Names quak builds from server data fit in 255 bytes (closes #205)

The default names of `quak get`, `quak get-thumb`, `downloadFile` and
`downloadThumbnail`, and the album folders of `backup-metadata`, could be
over 255 bytes, which a file system refuses. `shortenKeepingExtension` in
`src/filename.ts` cuts a name short, keeping its extension when it fits,
and can put text right before the extension; the backup's `withID` now
calls it instead of doing the same itself. The `get` defaults use it with
`thumb_` counted. A `backup-metadata` folder keeps its `<id>-` and has the
name after it cut short. `safeExtension` gives `.bin` for an extension of
more than 16 letters and digits, so a save path's name always fits.

Model: opus-5-5

test/filename/filename.test.ts

@@ -17,6 +17,7 @@ import { describe, expect, it } from "vitest";
 import {
     safeExtension,
     sanitizeFileName,
+    shortenKeepingExtension,
     shortenToBytes,
 } from "../../src/filename.js";
 
@@ -94,6 +95,12 @@ describe("safeExtension", () => {
         expect(safeExtension("x.jp g")).toBe(".bin");
         expect(safeExtension("x.jpg\0")).toBe(".bin");
     });
+
+    it("keeps an extension of 16 letters and digits, and uses .bin for a longer one", () => {
+        expect(safeExtension(`x.${"e".repeat(16)}`)).toBe(`.${"e".repeat(16)}`);
+        expect(safeExtension(`x.${"e".repeat(17)}`)).toBe(".bin");
+        expect(safeExtension(`b.${"e".repeat(300)}`)).toBe(".bin");
+    });
 });
 
 describe("shortenToBytes", () => {
@@ -118,3 +125,42 @@ describe("shortenToBytes", () => {
         );
     });
 });
+
+describe("shortenKeepingExtension", () => {
+    it("returns a name that fits unchanged", () => {
+        expect(shortenKeepingExtension("beach.jpg", 9)).toBe("beach.jpg");
+        expect(shortenKeepingExtension("beach", 5)).toBe("beach");
+    });
+
+    it("cuts the part before the extension short and keeps the extension", () => {
+        expect(shortenKeepingExtension(`${"t".repeat(296)}.jpg`, 255)).toBe(
+            `${"t".repeat(251)}.jpg`,
+        );
+        expect(shortenKeepingExtension("é".repeat(150), 255)).toBe(
+            "é".repeat(127),
+        );
+    });
+
+    it("cuts the extension short when it alone does not fit", () => {
+        expect(shortenKeepingExtension(`b.${"e".repeat(300)}`, 255)).toBe(
+            `.${"e".repeat(254)}`,
+        );
+        // "€" takes three bytes, so the extension is cut to 253 and the
+        // part before it keeps the two bytes left over, for the "a".
+        expect(shortenKeepingExtension(`a.${"€".repeat(90)}`, 255)).toBe(
+            `a.${"€".repeat(84)}`,
+        );
+    });
+
+    it("puts `insert` right before the extension and leaves room for it", () => {
+        expect(shortenKeepingExtension("beach.jpg", 255, " (7)")).toBe(
+            "beach (7).jpg",
+        );
+        expect(
+            shortenKeepingExtension(`${"t".repeat(296)}.jpg`, 255, " (7)"),
+        ).toBe(`${"t".repeat(247)} (7).jpg`);
+        expect(
+            shortenKeepingExtension(`b.${"e".repeat(300)}`, 255, " (7)"),
+        ).toBe(` (7).${"e".repeat(250)}`);
+    });
+});