fix(chase): enforce distinct start positions in round generation

Made-with: Cursor

src/game/chase/generator.ts

@@ -174,7 +174,13 @@ function generateGraphAttempt(seed: number, difficulty: Difficulty): ChaseRound
   const exitNodeId = farthest.id;
   const pathLength = shortestPathLength(graph, thiefStartNodeId, exitNodeId);
   const farthestFromExit = getFarthestNode(graph, exitNodeId);
-  const guardStartNodeId = farthestFromExit.id;
+  let guardStartNodeId = farthestFromExit.id;
+  if (guardStartNodeId === thiefStartNodeId) {
+    const fallbackCandidates = existingNodeIds.filter(
+      (nodeId) => nodeId !== thiefStartNodeId,
+    );
+    guardStartNodeId = fallbackCandidates[0] ?? thiefStartNodeId;
+  }
 
   return {
     snapshot: {
@@ -206,6 +212,7 @@ export function generateChaseRound(input: GenerateChaseRoundInput): ChaseRound {
     const isPlayable =
       isConnected(round.snapshot.graph) &&
       round.meta.hasEscapePath &&
+      round.snapshot.guardStartNodeId !== round.snapshot.thiefStartNodeId &&
       round.meta.pathLength >= minPathLength;
 
     if (isPlayable) {

tests/chase/chaseGenerator.test.ts

@@ -40,8 +40,8 @@ describe("chase round generator", () => {
           "2,-1",
           "1,-2",
         ],
-        "guardNodeId": "4,-5",
-        "guardStartNodeId": "4,-5",
+        "guardNodeId": "0,0",
+        "guardStartNodeId": "0,0",
         "hasEscapePath": true,
         "nodeCount": 20,
         "seed": 424242,
@@ -71,4 +71,23 @@ describe("chase round generator", () => {
     expect(pathLength).toBeGreaterThan(0);
     expect(pathLength).toBeLessThan(Infinity);
   });
+
+  it("ensures thief and guard do not start on the same node", () => {
+    const round = generateChaseRound({ seed: 123456, difficulty: "normal" });
+    expect(round.snapshot.thiefStartNodeId).not.toBe(round.snapshot.guardStartNodeId);
+  });
+
+  it("stays playable and stable across many seeds", () => {
+    for (let seed = 1; seed <= 60; seed += 1) {
+      const round = generateChaseRound({ seed, difficulty: "normal" });
+      const nodeCount = Object.keys(round.snapshot.graph.nodes).length;
+
+      expect(nodeCount).toBeGreaterThanOrEqual(20);
+      expect(nodeCount).toBeLessThanOrEqual(30);
+      expect(round.meta.hasEscapePath).toBe(true);
+      expect(round.snapshot.thiefStartNodeId).not.toBe(
+        round.snapshot.guardStartNodeId,
+      );
+    }
+  });
 });