refactor(ids): co-locate ID generation and validation in ids.ts to prevent contract drift

src/main/chapters.ts

@@ -9,6 +9,7 @@ import {
   makeChapterId,
   saveMeta,
 } from "./project.js";
+import { isSafeChapterId } from "./ids.js";
 import {
   getChapterDoc,
   setChapterDoc,
@@ -164,16 +165,14 @@ export interface ChapterSnapshot {
   index: number;
 }
 
-// Chapter ids are always slugified lowercase ids; enforcing that shape here
-// keeps renderer-supplied snapshots from escaping the book directory.
-const SAFE_ID = /^[a-z0-9][a-z0-9-]*$/;
-
 // Re-create a previously deleted chapter at its original position, restoring
 // its title, color, and content. The chapter is stored as Markdown so it reads
 // correctly regardless of the format it originally used. Idempotent: restoring
 // an id that still exists is a no-op instead of a duplicate.
 export function restoreChapter(bookPath: string, snap: ChapterSnapshot): ChapterEntry {
-  if (!snap || typeof snap.id !== "string" || !SAFE_ID.test(snap.id)) {
+  // Enforcing the shared id grammar (see ids.ts) keeps renderer-supplied
+  // snapshots from escaping the book directory.
+  if (!isSafeChapterId(snap?.id)) {
     throw new Error("Invalid chapter id");
   }
   const meta = loadBook(bookPath);

src/main/ids.ts

@@ -0,0 +1,48 @@
+// The chapter-id grammar and book-folder naming, in one place.
+//
+// Three concerns used to live scattered across project.ts (slug + id
+// generation), chapters.ts (the SAFE_ID validator) and main/index.ts (folder
+// sanitization). Splitting the grammar from its validator meant a future edit
+// to slugify could silently produce ids that isSafeChapterId rejects (e.g.
+// allowing underscores), breaking undo/restore for every new chapter.
+// Co-locating them makes the contract impossible to miss.
+
+/** Human-readable book folder name under ~/Documents. Keeps spaces and case;
+ *  strips characters hostile to filesystems, collapses whitespace, caps at
+ *  100 chars. Callers fall back to "Untitled" when the result is empty. */
+export function sanitizeBookFolderName(name: string): string {
+  return name
+    .replace(/[\\/:*?"<>|\x00-\x1f\x7f]/g, "")
+    .replace(/\s+/g, " ")
+    .trim()
+    .slice(0, 100);
+}
+
+/** Lowercase ASCII slug of a title; never empty ("chapter" fallback). */
+export function slugify(s: string): string {
+  return s
+    .toLowerCase()
+    .replace(/[^a-z0-9]+/g, "-")
+    .replace(/^-+|-+$/g, "") || "chapter";
+}
+
+/** Fresh chapter id: `<slug>-<4 random chars>`. Guaranteed to satisfy
+ *  isSafeChapterId — keep it that way if you ever touch either half. */
+export function makeChapterId(title: string): string {
+  const rand = Math.random().toString(36).slice(2, 6);
+  return `${slugify(title)}-${rand}`;
+}
+
+/** The id contract every persisted chapter must satisfy: starts with an
+ *  alphanumeric, continues with alphanumerics or dashes. This is also what
+ *  keeps renderer-supplied snapshots from escaping the book directory. */
+const CHAPTER_ID_RE = /^[a-z0-9][a-z0-9-]*$/;
+
+export function isSafeChapterId(id: unknown): id is string {
+  return typeof id === "string" && CHAPTER_ID_RE.test(id);
+}
+
+/** Markdown filename for a chapter id (legacy/native Markdown storage). */
+export function chapterFileName(id: string): string {
+  return `${id}.md`;
+}

src/main/index.ts

@@ -37,6 +37,7 @@ import {
   getRecoveryRoot,
   setRecoveryRoot,
 } from "./backup.js";
+import { sanitizeBookFolderName } from "./ids.js";
 import { buildDocx, buildOdt } from "./office-export.js";
 import { getSetting, setSetting } from "./settings.js";
 import { migrateChapterToNative, migrateBookToNative } from "./migration.js";
@@ -187,14 +188,6 @@ async function openBookAt(p: string): Promise<BookResult | ErrResult> {
   }
 }
 
-function sanitizeName(name: string): string {
-  return name
-    .replace(/[\\/:*?"<>|\x00-\x1f\x7f]/g, "")
-    .replace(/\s+/g, " ")
-    .trim()
-    .slice(0, 100);
-}
-
 // Create a new book as a dedicated folder <name>/ inside the user's Documents
 // directory, then open it. The folder name is the sanitized book title; if a
 // folder with that name already exists we append a numeric suffix.
@@ -203,7 +196,7 @@ ipcMain.handle("folio:newBookNamed", async (_e, name: string) => {
   if (!title) return { error: "Please provide a book name." };
   try {
     const base = app.getPath("documents");
-    const folder = sanitizeName(title) || "Untitled";
+    const folder = sanitizeBookFolderName(title) || "Untitled";
     let target = path.join(base, folder);
     let n = 2;
     while (hasBook(target) || fs.existsSync(target)) {

src/main/project.ts

@@ -53,21 +53,15 @@ export const META_FILE = "folio.json";
 export const CHAPTERS_DIR = "chapters";
 export const CURRENT_VERSION = 1;
 
-export function slugify(s: string): string {
-  return s
-    .toLowerCase()
-    .replace(/[^a-z0-9]+/g, "-")
-    .replace(/^-+|-+$/g, "") || "chapter";
-}
-
-export function makeChapterId(title: string): string {
-  const rand = Math.random().toString(36).slice(2, 6);
-  return `${slugify(title) || "chapter"}-${rand}`;
-}
-
-export function chapterFileName(id: string): string {
-  return `${id}.md`;
-}
+// The id grammar moved to ids.ts (with its validator, so the contract cannot
+// drift). Re-exported here for existing importers and test bundles.
+export {
+  slugify,
+  makeChapterId,
+  chapterFileName,
+  isSafeChapterId,
+  sanitizeBookFolderName,
+} from "./ids.js";
 
 export function defaultMeta(title: string): BookMeta {
   return {