added BoundingBox.intersection(:_) method and unit test.

Author: hfutrell

BezierKit/BezierKitTests/BoundingBoxTests.swift

@@ -111,4 +111,22 @@ class BoundingBoxTests: XCTestCase {
         XCTAssertEqual(result, BoundingBox(p1: CGPoint(x: 1.0, y: 1.0), p2: CGPoint(x: 3.0, y: 5.0)))
     }
 
+    func testIntersection() {
+        let box1 = BoundingBox(p1: CGPoint(x:0,y:0), p2: CGPoint(x:3,y:2))
+        let box2 = BoundingBox(p1: CGPoint(x:2,y:1), p2: CGPoint(x:4,y:5))      // overlaps box1
+        let box3 = BoundingBox(p1: CGPoint(x:2,y:4), p2: CGPoint(x:4,y:5))      // does not overlap box1
+        let box4 = BoundingBox(p1: CGPoint(x:3,y:0), p2: CGPoint(x:5,y:2))      // overlaps box1 exactly on x edge
+        let box5 = BoundingBox(p1: CGPoint(x:0,y:2), p2: CGPoint(x:3,y:4))      // overlaps box1 exactly on y edge
+        let box6 = BoundingBox(p1: CGPoint(x:0,y:0), p2: CGPoint(x:-5,y:-5))    // overlaps box1 only at (0,0)
+        let expectedBox = BoundingBox(p1: CGPoint(x:2,y:1), p2: CGPoint(x:3,y:2))
+        XCTAssertEqual(box1.intersection(box2), expectedBox)
+        XCTAssertEqual(box2.intersection(box1), expectedBox)
+        XCTAssertEqual(box1.intersection(box3), BoundingBox.empty)
+        XCTAssertEqual(box1.intersection(BoundingBox.empty), BoundingBox.empty)
+        XCTAssertEqual(BoundingBox.empty.intersection(box1), BoundingBox.empty)
+        XCTAssertEqual(BoundingBox.empty.intersection(BoundingBox.empty), BoundingBox.empty)
+        XCTAssertEqual(box1.intersection(box4), BoundingBox(p1: CGPoint(x:3,y:0), p2: CGPoint(x:3,y:2)))
+        XCTAssertEqual(box1.intersection(box5), BoundingBox(p1: CGPoint(x:0,y:2), p2: CGPoint(x:3,y:2)))
+        XCTAssertEqual(box1.intersection(box6), BoundingBox(p1: CGPoint.zero, p2: CGPoint.zero))
+    }
 }

BezierKit/Library/Types.swift

@@ -54,6 +54,17 @@ public struct BoundingBox: Equatable {
         self.max = CGPoint.max(self.max, other.max)
         return self
     }
+    public func intersection(_ other: BoundingBox) -> BoundingBox {
+        let box = BoundingBox(min: CGPoint.max(self.min, other.min),
+                              max: CGPoint.min(self.max, other.max))
+        guard box.max.x - box.min.x >= 0, box.max.y - box.min.y >= 0 else {
+            return BoundingBox.empty
+        }
+        return box
+    }
+    public var isEmpty: Bool {
+        return self.min.x > self.max.x || self.min.y > self.max.y
+    }
     public init(p1: CGPoint, p2: CGPoint) {
         self.min = CGPoint.min(p1, p2)
         self.max = CGPoint.max(p1, p2)