feat(geom): add offset() & initial impls

- add impls for circle, line, rect

packages/geom/src/index.ts

@@ -58,6 +58,7 @@ export * from "./ops/fit-into-bounds";
 export * from "./ops/flip";
 export * from "./ops/intersects";
 export * from "./ops/map-point";
+export * from "./ops/offset";
 export * from "./ops/point-at";
 export * from "./ops/point-inside";
 export * from "./ops/resample";

packages/geom/src/ops/offset.ts

@@ -0,0 +1,47 @@
+import type { IObjectOf } from "@thi.ng/api";
+import { defmulti, Implementation2 } from "@thi.ng/defmulti";
+import { IShape, Type } from "@thi.ng/geom-api";
+import {
+    addN2,
+    max2,
+    set2,
+    ZERO2,
+    normalCCW,
+    add2,
+    sub2,
+} from "@thi.ng/vectors";
+import { Circle } from "../api/circle";
+import { Rect } from "../api/rect";
+import { rectFromCentroid } from "../ctors/rect";
+import { copyAttribs } from "../internal/copy-attribs";
+import { dispatch } from "../internal/dispatch";
+import { centroid } from "./centroid";
+import { Line } from "../api/line";
+import { Quad } from "../api/quad";
+
+export const offset = defmulti<IShape, number, IShape>(dispatch);
+
+offset.addAll(<IObjectOf<Implementation2<unknown, number, IShape>>>{
+    [Type.CIRCLE]: ($: Circle, n) =>
+        new Circle(set2([], $.pos), Math.max($.r + n, 0)),
+
+    [Type.LINE]: ({ points: [a, b], attribs }: Line, n) => {
+        const norm = normalCCW([], a, b, n);
+        return new Quad(
+            [
+                add2([], a, norm),
+                add2([], b, norm),
+                sub2([], b, norm),
+                sub2([], a, norm),
+            ],
+            { ...attribs }
+        );
+    },
+
+    [Type.RECT]: ($: Rect, n) =>
+        rectFromCentroid(
+            centroid($)!,
+            max2(null, addN2([], $.size, n), ZERO2),
+            copyAttribs($)
+        ),
+});