/utils/bbox.ts

import type { Bounds } from "../excalidraw/element/bounds";
import type { Point } from "../excalidraw/types";

export type LineSegment = [Point, Point];

export function getBBox(line: LineSegment): Bounds {
  return [
    Math.min(line[0][0], line[1][0]),
    Math.min(line[0][1], line[1][1]),
    Math.max(line[0][0], line[1][0]),
    Math.max(line[0][1], line[1][1]),
  ];
}

export function crossProduct(a: Point, b: Point) {
  return a[0] * b[1] - b[0] * a[1];
}

export function doBBoxesIntersect(a: Bounds, b: Bounds) {
  return a[0] <= b[2] && a[2] >= b[0] && a[1] <= b[3] && a[3] >= b[1];
}

export function translate(a: Point, b: Point): Point {
  return [a[0] - b[0], a[1] - b[1]];
}

const EPSILON = 0.000001;

export function isPointOnLine(l: LineSegment, p: Point) {
  const p1 = translate(l[1], l[0]);
  const p2 = translate(p, l[0]);

  const r = crossProduct(p1, p2);

  return Math.abs(r) < EPSILON;
}

export function isPointRightOfLine(l: LineSegment, p: Point) {
  const p1 = translate(l[1], l[0]);
  const p2 = translate(p, l[0]);

  return crossProduct(p1, p2) < 0;
}

export function isLineSegmentTouchingOrCrossingLine(
  a: LineSegment,
  b: LineSegment,
) {
  return (
    isPointOnLine(a, b[0]) ||
    isPointOnLine(a, b[1]) ||
    (isPointRightOfLine(a, b[0])
      ? !isPointRightOfLine(a, b[1])
      : isPointRightOfLine(a, b[1]))
  );
}

// https://martin-thoma.com/how-to-check-if-two-line-segments-intersect/
export function doLineSegmentsIntersect(a: LineSegment, b: LineSegment) {
  return (
    doBBoxesIntersect(getBBox(a), getBBox(b)) &&
    isLineSegmentTouchingOrCrossingLine(a, b) &&
    isLineSegmentTouchingOrCrossingLine(b, a)
  );
}