feat: calculate tiles in range 2

 + unit test

```js
function tilesInRangeRec(
  tileId: TileID,
  range: number,
  acc: TileID[]
): TileID[] {
  const neighbourTiles = tilesInRange1Cached(tileId);
  if (range === 1) {
    return addAll(acc, neighbourTiles);
  }
  const tiles = neighbourTiles.flatMap((tileId) =>
    tilesInRangeRec(tileId, range - 1, acc)
  );
  return addAll(acc, tiles);
}
```

src/models/tiles-in-range.test.ts

@@ -0,0 +1,50 @@
+import { tilesInRange } from "./tiles-in-range";
+
+describe("tilesInRange()", () => {
+  /**
+   *     [ ] [X] [X]
+   *   [ ] [X] [.] [X]
+   * [ ] [ ] [X] [X] [ ]
+   *
+   */
+  it("returns tilesID[] in range 1 from origin tile", () => {
+    const got = tilesInRange("1,-2", { range: 1 });
+    const expected = ["1,-3", "2,-3", "0,-2", "2,-2", "1,-1", "2,-1"];
+    expect(got.sort()).toEqual(expected.sort());
+  });
+
+  /**
+   *       [ ] [ ] [X] [X] [X]
+   *     [ ] [ ] [X] [X] [X] [X]
+   *   [ ] [ ] [X] [X] [.] [X] [X]
+   * [ ] [ ] [ ] [X] [X] [X] [X] [ ]
+   *   [ ] [ ] [ ] [X] [X] [X] [ ]
+   *     [ ] [ ] [ ] [ ] [ ] [ ]
+   *       [ ] [ ] [ ] [ ] [ ]
+   *
+   */
+  it("returns tilesID[] in range 2 from origin tile", () => {
+    const got = tilesInRange("1,-1", { range: 2 });
+    const expected = [
+      "0,-3",
+      "1,-3",
+      "2,-3",
+      "-1,-2",
+      "0,-2",
+      "1,-2",
+      "2,-2",
+      "-1,-1",
+      "0,-1",
+      "2,-1",
+      "3,-1",
+      "-1,0",
+      "0,0",
+      "1,0",
+      "2,0",
+      "0,1",
+      "1,1",
+      "2,1",
+    ];
+    expect(got.sort()).toEqual(expected.sort());
+  });
+});

src/models/tiles-in-range.ts

@@ -0,0 +1,68 @@
+import { asTileID, coordinates } from "./tiles";
+
+const NEIGHBOUR_TILES_MEM_CACHE = {} as Record<TileID, TileID[]>;
+
+export function tilesInRange(
+  tileId: TileID,
+  { range }: { range?: number } = {}
+) {
+  return tilesInRangeRec(tileId, range || 1, []).filter((t) => t !== tileId);
+}
+
+function tilesInRangeRec(
+  tileId: TileID,
+  range: number,
+  acc: TileID[]
+): TileID[] {
+  const neighbourTiles = tilesInRange1Cached(tileId);
+  if (range === 1) {
+    return addAll(acc, neighbourTiles);
+  }
+  const tiles = neighbourTiles.flatMap((tileId) =>
+    tilesInRangeRec(tileId, range - 1, acc)
+  );
+  return addAll(acc, tiles);
+}
+
+function addAll<T>(a: T[], b: T[]) {
+  return Array.from(new Set([...a, ...b]));
+}
+
+function tilesInRange1Cached(tileId: TileID) {
+  if (!NEIGHBOUR_TILES_MEM_CACHE[tileId]) {
+    NEIGHBOUR_TILES_MEM_CACHE[tileId] = tilesInRange1(tileId);
+  }
+  return NEIGHBOUR_TILES_MEM_CACHE[tileId];
+}
+
+function tilesInRange1(tileId: TileID) {
+  const { x, y } = coordinates(tileId);
+
+  return range1Variance(y).flatMap((variance) => {
+    const tileIdStr = asTileID({
+      x: x + variance[0],
+      y: y + variance[1],
+    });
+    return tileIdStr ? [tileIdStr] : [];
+  });
+}
+
+function range1Variance(y: number) {
+  return y % 2 === 0
+    ? [
+        [0, -1],
+        [+1, -1],
+        [-1, 0],
+        [+1, 0],
+        [0, +1],
+        [+1, +1],
+      ]
+    : [
+        [-1, -1],
+        [0, -1],
+        [-1, 0],
+        [+1, 0],
+        [-1, +1],
+        [0, +1],
+      ];
+}

src/models/tiles.ts

@@ -8,7 +8,6 @@
  *                    [ ] [ ] [ ] [ ] [ ] [ ]
  *                      [ ] [ ] [ ] [ ] [ ]
  *
- *                            =>
  *
  *              [-2,-3] [-1,-3] [+0,-3] [1,-3] [2,-3]
  *
@@ -101,35 +100,3 @@ export function asTileID({ x, y }: { x: number; y: number }): TileID | null {
 export function row(n: -3 | -2 | -1 | 0 | 1 | 2 | 3): TileID[] {
   return tiles.filter((id) => coordinates(id).y === n);
 }
-
-export function tilesInRange(tile: TileID, { range } = { range: 1 }): TileID[] {
-  const { x, y } = coordinates(tile);
-
-  // TODO use range param instead of fixed array?
-  const range1Movements =
-    y % 2 === 0
-      ? [
-          [0, -1],
-          [+1, -1],
-          [-1, 0],
-          [+1, 0],
-          [0, +1],
-          [+1, +1],
-        ]
-      : [
-          [-1, -1],
-          [0, -1],
-          [-1, 0],
-          [+1, 0],
-          [-1, +1],
-          [0, +1],
-        ];
-
-  return range1Movements.flatMap((variance) => {
-    const tileIdStr = asTileID({
-      x: x + variance[0],
-      y: y + variance[1],
-    });
-    return tileIdStr ? [tileIdStr] : [];
-  });
-}