Use Bhijn's algorithm for corner assist

src/engine/datatypes.ts

@@ -144,6 +144,57 @@ export class Size {
   }
 }
 
+export class Circle {
+  readonly center: Point;
+  readonly radius: number;
+
+  constructor(center: Point, radius: number) {
+    this.center = center;
+    this.radius = radius;
+  }
+
+  getContactWithRect(rect: Rect): Point | null {
+    // port: https://www.jeffreythompson.org/collision-detection/circle-rect.php
+    let cx = this.center.x;
+    let cy = this.center.y;
+    let testX = this.center.x;
+    let testY = this.center.y;
+
+    let rx = rect.top.x;
+    let ry = rect.top.y;
+    let rw = rect.size.w;
+    let rh = rect.size.h;
+
+    if (cx < rx) { testX = rx; }
+    else if (cx > rx + rw) { testX = rx + rw; }
+    if (cy < ry) { testY = ry; }
+    else if (cy > ry + rh) { testY = ry + rh; }
+
+    let distX = cx - testX;
+    let distY = cy - testY;
+    let sqDistance = (distX * distX) + (distY * distY);
+
+    if (sqDistance <= this.radius * this.radius) {
+      return new Point(testX, testY);
+    }
+    return null;
+  }
+
+  overlappedCells(size: Size): Rect[] {
+    let meAsRect = new Rect(
+      this.center.offset(new Point(-this.radius, -this.radius)),
+      new Size(this.radius * 2, this.radius * 2)
+    );
+    let all: Rect[] = [];
+    for (let cell of meAsRect.overlappedCells(size).values()) {
+      if (this.getContactWithRect(cell) != null) {
+        all.push(cell);
+      }
+    }
+    return all;
+  }
+}
+
 export class Rect {
   readonly top: Point;
   readonly size: Size;