Avoid redundant path obfuscation hash stretching

src/string_and_binary/path.ts

@@ -82,12 +82,8 @@ export function expandDocumentIDPrefix(id: DocumentID): [string, FilePathWithPre
 const _hashString = memorizeFuncWithLRUCache(async (key: string) => {
     const buff = writeString(key);
     const webcrypto = await getWebCrypto();
-    let digest = await webcrypto.subtle.digest("SHA-256", buff);
-    const len = key.length;
-    for (let i = 0; i < len; i++) {
-        // Stretching
-        digest = await webcrypto.subtle.digest("SHA-256", buff);
-    }
+    // Derive the obfuscated document ID from this input.
+    const digest = await webcrypto.subtle.digest("SHA-256", buff);
     return uint8ArrayToHexString(new Uint8Array(digest));
 });
 

src/string_and_binary/path.unit.spec.ts

@@ -1,6 +1,7 @@
 import { beforeEach, describe, expect, it, vi } from "vitest";
 
 const minimatchStats = vi.hoisted(() => ({ constructions: 0 }));
+const webCryptoStats = vi.hoisted(() => ({ digests: 0 }));
 
 vi.mock("minimatch", async (importOriginal) => {
     const actual = await importOriginal<typeof import("minimatch")>();
@@ -18,6 +19,24 @@ vi.mock("minimatch", async (importOriginal) => {
     };
 });
 
+vi.mock("@lib/mods.ts", async (importOriginal) => {
+    const actual = await importOriginal<typeof import("@lib/mods.ts")>();
+    const webcrypto = await actual.getWebCrypto();
+    const countedWebCrypto = {
+        subtle: {
+            digest: (algorithm: AlgorithmIdentifier, data: BufferSource) => {
+                webCryptoStats.digests++;
+                return webcrypto.subtle.digest(algorithm, data);
+            },
+        },
+    } as Crypto;
+
+    return {
+        ...actual,
+        getWebCrypto: async () => countedWebCrypto,
+    };
+});
+
 import type { FilePath } from "@lib/common/types";
 import { isAccepted, path2id_base } from "./path";
 
@@ -37,6 +56,28 @@ describe("path2id_base path case", () => {
     });
 });
 
+describe("path2id_base path obfuscation", () => {
+    it("performs one digest for each uncached hash input", async () => {
+        webCryptoStats.digests = 0;
+        const passphrase = "digest-count-regression-secret";
+
+        await path2id_base("First.md" as FilePath, passphrase, false);
+        expect(webCryptoStats.digests).toBe(2);
+
+        await path2id_base("Second.md" as FilePath, passphrase, false);
+        expect(webCryptoStats.digests).toBe(3);
+    });
+
+    it.each([
+        ["資料/概要.md", true, "f:d17ef57666777963bdb6875c83e16b39fee9ca7f7c3593f1b50af691c7bc2fa8"],
+        ["_private/Calculus.md", false, "f:2a18fa03d734284a8ace4d3c0a7b65558209b8f092ae36be1fdfc3626bc99268"],
+    ])("keeps the established document ID for %s", async (path, caseInsensitive, expected) => {
+        await expect(
+            path2id_base(path as FilePath, "path-obfuscation-regression-secret", caseInsensitive)
+        ).resolves.toBe(expected);
+    });
+});
+
 describe("isAccepted matcher cache", () => {
     beforeEach(() => {
         minimatchStats.constructions = 0;