feat(FN-4956): complete Step 1 — add scope partition helper

Fusion-Task-Id: FN-4956
Fusion-Task-Lineage: 7105cd5e-27f4-4e84-b2ae-e811a183803f

packages/engine/src/__tests__/merger-layer3-scope-partition.test.ts

@@ -0,0 +1,77 @@
+import { describe, expect, it } from "vitest";
+
+import { partitionConflictsByFileScope } from "../merger.js";
+
+describe("partitionConflictsByFileScope", () => {
+  it("treats empty declared scope as no enforcement", () => {
+    const result = partitionConflictsByFileScope({
+      conflictFiles: ["AGENTS.md", "packages/core/src/store.ts"],
+      declaredScope: [],
+    });
+
+    expect(result).toEqual({
+      inScope: ["AGENTS.md", "packages/core/src/store.ts"],
+      outOfScope: [],
+    });
+  });
+
+  it("returns all in-scope files", () => {
+    const result = partitionConflictsByFileScope({
+      conflictFiles: ["packages/engine/src/merger.ts", "packages/engine/src/store.ts"],
+      declaredScope: ["packages/engine/src/**"],
+    });
+
+    expect(result).toEqual({
+      inScope: ["packages/engine/src/merger.ts", "packages/engine/src/store.ts"],
+      outOfScope: [],
+    });
+  });
+
+  it("returns all out-of-scope files", () => {
+    const result = partitionConflictsByFileScope({
+      conflictFiles: ["AGENTS.md", "packages/core/src/store.ts"],
+      declaredScope: ["packages/desktop/src/**"],
+    });
+
+    expect(result).toEqual({
+      inScope: [],
+      outOfScope: ["AGENTS.md", "packages/core/src/store.ts"],
+    });
+  });
+
+  it("partitions mixed files", () => {
+    const result = partitionConflictsByFileScope({
+      conflictFiles: ["packages/desktop/src/foo.ts", "packages/core/src/store.ts"],
+      declaredScope: ["packages/desktop/src/**"],
+    });
+
+    expect(result).toEqual({
+      inScope: ["packages/desktop/src/foo.ts"],
+      outOfScope: ["packages/core/src/store.ts"],
+    });
+  });
+
+  it("matches glob scope entries", () => {
+    const result = partitionConflictsByFileScope({
+      conflictFiles: ["packages/core/src/store.ts", "packages/core/test/store.test.ts"],
+      declaredScope: ["packages/core/src/**"],
+    });
+
+    expect(result).toEqual({
+      inScope: ["packages/core/src/store.ts"],
+      outOfScope: ["packages/core/test/store.test.ts"],
+    });
+  });
+
+  it("supports .changeset patterns", () => {
+    const result = partitionConflictsByFileScope({
+      conflictFiles: [".changeset/fn-4956.md", "AGENTS.md"],
+      declaredScope: [".changeset/*"],
+    });
+
+    expect(result).toEqual({
+      inScope: [".changeset/fn-4956.md"],
+      outOfScope: ["AGENTS.md"],
+    });
+  });
+});

packages/engine/src/merger.ts

@@ -3927,6 +3927,27 @@ export function matchesScope(filePath: string, scopePatterns: string[]): boolean
   return false;
 }
 
+export function partitionConflictsByFileScope(params: {
+  conflictFiles: string[];
+  declaredScope: string[];
+}): { inScope: string[]; outOfScope: string[] } {
+  const { conflictFiles, declaredScope } = params;
+  if (declaredScope.length === 0) {
+    return { inScope: [...conflictFiles], outOfScope: [] };
+  }
+
+  const inScope: string[] = [];
+  const outOfScope: string[] = [];
+  for (const file of conflictFiles) {
+    if (matchesScope(file, declaredScope)) {
+      inScope.push(file);
+    } else {
+      outOfScope.push(file);
+    }
+  }
+  return { inScope, outOfScope };
+}
+
 /**
  * Validate that the diff stays within the task's declared File Scope.
  * Returns warnings for out-of-scope changes, especially large deletions.