perf: shorten chunk hashes from 64 chars to 12 chars

packages/reflect-yjs/src/chunk-config.ts

@@ -0,0 +1,24 @@
+import * as base64 from 'base64-js';
+
+export const AVG_CHUNK_SIZE_B = 1024;
+export const MIN_CHUNK_SIZE_B = 256;
+export const MAX_CHUNK_SIZE_B = 2048;
+
+export const hashFn = async (chunk: Uint8Array) => {
+  const hashBuffer = await crypto.subtle.digest('SHA-256', chunk);
+  // Truncate the sha-256 hash from 32 bytes to 9 bytes.  This gives us
+  // plenty of collision resistance for the range of expected document sizes.
+  // If we assume a max document size of 100MB, the probability of having a
+  // hash collision in a document of this size is roughly 1 in a trillion
+  // (based on the approximation function p(n) = n^2 / (2H)
+  // from
+  // https://en.wikipedia.org/wiki/Birthday_attack#Simple_approximation),
+  // where p(n) is probability of collision, n is number of hashes, and H
+  // is number of possible hash outputs.  For 100MB document we have
+  // n = 100,000 (100,000 1KB chunks) and H = 2^(8*9),
+  // p(100,000) = (100,000^2 / (2*2^72)) = 1.0587912e-12
+  //
+  // In base64 9 bytes will encode to 12 chars with no padding (all chars
+  // contain information).
+  return base64.fromByteArray(new Uint8Array(hashBuffer.slice(0, 9)));
+};

packages/reflect-yjs/src/chunk.test.ts

@@ -2,19 +2,32 @@ import {expect, suite, test} from 'vitest';
 import {chunk, unchunk} from './chunk.js';
 import {TEST_TEXT_LEAR} from './chunk-test-text-lear.js';
 import {TEST_TEXT_MOBY_DICK} from './chunk-test-text-moby-dick.js';
+import {hashFn} from './chunk-config.js';
 
 suite('chunk', () => {
   test('chunk is pure', async () => {
     const source = new TextEncoder().encode(TEST_TEXT_LEAR);
     const {
       chunksByHash: chunksByHash1,
       sourceAsChunkHashes: sourceAsChunkHashes1,
-    } = await chunk(Math.pow(2, 10), Math.pow(2, 9), Math.pow(2, 11), source);
+    } = await chunk(
+      Math.pow(2, 10),
+      Math.pow(2, 9),
+      Math.pow(2, 11),
+      source,
+      hashFn,
+    );
 
     const {
       chunksByHash: chunksByHash2,
       sourceAsChunkHashes: sourceAsChunkHashes2,
-    } = await chunk(Math.pow(2, 10), Math.pow(2, 9), Math.pow(2, 11), source);
+    } = await chunk(
+      Math.pow(2, 10),
+      Math.pow(2, 9),
+      Math.pow(2, 11),
+      source,
+      hashFn,
+    );
 
     expect(chunksByHash1).toEqual(chunksByHash2);
     expect(sourceAsChunkHashes1).toEqual(sourceAsChunkHashes2);
@@ -33,6 +46,7 @@ suite('chunk', () => {
         min,
         max,
         source,
+        hashFn,
       );
 
       let totalExcludingLast = 0;

packages/reflect-yjs/src/chunk.ts

@@ -109,6 +109,7 @@ export async function chunk(
   minimum: number,
   maximum: number,
   source: Uint8Array,
+  hashFn: (chunk: Uint8Array) => Promise<string>,
 ): Promise<{
   chunksByHash: Map<string, Uint8Array>;
   sourceAsChunkHashes: string[];
@@ -159,10 +160,7 @@ export async function chunk(
     }
 
     const chunk = source.slice(sourceOffset, sourceOffset + chunkSize);
-    const hashBuffer = await crypto.subtle.digest('SHA-256', chunk);
-    const hash = Array.from(new Uint8Array(hashBuffer))
-      .map(b => b.toString(16).padStart(2, '0'))
-      .join('');
+    const hash = await hashFn(chunk);
     if (!chunksByHash.has(hash)) {
       chunksByHash.set(hash, chunk);
     }

packages/reflect-yjs/src/mutators.ts

@@ -8,6 +8,12 @@ import type {
 import * as base64 from 'base64-js';
 import * as Y from 'yjs';
 import {chunk, unchunk} from './chunk.js';
+import {
+  AVG_CHUNK_SIZE_B,
+  MAX_CHUNK_SIZE_B,
+  MIN_CHUNK_SIZE_B,
+  hashFn,
+} from './chunk-config.js';
 
 export const mutators = {
   yjsSetLocalStateField,
@@ -96,9 +102,6 @@ function setClientUpdate(
   return tx.set(yjsProviderClientUpdateKey(name, id), update);
 }
 
-const AVG_CHUNK_SIZE_B = 1024;
-const MIN_CHUNK_SIZE_B = 256;
-const MAX_CHUNK_SIZE_B = 2048;
 async function setServerUpdate(
   name: string,
   update: Uint8Array,
@@ -116,6 +119,7 @@ async function setServerUpdate(
     MIN_CHUNK_SIZE_B,
     MAX_CHUNK_SIZE_B,
     update,
+    hashFn,
   );
   const updateMeta: ChunkedUpdateMeta = {
     chunkHashes: chunkInfo.sourceAsChunkHashes,