Function edgesIntersection

src/offset-polygon.ts:103–132

(edgeA: Edge | OffsetEdge, edgeB: Edge | OffsetEdge)

// based on http://local.wasp.uwa.edu.au/~pbourke/geometry/lineline2d/, edgeA => "line a", edgeB => "line b"

function edgesIntersection(edgeA: Edge | OffsetEdge, edgeB: Edge | OffsetEdge) {
  const den =
    (edgeB.vertex2.y - edgeB.vertex1.y) * (edgeA.vertex2.x - edgeA.vertex1.x) -
    (edgeB.vertex2.x - edgeB.vertex1.x) * (edgeA.vertex2.y - edgeA.vertex1.y);

  if (den == 0) {
    return null; // lines are parallel or coincident
  }

  const ua =
    ((edgeB.vertex2.x - edgeB.vertex1.x) * (edgeA.vertex1.y - edgeB.vertex1.y) -
      (edgeB.vertex2.y - edgeB.vertex1.y) *
        (edgeA.vertex1.x - edgeB.vertex1.x)) /
    den;

  const ub =
    ((edgeA.vertex2.x - edgeA.vertex1.x) * (edgeA.vertex1.y - edgeB.vertex1.y) -
      (edgeA.vertex2.y - edgeA.vertex1.y) *
        (edgeA.vertex1.x - edgeB.vertex1.x)) /
    den;

  // Edges are not intersecting but the lines defined by them are
  const isIntersectionOutside = ua < 0 || ub < 0 || ua > 1 || ub > 1;

  return {
    x: edgeA.vertex1.x + ua * (edgeA.vertex2.x - edgeA.vertex1.x),
    y: edgeA.vertex1.y + ua * (edgeA.vertex2.y - edgeA.vertex1.y),
    isIntersectionOutside,
  };
}

function appendArc(
  arcSegments: number,