A live photo's default names fit, and no cut name starts with a dot

`defaultFileName` in `src/filename.ts` now gives the name a file is saved
under when the caller gives no path, and `originalName`, `thumbnailName`,
`downloadFile` and `downloadThumbnail` all call it. For a live photo's image
and video it puts the part's own extension in before cutting the name short,
so those names fit too; a path from `--out` or `outPath` is kept as given.
`shortenKeepingExtension` keeps the first character before the extension,
cutting a long extension short to leave room for it, so a cut name no longer
starts with a dot. The backup's links for such a title change to match.
TODO.md names the open bug and describes what the code does.

Model: opus-5-5

README.md

@@ -687,24 +687,26 @@ Without `--out`, `get` writes the file to the working directory under its title,
 and `get-thumb` writes the thumbnail there under `thumb_` followed by the title,
 each with unsafe characters replaced. A name over 255 bytes, which most file
 systems refuse, is cut short between two characters before its extension, so the
-extension is kept whenever it fits.
+extension is kept whenever it fits. An extension too long to leave room for the
+name's first character is cut short too, so the name never starts with a dot.
 
 `get` and `get-thumb` resolve the file by ID directly, so `--collection` is
 accepted for backward compatibility but ignored. For a live photo, `get` writes
 its image and its video, each named after the title with its own extension, as
-Ente's clients name them (`IMG_0001.heic` and `IMG_0001.mov`). With
+Ente's clients name them (`IMG_0001.heic` and `IMG_0001.mov`), and each name is
-`--out PATH`, the image is written to `PATH` and the video beside it, with
+cut short as above once its own extension is in it. With `--out PATH`, the image
-`PATH`'s name and the video's extension; a `PATH` with the video's extension is
+is written to `PATH` and the video beside it, with `PATH`'s name and the video's
-refused. `backup-metadata --exif` (alias `--all`) additionally fetches each
+extension; a `PATH` with the video's extension is refused.
-file's original through the cache and records, from it or a live photo's image,
+`backup-metadata --exif` (alias `--all`) additionally fetches each file's
-its XMP metadata, its EXIF metadata and, for a JPEG, its dimensions. EXIF is
+original through the cache and records, from it or a live photo's image, its XMP
-read with [exifreader](https://github.com/mattiasw/ExifReader) from any image
+metadata, its EXIF metadata and, for a JPEG, its dimensions. EXIF is read with
-format it reads, JPEG, HEIC/HEIF, AVIF, PNG and WebP among them. The record's
+[exifreader](https://github.com/mattiasw/ExifReader) from any image format it
-`exif` field is exifreader's EXIF tag output: each tag by name, with its
+reads, JPEG, HEIC/HEIF, AVIF, PNG and WebP among them. The record's `exif` field
-`value`, `description` and `computed` value. An EXIF block exifreader finds but
+is exifreader's EXIF tag output: each tag by name, with its `value`,
-reads no tag from is recorded, base64, as `exifRaw`, with the reason in
+`description` and `computed` value. An EXIF block exifreader finds but reads no
-`exifError`. `collections`, `files`, `backup`, `helper list-missing-thumbnails`
+tag from is recorded, base64, as `exifRaw`, with the reason in `exifError`.
-and `helper fix-missing-thumbnails` take `--json` for machine-readable output.
+`collections`, `files`, `backup`, `helper list-missing-thumbnails` and
+`helper fix-missing-thumbnails` take `--json` for machine-readable output.
 
 `backup --verify` also hashes the originals already in the backup and replaces
 any that do not match the content hash Ente records; one it cannot replace stays
@@ -864,11 +866,12 @@ gets its ID added as well: beside an album `x`, whose JSON is `x.json`, an album
 named `x.json` gets the directory `x.json (12)/`. A name too long for a file
 name, which most file systems limit to 255 bytes, gets its ID added too. A name
 with its ID added is cut short between two characters in the part before the ID,
-just far enough that it fits. When a symlink's extension alone leaves no room
+just far enough that it fits. When a symlink's extension leaves no room for the
-for the ID, the extension is cut short and the part before the ID keeps only the
+ID and the first character of the name, the extension is cut short, and the part
-bytes that cut leaves over. A collection's directory name also leaves room for
+before the ID keeps that character and any bytes the cut leaves over. A
-the `.json` its JSON adds. IDs never change, so a name stays the same from run
+collection's directory name also leaves room for the `.json` its JSON adds. IDs
-to run until such a clash appears or goes away.
+never change, so a name stays the same from run to run until such a clash
+appears or goes away.
 
 A collection whose directory or JSON cannot be written, such as one whose
 directory name is taken by a file you put in `collections/`, does not stop the

TODO.md

@@ -17,9 +17,11 @@ pre-1.0
 
 # Next Step
 
-No bug is open. The check of quak against a live Ente account,
+No bug is open. Two changes sneak asked for are open:
-https://git.eeqj.de/sneak/quak/issues/187, needs an account only sneak has, so
+https://git.eeqj.de/sneak/quak/issues/211 and
-it waits on him.
+https://git.eeqj.de/sneak/quak/issues/212. The check of quak against a live Ente
+account, https://git.eeqj.de/sneak/quak/issues/187, needs an account only sneak
+has, so it waits on him.
 
 Tagging and releases are decided by sneak alone, and happen only when he
 declares one.
@@ -27,13 +29,17 @@ declares one.
 # Completed Steps
 
 - 2026-10-18: Names quak builds from an album name or a title fit in a file name
-  (issue 205). `shortenKeepingExtension` in `src/filename.ts` cuts a name short
+  (https://git.eeqj.de/sneak/quak/issues/205). `shortenKeepingExtension` in
-  to a number of bytes, keeping its extension when it fits, and can put text
+  `src/filename.ts` cuts a name short to a number of bytes, keeping its
-  right before the extension; the backup's `withID` uses it for an ID. The
+  extension when it fits and always its first character, and can put text right
-  default names of `quak get`, `quak get-thumb`, `downloadFile` and
+  before the extension; the backup's `withID` uses it for an ID.
-  `downloadThumbnail` are cut short with it to 255 bytes, `thumb_` included.
+  `defaultFileName` there gives the name a file is saved under when the caller
-  `backup-metadata` cuts an album's name short so that its folder,
+  gives no path: a prefix such as `thumb_` and the title with unsafe characters
-  `<id>-<name>`, fits in 255 bytes. `safeExtension` gives `.bin` for an
+  replaced, cut short with it to 255 bytes, and, for a live photo's image or
+  video, with that part's own extension before the cut. `quak get`,
+  `quak get-thumb`, `downloadFile` and `downloadThumbnail` use it without
+  `--out` or `outPath`. `backup-metadata` cuts an album's name short so that its
+  folder, `<id>-<name>`, fits in 255 bytes. `safeExtension` gives `.bin` for an
   extension of more than 16 letters and digits, so a save path's name always
   fits.
 

src/backup.ts

@@ -290,9 +290,10 @@ const rebuildSymlink = (linkPath: string, target: string): void => {
 // case (two names that differ only in case are one entry on a case-insensitive
 // file system), or it is too long for a file name; then it gets ` (<id>)`,
 // right before the extension when `beforeExtension` is set. To fit, the part
-// before the ID is cut short as far as it must be. When the extension alone
+// before the ID is cut short as far as it must be. When the extension leaves
-// leaves no room for the ID, the extension is cut short and the part before
+// no room for the ID and the first character of the name, the extension is
-// the ID keeps only the bytes that cut leaves over. With `withJSON`, each
+// cut short, and the part before the ID keeps that character and any bytes
+// the cut leaves over (see `shortenKeepingExtension`). With `withJSON`, each
 // entry also has the JSON file `<name>.json` beside it, as an album does: a
 // name then leaves room for `.json`, and one that is another entry's JSON name
 // gets its ID added too.

src/cli-commands.ts

@@ -332,8 +332,8 @@ export const getCommand = async (
         const result = await photo.original();
         // Default name is the file's own title, as the pre-library CLI used
         // (not the editedName-preferring projection title) (issue #52).
-        const outPath = opts.out ?? originalName(file);
         if (result.videoPath === undefined) {
+            const outPath = opts.out ?? originalName(file);
             copyFileSync(result.path, outPath);
             ctx.stderr.write(`${result.bytes} bytes -> ${outPath}\n`);
             return 0;
@@ -341,9 +341,13 @@ export const getCommand = async (
         // A live photo is written as its image and its video, each named after
         // the title with its own extension, as Ente's clients name them. With
         // --out, the image goes there and the video beside it.
-        const imageOut =
+        const imageExt = extname(result.path);
-            opts.out ?? withExtension(outPath, extname(result.path));
+        const videoExt = extname(result.videoPath);
-        const videoOut = withExtension(outPath, extname(result.videoPath));
+        const imageOut = opts.out ?? originalName(file, imageExt);
+        const videoOut =
+            opts.out === undefined
+                ? originalName(file, videoExt)
+                : withExtension(opts.out, videoExt);
         if (imageOut.toLowerCase() === videoOut.toLowerCase()) {
             ctx.stderr.write(
                 `File ${fileID} is a live photo, and its video would also be written to ${imageOut}\n`,

src/cli-output.ts

@@ -9,11 +9,7 @@
 // `metadata.title`, and issue #52 requires that output stay byte-identical, so
 // the commands shape their output from the raw `EnteFile` through here.
 
-import {
+import { defaultFileName } from "./filename.js";
-    MAX_NAME_BYTES,
-    sanitizeFileName,
-    shortenKeepingExtension,
-} from "./filename.js";
 import type { EnteFile, FileType, Microseconds } from "./model/types.js";
 
 // One row of `quak files --json`.
@@ -37,20 +33,13 @@ export const fileListRow = (file: EnteFile): FileListRow => ({
 export const fileListLine = (file: EnteFile): string =>
     `${file.id}\t${file.metadata.fileType}\t${file.metadata.title}`;
 
-// Default output path for `quak get` when `--out` is not given. The title comes
+// Default output path for `quak get` when `--out` is not given, made from the
-// from the server, so it is sanitized, and cut short to fit in a file name,
+// title (see `defaultFileName`), with `ext` in place of its extension for a
-// keeping its extension; `--out` is the user's and is used as is.
+// live photo's image or video; `--out` is the user's and is used as is.
-export const originalName = (file: EnteFile): string =>
+export const originalName = (file: EnteFile, ext?: string): string =>
-    shortenKeepingExtension(
+    defaultFileName(file.metadata.title, file.id, { ext });
-        sanitizeFileName(file.metadata.title, `file-${file.id}`),
-        MAX_NAME_BYTES,
-    );
 
 // Default output path for `quak get-thumb` when `--out` is not given: `thumb_`
-// and the title, cut short so that the two fit in a file name.
+// and the title.
 export const thumbnailName = (file: EnteFile): string =>
-    "thumb_" +
+    defaultFileName(file.metadata.title, file.id, { prefix: "thumb_" });
-    shortenKeepingExtension(
-        sanitizeFileName(file.metadata.title, `file-${file.id}`),
-        MAX_NAME_BYTES - "thumb_".length,
-    );

src/download/index.ts

@@ -16,13 +16,7 @@ import {
     streamTagFinal,
 } from "../crypto/index.js";
 import { TruncatedStreamError } from "../errors.js";
-import {
+import { defaultFileName, safeExtension, withExtension } from "../filename.js";
-    MAX_NAME_BYTES,
-    safeExtension,
-    sanitizeFileName,
-    shortenKeepingExtension,
-    withExtension,
-} from "../filename.js";
 import { withRetry } from "../retry.js";
 import type { ApiClient } from "../api/client.js";
 import type { EnteFile } from "../model/types.js";
@@ -371,10 +365,14 @@ const openPart = async (
 // name starts with `image` as the image and the one whose name starts with
 // `video` as the video, and refuses a ZIP holding a second of either. It is
 // written unpacked: each part is named `destination` with the extension
-// replaced by its own entry's, and the two must differ ignoring case. When the
+// replaced by its own entry's, and the two must differ ignoring case. When
-// file records a hash, `<imageHash>:<videoHash>` must match it, each over that
+// `defaultName` is set, `destination` is the name `defaultFileName` made from
-// part's own bytes. Only then are the image, then the video, renamed into
+// the title rather than a path the caller gave, and each part is named as
-// place; on any failure neither is stored.
+// `defaultFileName` names it with its own extension, so that the name is cut
+// short to fit with that extension. When the file records a hash,
+// `<imageHash>:<videoHash>` must match it, each over that part's own bytes.
+// Only then are the image, then the video, renamed into place; on any failure
+// neither is stored.
 //
 // The ZIP is chosen by its uploader and may expand enormously, so each part is
 // written as it decompresses and never held, and the ZIP is refused once the
@@ -394,6 +392,7 @@ const decryptLivePhoto = async (
     key: Uint8Array,
     onProgress: ProgressCallback | undefined,
     file: EnteFile,
+    defaultName: boolean,
 ): Promise<DownloadResult> => {
     const sliceSize = 4096;
     const dir = dirname(destination);
@@ -492,8 +491,12 @@ const decryptLivePhoto = async (
                 `live photo's image and video have the same extension, ${video.ext}`,
             );
         }
-        const path = withExtension(destination, image.ext);
+        const partPath = (ext: string): string =>
-        const videoPath = withExtension(destination, video.ext);
+            defaultName
+                ? defaultFileName(file.metadata.title, file.id, { ext })
+                : withExtension(destination, ext);
+        const path = partPath(image.ext);
+        const videoPath = partPath(video.ext);
         for (const part of parts) {
             await part.handle.sync();
             await part.handle.close();
@@ -537,6 +540,9 @@ const decryptLivePhoto = async (
 // and then removes its own temp file, and only the attempt that reaches
 // TAG_FINAL renames one into place, so a download that needed three tries still
 // performs exactly one rename over the destination.
+//
+// `defaultName` says `destination` is the name `defaultFileName` made, not a
+// path the caller gave (see `decryptLivePhoto`).
 const fetchAndDecrypt = async (
     api: ApiClient,
     openStream: () => Promise<ReadableStream<Uint8Array>>,
@@ -545,6 +551,7 @@ const fetchAndDecrypt = async (
     destination: string,
     onProgress?: ProgressCallback,
     original?: EnteFile,
+    defaultName = false,
 ): Promise<DownloadResult> =>
     withRetry(async () => {
         const stream = await openStream();
@@ -557,6 +564,7 @@ const fetchAndDecrypt = async (
                     key,
                     onProgress,
                     original,
+                    defaultName,
                 );
             }
             const bytesWritten = await decryptToTemp(
@@ -588,15 +596,11 @@ export const downloadFile = async (
     outPath?: string,
     onProgress?: ProgressCallback,
 ): Promise<DownloadResult> => {
-    // `outPath` is the caller's and is used as is; the title is the server's
+    // `outPath` is the caller's and is used as is. Without it, the file is
-    // and is sanitized so it can only name a file in the current directory,
+    // written in the current directory under the name `defaultFileName` makes
-    // and cut short to fit in a file name, keeping its extension.
+    // from the title, which is the server's.
     const resolvedPath =
-        outPath ??
+        outPath ?? defaultFileName(file.metadata.title, file.id);
-        shortenKeepingExtension(
-            sanitizeFileName(file.metadata.title, `file-${file.id}`),
-            MAX_NAME_BYTES,
-        );
     const header = fromBase64(file.file.decryptionHeader);
     return fetchAndDecrypt(
         api,
@@ -606,6 +610,7 @@ export const downloadFile = async (
         resolvedPath,
         onProgress,
         file,
+        outPath === undefined,
     );
 };
 
@@ -618,11 +623,7 @@ export const downloadThumbnail = async (
     // As in `downloadFile`, with `thumb_` before the title.
     const resolvedPath =
         outPath ??
-        "thumb_" +
+        defaultFileName(file.metadata.title, file.id, { prefix: "thumb_" });
-            shortenKeepingExtension(
-                sanitizeFileName(file.metadata.title, `file-${file.id}`),
-                MAX_NAME_BYTES - "thumb_".length,
-            );
     const header = fromBase64(file.thumbnail.decryptionHeader);
     return fetchAndDecrypt(
         api,

src/filename.ts

@@ -6,6 +6,8 @@
 // directory the caller chose: never a path, never `..`, never hidden, never a
 // Windows device name. Such a name can still be too long for a file name, and
 // `shortenToBytes` or `shortenKeepingExtension` cuts one short.
+// `defaultFileName` does both for the name a file is saved under when the
+// caller gives no path.
 //
 // A path the user typed (`--out`, `outPath`) is not passed through here: the
 // caller is trusted, the server is not.
@@ -61,10 +63,12 @@ export const shortenToBytes = (name: string, maxBytes: number): string => {
 // `name` cut short to fit in `maxBytes` bytes with `insert` put right before
 // its extension (`IMG_0001 (12).jpg`), each cut made as `shortenToBytes`
 // makes it. The part before the extension is cut first, so the extension is
-// kept whenever it fits. When the extension alone leaves no room for
+// kept whenever it fits, but that part always keeps its first character:
-// `insert`, the extension is cut short too, and the part before it keeps only
+// when the extension leaves no room for `insert` and that character, the
-// the bytes that cut leaves over. `insert` is never cut. With no `insert`, a
+// extension is cut short too, and the part before it keeps the bytes that cut
-// name that fits comes back unchanged.
+// leaves over. So a name that does not start with a dot never comes back
+// starting with one, which would hide it. `insert` is never cut. With no
+// `insert`, a name that fits comes back unchanged.
 export const shortenKeepingExtension = (
     name: string,
     maxBytes: number,
@@ -72,7 +76,36 @@ export const shortenKeepingExtension = (
 ): string => {
     const fullExt = extname(name);
     const stem = name.slice(0, name.length - fullExt.length);
-    const ext = shortenToBytes(fullExt, maxBytes - Buffer.byteLength(insert));
+    const first = Array.from(stem)[0] ?? "";
+    const ext = shortenToBytes(
+        fullExt,
+        maxBytes - Buffer.byteLength(insert + first),
+    );
     const room = maxBytes - Buffer.byteLength(insert + ext);
     return shortenToBytes(stem, room) + insert + ext;
 };
+
+// The name a file is saved under in the working directory when the caller
+// gives no path (`quak get` without `--out`, `downloadFile` without
+// `outPath`): `prefix` (`thumb_` for a thumbnail), then the file's title with
+// unsafe characters replaced, or `file-<id>` when the title is empty, cut
+// short so that the whole fits in a file name, keeping the title's extension
+// whenever it fits. `ext` (".mov"), when given, takes the place of the
+// title's extension before the name is cut short; a live photo's image and
+// video are each named with their own.
+export const defaultFileName = (
+    title: string,
+    fileID: number,
+    opts: { prefix?: string; ext?: string } = {},
+): string => {
+    const { prefix = "", ext } = opts;
+    const safe = sanitizeFileName(title, `file-${fileID}`);
+    const name = ext === undefined ? safe : withExtension(safe, ext);
+    return (
+        prefix +
+        shortenKeepingExtension(
+            name,
+            MAX_NAME_BYTES - Buffer.byteLength(prefix),
+        )
+    );
+};

test/cli/backup.test.ts

@@ -1795,12 +1795,11 @@ describe("backup album folders", () => {
         expect(readLedger(outDir).files["5"]).toMatchObject({ attempts: 1 });
     });
 
-    // The extensions of these titles leave no room for the ID, so each is cut
+    // The extensions of these titles leave no room for the ID and the "a"
-    // short and the name before the ID keeps only the bytes that cut leaves
+    // before them, so each is cut short to 250 bytes, which leaves one, for
-    // over. File 1's extension is 303 bytes and is cut to 251, which leaves
+    // the "a". File 1's extension is 303 bytes and file 2's 271. Neither
-    // none. File 2's is 271 bytes and is cut to 250, since "€" takes three
+    // extension is only letters and digits, so both originals are saved with
-    // bytes, which leaves one, for the "a". Neither extension is only letters
+    // `.bin`, a name that fits.
-    // and digits, so both originals are saved with `.bin`, a name that fits.
     it("cuts the extension of a title short when it leaves no room for the ID", async () => {
         const outDir = join(root, "backup");
         const lib = libraryOf([
@@ -1815,7 +1814,7 @@ describe("backup album folders", () => {
 
         const result = await runBackup(lib, { downloadDirectory: outDir });
 
-        const link = ` (1).${"L".repeat(250)}`;
+        const link = `a (1).${"L".repeat(249)}`;
         const link2 = `a (2).${"€".repeat(83)}`;
         expect(Buffer.byteLength(link)).toBe(255);
         expect(Buffer.byteLength(link2)).toBe(255);
@@ -1830,9 +1829,9 @@ describe("backup album folders", () => {
 
     // Files 1 and 3 share a title, so each gets its ID before the
     // extension. File 2's title is too long, and its extension leaves no
-    // room for the ID. Were the ID put at the end of file 2's title cut
+    // room for the ID and the "x" before it. Were the ID put at the end of
-    // short, its link would have file 1's name, and the link made second
+    // file 2's title cut short, its link would have file 1's name, and the
-    // would replace the first.
+    // link made second would replace the first.
     it("keeps names unique when a title's extension leaves no room for the ID", async () => {
         const outDir = join(root, "backup");
         const shared = `x.${"Q".repeat(245)} (2)`;
@@ -1852,8 +1851,8 @@ describe("backup album folders", () => {
         expect(result.failed).toBe(0);
         expect(tree(outDir)).toEqual([
             "Trip/",
-            `Trip/ (2).${"Q".repeat(250)} -> ${linkTo("2.bin")}`,
             `Trip/x (1).${"Q".repeat(245)} (2) -> ${linkTo("1.bin")}`,
+            `Trip/x (2).${"Q".repeat(249)} -> ${linkTo("2.bin")}`,
             `Trip/x (3).${"Q".repeat(245)} (2) -> ${linkTo("3.bin")}`,
             "Trip.json",
         ]);

test/cli/commands.test.ts

@@ -187,6 +187,9 @@ const FETCHED_BOTH_ALBUMS =
     "Fetched the files of 1 of 2 collections\n" +
     "Fetched the files of 2 of 2 collections\n";
 
+// The same for an account with one album.
+const FETCHED_ONE_ALBUM = "Fetched the files of 1 of 1 collections\n";
+
 // Collects everything written to it.
 class Output {
     text = "";
@@ -605,7 +608,7 @@ describe("names from an album name or a title over 255 bytes", () => {
         expect(Buffer.byteLength(name)).toBe(255);
         expect(code).toBe(0);
         expect(readdirSync(dir)).toEqual([name]);
-        expect(stderr.text).toBe(`7 bytes -> ${name}\n`);
+        expect(stderr.text).toBe(FETCHED_ONE_ALBUM + `7 bytes -> ${name}\n`);
     });
 
     it("get-thumb cuts the title short so that thumb_ and it fit in 255 bytes, and exits 0", async () => {
@@ -617,7 +620,7 @@ describe("names from an album name or a title over 255 bytes", () => {
         expect(Buffer.byteLength(name)).toBe(255);
         expect(code).toBe(0);
         expect(readdirSync(dir)).toEqual([name]);
-        expect(stderr.text).toBe(`3 bytes -> ${name}\n`);
+        expect(stderr.text).toBe(FETCHED_ONE_ALBUM + `3 bytes -> ${name}\n`);
     });
 
     it("backup-metadata cuts the album name short so that its folder's name fits in 255 bytes, and exits 0", async () => {
@@ -659,12 +662,12 @@ describe("names from an album name or a title over 255 bytes", () => {
 // one live photo, 300, downloaded through the real download layer
 // (test/live-photo.ts).
 describe("a live photo", () => {
-    // What `get` prints on stderr while its refresh fetches Vacation's files.
+    const livePhotoClient = async (
-    const FETCHED_ONE_ALBUM = "Fetched the files of 1 of 1 collections\n";
+        image = IMAGE,
-
+        title = "IMG_0300.HEIC",
-    const livePhotoClient = async (image = IMAGE): Promise<Client> => {
+    ): Promise<Client> => {
         const { file: live, body } = await asLivePhoto(
-            file(300, 1, "IMG_0300.HEIC"),
+            file(300, 1, title),
             livePhotoZip({ "image.heic": image, "video.mov": VIDEO }),
             livePhotoHash(image, VIDEO),
         );
@@ -712,6 +715,44 @@ describe("a live photo", () => {
         );
     });
 
+    // Run `get` without `--out` in a new directory for a live photo titled
+    // `title`, and return its exit code and the names it wrote.
+    const getInNewDirectory = async (
+        title: string,
+    ): Promise<{ code: number; names: string[] }> => {
+        const ctx = context(await livePhotoClient(IMAGE, title));
+        const dir = mkdtempSync(join(root, "cwd-"));
+        const previous = process.cwd();
+        process.chdir(dir);
+        try {
+            const code = await getCommand(ctx, "300", {});
+            return { code, names: readdirSync(dir).sort() };
+        } finally {
+            process.chdir(previous);
+        }
+    };
+    const imageName = `${"t".repeat(250)}.heic`;
+    const videoName = `${"t".repeat(251)}.mov`;
+
+    // The title fits, but ".heic" is one byte longer than ".jpg", so the
+    // image's name is cut short once its extension is in it.
+    it("get cuts its image's name short for a 255-byte title ending in .jpg, and exits 0", async () => {
+        const title = `${"t".repeat(251)}.jpg`;
+
+        const result = await getInNewDirectory(title);
+
+        expect(Buffer.byteLength(title)).toBe(255);
+        expect(Buffer.byteLength(imageName)).toBe(255);
+        expect(Buffer.byteLength(videoName)).toBe(255);
+        expect(result).toEqual({ code: 0, names: [imageName, videoName] });
+    });
+
+    it("get cuts the names of its image and video short for a 300-byte title with no extension, and exits 0", async () => {
+        const result = await getInNewDirectory("t".repeat(300));
+
+        expect(result).toEqual({ code: 0, names: [imageName, videoName] });
+    });
+
     it("get --out writes the image there and the video beside it", async () => {
         const out = join(root, "photo.jpg");
         const video = join(root, "photo.mov");

test/download/download.test.ts

@@ -1923,6 +1923,53 @@ describe("downloadFile live photos", () => {
         ]);
     });
 
+    // A file system refuses a file name over 255 bytes. Each name is cut
+    // short once its own extension is in it.
+    const imageName = `${"t".repeat(250)}.heic`;
+    const videoName = `${"t".repeat(251)}.mov`;
+
+    // The title fits, but ".heic" is one byte longer than ".jpg".
+    it("cuts the image's name short for a 255-byte title ending in .jpg when no outPath is given", async () => {
+        const title = `${"t".repeat(251)}.jpg`;
+        const t = setup(livePhotoZip(), { ...livePhoto, title });
+
+        const result = await inDirectory(t.dir, () =>
+            downloadFile(t.api, t.file),
+        );
+
+        expect(Buffer.byteLength(title)).toBe(255);
+        expect(Buffer.byteLength(imageName)).toBe(255);
+        expect(Buffer.byteLength(videoName)).toBe(255);
+        expect(result.path).toBe(imageName);
+        expect(result.videoPath).toBe(videoName);
+        expect(readdirSync(t.dir).sort()).toEqual([imageName, videoName]);
+    });
+
+    it("cuts the names of the image and the video short for a 300-byte title with no extension when no outPath is given", async () => {
+        const t = setup(livePhotoZip(), {
+            ...livePhoto,
+            title: "t".repeat(300),
+        });
+
+        const result = await inDirectory(t.dir, () =>
+            downloadFile(t.api, t.file),
+        );
+
+        expect(result.path).toBe(imageName);
+        expect(result.videoPath).toBe(videoName);
+        expect(readdirSync(t.dir).sort()).toEqual([imageName, videoName]);
+    });
+
+    // The caller's path is used as is; only the extension is each part's own.
+    it("keeps an outPath too long for a file name as given", async () => {
+        const t = setup(livePhotoZip(), livePhoto);
+        const outPath = join(t.dir, `${"t".repeat(251)}.jpg`);
+
+        await expect(downloadFile(t.api, t.file, outPath)).rejects.toThrow(
+            /ENAMETOOLONG/,
+        );
+    });
+
     it("gives each part its own extension from the ZIP, letters and digits only", async () => {
         const zip = livePhotoZip({ "image.JPG": IMAGE, "video.m-4v": VIDEO });
         const t = setup(zip, livePhoto);

test/filename/filename.test.ts

@@ -15,6 +15,7 @@
 import { describe, expect, it } from "vitest";
 
 import {
+    defaultFileName,
     safeExtension,
     sanitizeFileName,
     shortenKeepingExtension,
@@ -141,9 +142,17 @@ describe("shortenKeepingExtension", () => {
         );
     });
 
-    it("cuts the extension short when it alone does not fit", () => {
+    // A name cut down to its extension would start with a dot, and so be
+    // hidden.
+    it("keeps the first character before the extension, cutting the extension short to leave room for it", () => {
         expect(shortenKeepingExtension(`b.${"e".repeat(300)}`, 255)).toBe(
-            `.${"e".repeat(254)}`,
+            `b.${"e".repeat(253)}`,
+        );
+        expect(shortenKeepingExtension(`ab.${"e".repeat(300)}`, 255)).toBe(
+            `a.${"e".repeat(253)}`,
+        );
+        expect(shortenKeepingExtension(`é.${"e".repeat(300)}`, 255)).toBe(
+            `é.${"e".repeat(252)}`,
         );
         // "€" takes three bytes, so the extension is cut to 253 and the
         // part before it keeps the two bytes left over, for the "a".
@@ -161,6 +170,50 @@ describe("shortenKeepingExtension", () => {
         ).toBe(`${"t".repeat(247)} (7).jpg`);
         expect(
             shortenKeepingExtension(`b.${"e".repeat(300)}`, 255, " (7)"),
-        ).toBe(` (7).${"e".repeat(250)}`);
+        ).toBe(`b (7).${"e".repeat(249)}`);
+    });
+});
+
+describe("defaultFileName", () => {
+    it("is the title when it is safe and fits", () => {
+        expect(defaultFileName("IMG_0001.HEIC", 42)).toBe("IMG_0001.HEIC");
+    });
+
+    it("replaces unsafe characters, and is file-<id> for an empty title", () => {
+        expect(defaultFileName("../x.jpg", 42)).toBe("__x.jpg");
+        expect(defaultFileName("", 42)).toBe("file-42");
+    });
+
+    it("cuts a title over 255 bytes short, keeping its extension", () => {
+        expect(defaultFileName(`${"t".repeat(296)}.jpg`, 42)).toBe(
+            `${"t".repeat(251)}.jpg`,
+        );
+        expect(defaultFileName(`b.${"e".repeat(300)}`, 42)).toBe(
+            `b.${"e".repeat(253)}`,
+        );
+    });
+
+    it("puts `prefix` before the title and leaves room for it", () => {
+        expect(defaultFileName("beach.jpg", 42, { prefix: "thumb_" })).toBe(
+            "thumb_beach.jpg",
+        );
+        expect(
+            defaultFileName(`${"t".repeat(296)}.jpg`, 42, { prefix: "thumb_" }),
+        ).toBe(`thumb_${"t".repeat(245)}.jpg`);
+    });
+
+    // A live photo's image and video are each named with their own
+    // extension, and the name is cut short once that extension is in it.
+    it("puts `ext` in place of the title's extension before cutting the name short", () => {
+        expect(defaultFileName("IMG_0001.HEIC", 42, { ext: ".mov" })).toBe(
+            "IMG_0001.mov",
+        );
+        expect(defaultFileName("", 42, { ext: ".mov" })).toBe("file-42.mov");
+        expect(
+            defaultFileName(`${"t".repeat(251)}.jpg`, 42, { ext: ".heic" }),
+        ).toBe(`${"t".repeat(250)}.heic`);
+        expect(defaultFileName("t".repeat(300), 42, { ext: ".heic" })).toBe(
+            `${"t".repeat(250)}.heic`,
+        );
     });
 });