Fix paste ghost off-grid for odd-sized copies

packages/renderer/package.json

@@ -6,7 +6,7 @@
   "main": "src/index.ts",
   "scripts": {
     "check": "tsc --noEmit",
-    "test": "tsx test/wires.test.ts && tsx test/wireEdit.test.ts && tsx test/beltAnimation.test.ts && tsx test/animProfile.test.ts && tsx test/pipe.test.ts && tsx test/beltGraph.test.ts && tsx test/spatial-camera.test.ts && tsx test/redraw.test.ts && tsx test/recordingSummary.test.ts && tsx test/entityPreview.test.ts && tsx test/heatNetwork.test.ts && tsx test/inserterLayering.test.ts && tsx test/bake.test.ts && tsx test/undergroundPairing.test.ts && tsx test/buildOver.test.ts && tsx test/fusionPorts.test.ts && tsx test/fluidArt.test.ts && tsx test/railGeometry.test.ts && tsx test/railArt.test.ts && tsx test/railPlanner.test.ts && tsx test/collision.test.ts && tsx test/railBlocks.test.ts && tsx test/railSignals.test.ts && tsx test/tiles.test.ts && tsx test/cargoBay.test.ts && tsx test/sceneCache.test.ts"
+    "test": "tsx test/wires.test.ts && tsx test/wireEdit.test.ts && tsx test/beltAnimation.test.ts && tsx test/animProfile.test.ts && tsx test/pipe.test.ts && tsx test/beltGraph.test.ts && tsx test/spatial-camera.test.ts && tsx test/redraw.test.ts && tsx test/recordingSummary.test.ts && tsx test/entityPreview.test.ts && tsx test/heatNetwork.test.ts && tsx test/inserterLayering.test.ts && tsx test/bake.test.ts && tsx test/undergroundPairing.test.ts && tsx test/buildOver.test.ts && tsx test/fusionPorts.test.ts && tsx test/fluidArt.test.ts && tsx test/railGeometry.test.ts && tsx test/railArt.test.ts && tsx test/railPlanner.test.ts && tsx test/collision.test.ts && tsx test/railBlocks.test.ts && tsx test/railSignals.test.ts && tsx test/tiles.test.ts && tsx test/cargoBay.test.ts && tsx test/sceneCache.test.ts && tsx test/pasteSnap.test.ts"
   },
   "dependencies": {
     "@factoriotools/engine": "*"

packages/renderer/src/pasteSnap.ts

@@ -0,0 +1,13 @@
+/** Where a paste ghost's anchor lands for a cursor at `cursor`, when the
+ *  ghost snaps to whole tiles: the copied position shifted by a whole number
+ *  of tiles. The anchor is the group's centre, which is NOT on a tile corner
+ *  for a group of odd width (one belt, one assembler: x.5), and rounding it
+ *  to a whole tile would slide the stamp half a tile off the grid hand
+ *  placement uses. Keeping the anchor's own fraction keeps every stamped
+ *  entity exactly as far from the grid as the original was. */
+export function snapAnchorToTiles(cursor: { x: number; y: number }, anchor: { x: number; y: number }): { x: number; y: number } {
+  return {
+    x: anchor.x + Math.round(cursor.x - anchor.x),
+    y: anchor.y + Math.round(cursor.y - anchor.y),
+  };
+}

packages/renderer/src/render.ts

@@ -22,6 +22,7 @@ import { drawInserter } from "./sprites/inserter.js";
 import { SpatialIndex, type IndexedBox } from "./spatialIndex.js";
 import { boxHitsEntity, entitiesCollide } from "./collision.js";
 import { RAIL_BLOCK_COLORS } from "./railBlocks.js";
+import { snapAnchorToTiles } from "./pasteSnap.js";
 import { isElevatedRail, isRail, isRollingStock, onRailGrid, openEnds, RAIL_DECK_HEIGHT, RAIL_END_CAP, railCentreline, railHighlightBox, railJoints, snapToRailGrid, type RailEnd, type RailPiece, type RailSlot } from "./railGeometry.js";
 import {
   buildRailIndex,
@@ -853,7 +854,8 @@ export function mountRenderer(container: HTMLElement, data: GameData, catalog: R
 
   function snapPasteAnchorToTiles(cursor: { x: number; y: number }): { x: number; y: number } {
     const cell = pasteCell();
-    if (!cell || mode.kind !== "paste" || !mode.snap) return { x: Math.round(cursor.x), y: Math.round(cursor.y) };
+    if (mode.kind !== "paste") return { x: Math.round(cursor.x), y: Math.round(cursor.y) };
+    if (!cell || !mode.snap) return snapAnchorToTiles(cursor, mode.anchor);
     const { offset, size } = cell;
     const origin = mode.snap.absolute ? mode.snap.offset : relativeSnapOrigin;
     // The cell's top-left if the ghost were centred on the cursor.

packages/renderer/test/pasteSnap.test.ts

@@ -0,0 +1,54 @@
+import assert from "node:assert/strict";
+import { snapAnchorToTiles } from "../src/pasteSnap.js";
+
+let passed = 0;
+function test(name: string, fn: () => void) {
+  try {
+    fn();
+    passed++;
+    console.log(`  ok  ${name}`);
+  } catch (error) {
+    console.error(`FAIL  ${name}`);
+    console.error(error);
+    process.exitCode = 1;
+  }
+}
+
+/** Where hand placement puts a footprint's centre for a cursor (render.ts's
+ *  snapAxis): odd widths on a tile centre, even widths on a tile corner. */
+const handPlaced = (cursor: number, width: number) => (width % 2 === 1 ? Math.floor(cursor) + 0.5 : Math.round(cursor));
+
+// Issue #61: copy one building, stamp the ghost anywhere, and it must sit
+// where the same building would if placed by hand.
+for (const width of [1, 2, 3, 4, 5]) {
+  test(`${width}x${width} copy stamps where hand placement would`, () => {
+    const copiedAt = handPlaced(10.3, width);
+    for (let i = -70; i <= 70; i++) {
+      const cursor = i / 10 + 0.003;
+      const snapped = snapAnchorToTiles({ x: cursor, y: cursor }, { x: copiedAt, y: copiedAt });
+      assert.ok(Math.abs(snapped.x - handPlaced(cursor, width)) < 1e-9, `cursor ${cursor}: ${snapped.x} vs ${handPlaced(cursor, width)}`);
+      assert.ok(Math.abs(snapped.y - handPlaced(cursor, width)) < 1e-9);
+    }
+  });
+}
+
+test("a group moves by whole tiles from where it was copied", () => {
+  const anchor = { x: 1.75, y: -3.5 };
+  for (let i = -13; i <= 13; i++) {
+    const cx = i * 0.37;
+    const snapped = snapAnchorToTiles({ x: cx, y: cx * 1.3 }, anchor);
+    assert.ok(Math.abs(snapped.x - anchor.x - Math.round(snapped.x - anchor.x)) < 1e-9);
+    assert.ok(Math.abs(snapped.y - anchor.y - Math.round(snapped.y - anchor.y)) < 1e-9);
+  }
+});
+
+test("the ghost stays within half a tile of the cursor", () => {
+  const anchor = { x: 0.5, y: 0.5 };
+  for (let i = -38; i <= 38; i++) {
+    const c = i * 0.13;
+    const snapped = snapAnchorToTiles({ x: c, y: c }, anchor);
+    assert.ok(Math.abs(snapped.x - c) <= 0.5 + 1e-9);
+  }
+});
+
+console.log(`${passed} passed`);