import type { LineSegment } from './line-segment';
import type { Vector } from './vector';

/**
 * A 2D ray that can be cast into the scene to do collision detection
 */

export class Ray {
  public pos: Vector;
  public dir: Vector;

  /**
   * @param pos The starting position for the ray
   * @param dir The vector indicating the direction of the ray
   */
  constructor(pos: Vector, dir: Vector) {
    this.pos = pos;
    this.dir = dir.normalize();
  }

  /**
   * Tests a whether this ray intersects with a line segment. Returns a number greater than or equal to 0 on success.
   * This number indicates the mathematical intersection time.
   * @param line  The line to test
   */
  public intersect(line: LineSegment): number {
    const numerator = line.begin.sub(this.pos);

    // Test is line and ray are parallel and non intersecting
    if (this.dir.cross(line.getSlope()) === 0 && numerator.cross(this.dir) !== 0) {
      return -1;
    }

    // Lines are parallel
    const divisor = this.dir.cross(line.getSlope());
    if (divisor === 0) {
      return -1;
    }

    const t = numerator.cross(line.getSlope()) / divisor;

    if (t >= 0) {
      const u = numerator.cross(this.dir) / divisor / line.getLength();
      if (u >= 0 && u <= 1) {
        return t;
      }
    }
    return -1;
  }

  public intersectPoint(line: LineSegment): Vector | null {
    const time = this.intersect(line);
    if (time < 0) {
      return null;
    }
    return this.getPoint(time);
  }

  /**
   * Returns the point of intersection given the intersection time
   */
  public getPoint(time: number): Vector {
    return this.pos.add(this.dir.scale(time));
  }
}