tldraw/utils/intersections.ts

import { Bounds } from "types"
import * as vec from "utils/vec"

interface Intersection {
  didIntersect: boolean
  message: string
  points: number[][]
}

function getIntersection(message: string, ...points: number[][]) {
  return { didIntersect: points.length > 0, message, points }
}

export function intersectLineSegments(
  a1: number[],
  a2: number[],
  b1: number[],
  b2: number[]
) {
  const AB = vec.sub(a1, b1)
  const BV = vec.sub(b2, b1)
  const AV = vec.sub(a2, a1)

  const ua_t = BV[0] * AB[1] - BV[1] * AB[0]
  const ub_t = AV[0] * AB[1] - AV[1] * AB[0]
  const u_b = BV[1] * AV[0] - BV[0] * AV[1]

  if (ua_t === 0 || ub_t === 0) {
    return getIntersection("coincident")
  }

  if (u_b === 0) {
    return getIntersection("parallel")
  }

  if (u_b != 0) {
    const ua = ua_t / u_b
    const ub = ub_t / u_b
    if (0 <= ua && ua <= 1 && 0 <= ub && ub <= 1) {
      return getIntersection("intersection", vec.add(a1, vec.mul(AV, ua)))
    }
  }

  return getIntersection("no intersection")
}

export function intersectCircleLineSegment(
  c: number[],
  r: number,
  a1: number[],
  a2: number[]
): Intersection {
  const a =
    (a2[0] - a1[0]) * (a2[0] - a1[0]) + (a2[1] - a1[1]) * (a2[1] - a1[1])
  const b =
    2 * ((a2[0] - a1[0]) * (a1[0] - c[0]) + (a2[1] - a1[1]) * (a1[1] - c[1]))
  const cc =
    c[0] * c[0] +
    c[1] * c[1] +
    a1[0] * a1[0] +
    a1[1] * a1[1] -
    2 * (c[0] * a1[0] + c[1] * a1[1]) -
    r * r

  const deter = b * b - 4 * a * cc

  if (deter < 0) {
    return getIntersection("outside")
  }

  if (deter === 0) {
    return getIntersection("tangent")
  }

  var e = Math.sqrt(deter)
  var u1 = (-b + e) / (2 * a)
  var u2 = (-b - e) / (2 * a)
  if ((u1 < 0 || u1 > 1) && (u2 < 0 || u2 > 1)) {
    if ((u1 < 0 && u2 < 0) || (u1 > 1 && u2 > 1)) {
      return getIntersection("outside")
    } else {
      return getIntersection("inside")
    }
  }

  const results: number[][] = []
  if (0 <= u1 && u1 <= 1) results.push(vec.lrp(a1, a2, u1))
  if (0 <= u2 && u2 <= 1) results.push(vec.lrp(a1, a2, u2))

  return getIntersection("intersection", ...results)
}

export function intersectEllipseLineSegment(
  center: number[],
  rx: number,
  ry: number,
  a1: number[],
  a2: number[],
  rotation = 0
) {
  // If the ellipse or line segment are empty, return no tValues.
  if (rx === 0 || ry === 0 || vec.isEqual(a1, a2)) {
    return getIntersection("No intersection")
  }

  // Get the semimajor and semiminor axes.
  rx = rx < 0 ? rx : -rx
  ry = ry < 0 ? ry : -ry

  // Rotate points and translate so the ellipse is centered at the origin.
  a1 = vec.sub(vec.rotWith(a1, center, -rotation), center)
  a2 = vec.sub(vec.rotWith(a2, center, -rotation), center)

  // Calculate the quadratic parameters.
  const diff = vec.sub(a2, a1)

  var A = (diff[0] * diff[0]) / rx / rx + (diff[1] * diff[1]) / ry / ry
  var B = (2 * a1[0] * diff[0]) / rx / rx + (2 * a1[1] * diff[1]) / ry / ry
  var C = (a1[0] * a1[0]) / rx / rx + (a1[1] * a1[1]) / ry / ry - 1

  // Make a list of t values (normalized points on the line where intersections occur).
  var tValues: number[] = []

  // Calculate the discriminant.
  var discriminant = B * B - 4 * A * C

  if (discriminant === 0) {
    // One real solution.
    tValues.push(-B / 2 / A)
  } else if (discriminant > 0) {
    const root = Math.sqrt(discriminant)
    // Two real solutions.
    tValues.push((-B + root) / 2 / A)
    tValues.push((-B - root) / 2 / A)
  }

  // Filter to only points that are on the segment.
  // Solve for points, then counter-rotate points.
  const points = tValues
    .filter((t) => t >= 0 && t <= 1)
    .map((t) => vec.add(center, vec.add(a1, vec.mul(vec.sub(a2, a1), t))))
    .map((p) => vec.rotWith(p, center, rotation))

  return getIntersection("intersection", ...points)
}

export function intersectCircleRectangle(
  c: number[],
  r: number,
  point: number[],
  size: number[]
): Intersection[] {
  const tl = point
  const tr = vec.add(point, [size[0], 0])
  const br = vec.add(point, size)
  const bl = vec.add(point, [0, size[1]])

  const intersections: Intersection[] = []

  const topIntersection = intersectCircleLineSegment(c, r, tl, tr)
  const rightIntersection = intersectCircleLineSegment(c, r, tr, br)
  const bottomIntersection = intersectCircleLineSegment(c, r, bl, br)
  const leftIntersection = intersectCircleLineSegment(c, r, tl, bl)

  if (topIntersection.didIntersect) {
    intersections.push({ ...topIntersection, message: "top" })
  }

  if (rightIntersection.didIntersect) {
    intersections.push({ ...rightIntersection, message: "right" })
  }

  if (bottomIntersection.didIntersect) {
    intersections.push({ ...bottomIntersection, message: "bottom" })
  }

  if (leftIntersection.didIntersect) {
    intersections.push({ ...leftIntersection, message: "left" })
  }

  return intersections
}

export function intersectEllipseRectangle(
  c: number[],
  rx: number,
  ry: number,
  point: number[],
  size: number[],
  rotation = 0
): Intersection[] {
  const tl = point
  const tr = vec.add(point, [size[0], 0])
  const br = vec.add(point, size)
  const bl = vec.add(point, [0, size[1]])

  const intersections: Intersection[] = []

  const topIntersection = intersectEllipseLineSegment(
    c,
    rx,
    ry,
    tl,
    tr,
    rotation
  )
  const rightIntersection = intersectEllipseLineSegment(
    c,
    rx,
    ry,
    tr,
    br,
    rotation
  )
  const bottomIntersection = intersectEllipseLineSegment(
    c,
    rx,
    ry,
    bl,
    br,
    rotation
  )
  const leftIntersection = intersectEllipseLineSegment(
    c,
    rx,
    ry,
    tl,
    bl,
    rotation
  )

  if (topIntersection.didIntersect) {
    intersections.push({ ...topIntersection, message: "top" })
  }

  if (rightIntersection.didIntersect) {
    intersections.push({ ...rightIntersection, message: "right" })
  }

  if (bottomIntersection.didIntersect) {
    intersections.push({ ...bottomIntersection, message: "bottom" })
  }

  if (leftIntersection.didIntersect) {
    intersections.push({ ...leftIntersection, message: "left" })
  }

  return intersections
}

export function intersectRectangleLineSegment(
  point: number[],
  size: number[],
  a1: number[],
  a2: number[]
) {
  const tl = point
  const tr = vec.add(point, [size[0], 0])
  const br = vec.add(point, size)
  const bl = vec.add(point, [0, size[1]])

  const intersections: Intersection[] = []

  const topIntersection = intersectLineSegments(a1, a2, tl, tr)
  const rightIntersection = intersectLineSegments(a1, a2, tr, br)
  const bottomIntersection = intersectLineSegments(a1, a2, bl, br)
  const leftIntersection = intersectLineSegments(a1, a2, tl, bl)

  if (topIntersection.didIntersect) {
    intersections.push({ ...topIntersection, message: "top" })
  }

  if (rightIntersection.didIntersect) {
    intersections.push({ ...rightIntersection, message: "right" })
  }

  if (bottomIntersection.didIntersect) {
    intersections.push({ ...bottomIntersection, message: "bottom" })
  }

  if (leftIntersection.didIntersect) {
    intersections.push({ ...leftIntersection, message: "left" })
  }

  return intersections
}

/* -------------------------------------------------- */
/*                  Shape vs. Bounds                  */
/* -------------------------------------------------- */

export function intersectCircleBounds(
  c: number[],
  r: number,
  bounds: Bounds
): Intersection[] {
  const { minX, minY, width, height } = bounds
  return intersectCircleRectangle(c, r, [minX, minY], [width, height])
}

export function intersectEllipseBounds(
  c: number[],
  rx: number,
  ry: number,
  bounds: Bounds,
  rotation = 0
): Intersection[] {
  const { minX, minY, width, height } = bounds
  return intersectEllipseRectangle(
    c,
    rx,
    ry,
    [minX, minY],
    [width, height],
    rotation
  )
}

export function intersectLineSegmentBounds(
  a1: number[],
  a2: number[],
  bounds: Bounds
) {
  const { minX, minY, width, height } = bounds
  return intersectRectangleLineSegment([minX, minY], [width, height], a1, a2)
}

export function intersectPolylineBounds(points: number[][], bounds: Bounds) {
  const { minX, minY, width, height } = bounds
  const intersections: Intersection[] = []

  for (let i = 1; i < points.length; i++) {
    intersections.push(
      ...intersectRectangleLineSegment(
        [minX, minY],
        [width, height],
        points[i - 1],
        points[i]
      )
    )
  }

  return intersections
}

export function intersectPolygonBounds(points: number[][], bounds: Bounds) {
  const { minX, minY, width, height } = bounds
  const intersections: Intersection[] = []

  for (let i = 1; i < points.length + 1; i++) {
    intersections.push(
      ...intersectRectangleLineSegment(
        [minX, minY],
        [width, height],
        points[i - 1],
        points[i % points.length]
      )
    )
  }

  return intersections
}