A minimal ray-casting function.

castRay.tsx

export type RayOptions = {
  max?: number
  min?: number
  start?: number
}

/**
 * Casts a ray in a given direction and check if it hits something.
 * @param from - [x, y]
 * @param delta - [deltaX, deltaY]
 * @param hitTest - (point) => hit
 * @param options - {max: number, min: number, start: number}
 * @example
 * ```ts
 * const hitPolygon = castRay([0,0], [.25, .5], (point) => {
 *   return myPolys.find(poly => pointInPoly(point, poly))
 * }
 * ```
 */
export function castRay<T = any>(
  from: number[],
  delta: number[],
  hitTest: (point: number[]) => T,
  options: RayOptions = {}
): {
  hit?: T
  point: number[]
} {
  const { min = 0, max = 9999, start = 1 } = options

  const step = Math.hypot(delta[0], delta[1]),
    dx = delta[0] / step,
    dy = delta[1] / step,
    point = [from[0] + start * dx, from[1] + start * dy]

  let hit: T | undefined = undefined

  if (step > min) {
    for (let dist = 0; dist <= max; dist++) {
      point[0] += dx
      point[1] += dy

      hit = hitTest(point)

      if (hit !== undefined) {
        return { hit, point }
      }
    }
  }

  return { hit, point } // No hit
}