forum scraper: dedup downloads by content hash, not just filename

Mirror of a627388 but for the forum image path. The same image is often
re-uploaded under different filenames across pages/posts, so existsSync
on the target name can't catch content-duplicates. After fetching the
buffer, hash the first 64KB and compare against existing same-size files
in the target folder (same md5+size signature as gallery's duplicate
scanner). Confirmed against a known dani-speegle-2 pair:

  skip IMG_79695f8914f20ce38b07.jpg — same content as
       72759c89-7e53-4976-839a-7d952c444579.jpg

buildSizeIndex is built once per job in runForumScrape and threaded
through scrapeForumPage → downloadImage; the hash cache amortizes across
all pages in the job.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

server/scrapers/forum.js

@@ -1,15 +1,54 @@
 import * as cheerio from 'cheerio';
-import { createWriteStream, existsSync, mkdirSync, statSync, writeFileSync } from 'fs';
+import { createReadStream, createWriteStream, existsSync, mkdirSync, readdirSync, statSync, writeFileSync } from 'fs';
 import { basename, join, extname } from 'path';
 import { pipeline } from 'stream/promises';
 import { execFile } from 'child_process';
 import { promisify } from 'util';
+import { createHash } from 'crypto';
 import { upsertMediaFile } from '../db.js';
 import { fsGet } from '../flaresolverr.js';
 import { isTurboUrl, downloadTurbo } from './turbo.js';
 
 const execFileAsync = promisify(execFile);
 
+// Match the duplicate scanner in gallery.js and turbo.js — md5 of first 64KB + exact size.
+const HASH_BYTES = 65536;
+
+function hashFirst64kSync(filePath) {
+  return new Promise((resolve, reject) => {
+    const hash = createHash('md5');
+    const s = createReadStream(filePath, { start: 0, end: HASH_BYTES - 1 });
+    s.on('data', (c) => hash.update(c));
+    s.on('end', () => resolve(hash.digest('hex')));
+    s.on('error', reject);
+  });
+}
+
+// Build size -> [{filename, path, hash:null}] index for the folder. Hashes are
+// computed lazily only when a size collision is found.
+export function buildSizeIndex(folderPath) {
+  const idx = new Map();
+  let entries;
+  try { entries = readdirSync(folderPath); } catch { return idx; }
+  for (const name of entries) {
+    if (name.startsWith('.')) continue;
+    const p = join(folderPath, name);
+    try {
+      const st = statSync(p);
+      if (!st.isFile()) continue;
+      if (!idx.has(st.size)) idx.set(st.size, []);
+      idx.get(st.size).push({ filename: name, path: p, hash: null });
+    } catch {}
+  }
+  return idx;
+}
+
+async function ensureCandidateHash(c) {
+  if (c.hash != null) return c.hash;
+  try { c.hash = await hashFirst64kSync(c.path); } catch { c.hash = ''; }
+  return c.hash;
+}
+
 const UA = 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36';
 const SERVER_IP = '47.185.183.191';
 
@@ -150,7 +189,7 @@ function tryFullSizeUrl(thumbUrl) {
   return candidates;
 }
 
-async function downloadImage(url, outputDir, downloadedSet, logFn, cookies, userAgent) {
+async function downloadImage(url, outputDir, downloadedSet, logFn, cookies, userAgent, sizeIndex) {
   if (downloadedSet.has(url)) return false;
   if (!isImageUrl(url)) return false;
   const lower = url.toLowerCase();
@@ -183,11 +222,30 @@ async function downloadImage(url, outputDir, downloadedSet, logFn, cookies, user
       return false;
     }
 
+    // Content-hash dedup: same image often re-uploaded under different names.
+    let newHash = null;
+    if (sizeIndex && sizeIndex.has(buf.length)) {
+      const head = buf.subarray(0, Math.min(buf.length, HASH_BYTES));
+      newHash = createHash('md5').update(head).digest('hex');
+      for (const cand of sizeIndex.get(buf.length)) {
+        const ch = await ensureCandidateHash(cand);
+        if (ch && ch === newHash) {
+          logFn(`skip ${filename} — same content as ${cand.filename}`);
+          return false;
+        }
+      }
+    }
+
     writeFileSync(filepath, buf);
     const savedName = basename(filepath);
     const folderName = basename(outputDir);
     try { upsertMediaFile(folderName, savedName, 'image', buf.length, Date.now(), null); } catch {}
 
+    if (sizeIndex) {
+      if (!sizeIndex.has(buf.length)) sizeIndex.set(buf.length, []);
+      sizeIndex.get(buf.length).push({ filename: savedName, path: filepath, hash: newHash });
+    }
+
     const sizeKb = (buf.length / 1024).toFixed(1);
     logFn(`Downloaded: ${savedName} (${sizeKb} KB)`);
     return true;
@@ -273,7 +331,7 @@ async function downloadFromExternalHost(url, outputDir, downloadedSet, logFn, us
   }
 }
 
-export async function scrapeForumPage(pageUrl, outputDir, downloadedSet, logFn, cookies, userAgent, fsSession) {
+export async function scrapeForumPage(pageUrl, outputDir, downloadedSet, logFn, cookies, userAgent, fsSession, sizeIndex) {
   logFn(`Fetching page: ${pageUrl}${fsSession ? ' [via FlareSolverr]' : ''}`);
 
   let html;
@@ -403,7 +461,7 @@ export async function scrapeForumPage(pageUrl, outputDir, downloadedSet, logFn,
 
   // Download images
   for (const imgUrl of imageUrls) {
-    if (await downloadImage(imgUrl, outputDir, downloadedSet, logFn, cookies, userAgent)) {
+    if (await downloadImage(imgUrl, outputDir, downloadedSet, logFn, cookies, userAgent, sizeIndex)) {
       count++;
     }
   }