Fix position challenges to use the selected move-tree position

src/lib/challengeStartPosition.js

@@ -1,4 +1,4 @@
-import { getMainLineTipState } from "./Lab/exploration";
+import { getFocusNode } from "./Lab/exploration";
 import { isTeachingChallengeEligible } from "./teachingModeChallenge";
 
 /** Mirrors backend teaching eligibility for start-position challenges. */
@@ -28,7 +28,7 @@ export function buildStartPositionFromLabSession({ game, nodes, focus }) {
   if (!game?.metaGame || !nodes?.length || !focus) {
     throw new Error("Cannot build start position from this session.");
   }
-  const state = getMainLineTipState(nodes, game);
+  const state = getFocusNode(nodes, game, focus)?.state;
   if (!state) {
     throw new Error("Start position state is missing.");
   }

src/lib/challengeStartPosition.test.js

@@ -3,6 +3,7 @@ import { GameFactory } from "@abstractplay/gameslib";
 import {
   buildStartPositionPayload,
   buildStartPositionFromLabSession,
+  buildStartPositionFromGameMoveSession,
   isStartPositionChallengeEligible,
 } from "./challengeStartPosition";
 
@@ -19,7 +20,7 @@ describe("challengeStartPosition", () => {
     expect(isStartPositionChallengeEligible("saltire", 3, [])).toBe(false);
   });
 
-  it("buildStartPositionFromLabSession uses tip state at focus", () => {
+  it("buildStartPositionFromLabSession uses the selected main-line tip", () => {
     const engine = GameFactory("saltire", undefined, []);
     const opening = engine.serialize();
     engine.move(engine.moves()[0]);
@@ -39,3 +40,94 @@ describe("challengeStartPosition", () => {
     expect(draft.playerCount).toBe(2);
   });
 });
+
+describe.each([
+  ["Lab", buildStartPositionFromLabSession],
+  ["Game Move", buildStartPositionFromGameMoveSession],
+])("%s start-position selection", (_name, buildStartPosition) => {
+  it("uses an earlier main-line position, hydrating its snapshot", () => {
+    const engine = GameFactory("saltire", undefined, []);
+    engine.move(engine.moves()[0]);
+    const selected = engine.serialize();
+    engine.move(engine.moves()[0]);
+    const tip = engine.serialize();
+    const nodes = [
+      { state: null, children: [] },
+      { state: null, children: [] },
+      { state: tip, children: [] },
+    ];
+    const draft = buildStartPosition({
+      game: { metaGame: "saltire", state: tip, selectedVariants: [] },
+      nodes,
+      focus: { moveNumber: 1, exPath: [] },
+    });
+    const actual = GameFactory("saltire", draft.state);
+    const expected = GameFactory("saltire", selected);
+    expect(actual.stack).toHaveLength(2);
+    expect(actual.board).toEqual(expected.board);
+    expect(actual.currplayer).toBe(expected.currplayer);
+  });
+
+  it("uses the nested Arimaa exploration branch from the selected focus", () => {
+    const main = GameFactory("arimaa", undefined, ["free"]);
+    const opening = main.serialize();
+    main.move("Re1, Ee2, Md2, Hc2, Hb2, Da2, Da1, Cc1, Cb1");
+    const mainGold = main.serialize();
+    main.move(
+      "ra8, rb8, rc8, rd8, re8, rf8, rg8, rh8, rh7, rg7, rf7, re7, rd7, rc7, rb7, ra7, ra6, rb6, rc6, rd6, re6, rf6, rg6, rh6, rh5, rg5, rf5, re5, rd5, rc5, rb5, ra5"
+    );
+    const tip = main.serialize();
+
+    const branch = GameFactory("arimaa", opening);
+    branch.move(
+      "Ee2, Hb2, Dg2, Cd2, Ra2, Ra1, Rb1, Rc1, Rc2, Rd1, Re1, Rf1, Rf2, Rh1, Rg1, Rh2"
+    );
+    const branchGold = branch.serialize();
+    branch.move(
+      "ed7, me7, hg7, hb7, dd8, de8, cf7, cc7, ra7, ra8, rb8, rc8, rf8, rg8, rh8, rh7"
+    );
+    const selected = branch.serialize();
+    const siblings = Array.from({ length: 6 }, () => ({
+      state: opening,
+      children: [],
+    }));
+    const nodes = [
+      {
+        state: opening,
+        children: [
+          ...siblings,
+          {
+            state: branchGold,
+            children: [{ state: selected, children: [] }],
+          },
+        ],
+      },
+      { state: mainGold, children: [] },
+      { state: tip, children: [] },
+    ];
+    const draft = buildStartPosition({
+      game: { metaGame: "arimaa", state: tip, selectedVariants: ["free"] },
+      nodes,
+      focus: { moveNumber: 0, exPath: [6, 0] },
+    });
+
+    expect(draft.state).toBe(selected);
+    expect(buildStartPositionPayload(draft.state)).toEqual({
+      startPosition: { state: selected },
+    });
+    expect(draft.variants).toEqual(["free"]);
+    const actual = GameFactory("arimaa", draft.state);
+    expect(actual.board.get("d7")).toEqual(["E", 2]);
+    expect(actual.board.size).toBe(32);
+  });
+
+  it("rejects a missing focus node instead of using the main-line tip", () => {
+    expect(() =>
+      buildStartPosition({
+        game: { metaGame: "saltire" },
+        nodes: [{ state: "tip", children: [] }],
+        focus: { moveNumber: 0, exPath: [0] },
+      })
+    ).toThrow("Start position state is missing.");
+  });
+});