ЧЗ scanner: switch decoder to zxing-wasm (ZXing-C++) for reliable DataMatrix

The pure-JS @zxing/library locator is weak on real ЧЗ codes (decoded 1/4 of the
test photos with correct grayscale input; libdmtx and zxing-wasm read 3/4 — the
4th is physically undecodable). Symptom: scanner takes very long to even find a
code, 2 of 4 never read.

- Bundle zxing-wasm reader as an IIFE (frontend/lib/zxing-wasm.js, global
  ZXingWASM) + zxing_reader.wasm; drop frontend/lib/zxing.js. Init fetches the
  .wasm as an arrayBuffer and hands it over as wasmBinary (no streaming/MIME/CDN).
- ChZ.zxReady()/ChZ.decodeImageData() (chzn.js): decode an ImageData -> raw GS1
  bytes (result.bytes keeps the GS separators) with tryHarder/tryRotate/tryInvert.
- Scanner (main.js) rewritten to a canvas loop: crop the central ROI, upscale to
  540px, decode each frame — full-frame search was the slow/unreliable part.
  Photo path decodes the picked image the same way.

Not yet verified on-device (WebView WASM instantiation); decoder logic validated
in a Node harness against the 4 sample photos.

frontend/index.html

@@ -18,7 +18,7 @@
   <script src="lib/chart.min.js"></script>
   <script src="lib/chartjs-plugin-zoom.min.js"></script>
   <script src="lib/jszip.min.js"></script>
-  <script src="lib/zxing.js"></script>
+  <script src="lib/zxing-wasm.js"></script>
   <script src="js/db.js"></script>
   <script src="js/bmr.js"></script>
   <script src="js/tilecache.js"></script>

frontend/js/chzn.js

@@ -270,9 +270,40 @@ let ChZ = (function () {
     return prefill;
   }
 
+  // ------------------------------------------------------------------
+  // DataMatrix decoding via bundled zxing-wasm (ZXing-C++). Far better at
+  // LOCATING codes than the pure-JS port. The .wasm is fetched once as an
+  // arrayBuffer and handed to the module (wasmBinary) — no streaming/MIME issues.
+  // ------------------------------------------------------------------
+  let _zxReadyP = null;
+  function zxReady() {
+    if (_zxReadyP) return _zxReadyP;
+    _zxReadyP = (async () => {
+      if (typeof ZXingWASM === "undefined") throw new Error("ZXingWASM не загружен (пересобери приложение)");
+      let buf = await (await fetch("lib/zxing_reader.wasm")).arrayBuffer();
+      await ZXingWASM.prepareZXingModule({ overrides: { wasmBinary: new Uint8Array(buf) }, fireImmediately: true });
+      return true;
+    })();
+    return _zxReadyP;
+  }
+
+  // Decode one ImageData -> raw GS1 code string (with GS 0x1D), or null.
+  async function decodeImageData(imageData, formats) {
+    let res = await ZXingWASM.readBarcodes(imageData, {
+      formats: formats || ["DataMatrix"],
+      tryHarder: true, tryRotate: true, tryInvert: true, maxNumberOfSymbols: 1,
+    });
+    if (!res || !res.length || !res[0].isValid) return null;
+    let r = res[0];
+    // raw payload from bytes keeps the GS separators; fall back to text
+    if (r.bytes && r.bytes.length) return String.fromCharCode.apply(null, r.bytes);
+    return r.text || null;
+  }
+
   return {
     getSession, setSession, hasSession, importFromText, refresh, ensureSid,
     check, nutritionInfo, normalizeCode, extractGtin, parseCard, resolve,
+    zxReady, decodeImageData,
     _raw: raw,
   };
 })();

frontend/js/main.js

@@ -1844,16 +1844,16 @@ function showAddProductModal(onSaved, prefill) {
   });
 }
 
-// Честный знак scanner: camera + photo/manual, decoded by bundled ZXing
-// (frontend/lib/zxing.js) — fully offline, no Google Play Services (the native
-// BarcodeDetector needs GMS and is absent on de-Googled ROMs). On a decoded
-// DataMatrix -> ChZ.resolve -> prefill the new-product modal.
+// Честный знак scanner: camera + photo/manual, decoded by bundled zxing-wasm
+// (ZXing-C++: frontend/lib/zxing-wasm.js + zxing_reader.wasm) — fully offline,
+// no Google Play Services, and far better at LOCATING a DataMatrix than the
+// pure-JS port. Live frames are cropped to the central ROI and upscaled for
+// speed/reliability. On decode -> ChZ.resolve -> prefill the new-product modal.
 function showChznScanModal(onSaved) {
   if (!ChZ.hasSession()) {
     appAlert("Сессия Честного знака не импортирована.\nНастройки → Честный знак → Импорт сессии (файл chzn_session.json из tools/chzn_export.sh).");
     return;
   }
-  let hasZX = (typeof ZXing !== "undefined" && ZXing.BrowserDatamatrixCodeReader);
 
   let ov = document.createElement("div");
   ov.className = "modal-overlay";
@@ -1864,7 +1864,7 @@ function showChznScanModal(onSaved) {
         <h3>Скан «Честный знак»</h3>
         <div style="position:relative;background:#000;border-radius:8px;overflow:hidden;aspect-ratio:1/1;max-height:60vh">
           <video id="cz-video" playsinline muted style="width:100%;height:100%;object-fit:cover"></video>
-          <div style="position:absolute;inset:15%;border:2px solid rgba(255,255,255,.7);border-radius:8px;pointer-events:none"></div>
+          <div style="position:absolute;inset:14%;border:2px solid rgba(255,255,255,.7);border-radius:8px;pointer-events:none"></div>
         </div>
         <div id="cz-status" style="margin:10px 0;font-size:13px;color:var(--muted);min-height:18px">Запуск камеры...</div>
         <div style="display:flex;gap:8px">
@@ -1875,21 +1875,15 @@ function showChznScanModal(onSaved) {
     </div>`;
   document.body.appendChild(ov);
 
-  let controls = null, busy = false;
+  let stream = null, scanning = false, busy = false;
   let base = winStack.length;
   let statusEl = ov.querySelector("#cz-status");
   let video = ov.querySelector("#cz-video");
-
-  let reader = null;
-  if (hasZX) {
-    let hints = new Map();
-    hints.set(ZXing.DecodeHintType.TRY_HARDER, true); // better on curved/low-contrast codes
-    reader = new ZXing.BrowserDatamatrixCodeReader(hints, { delayBetweenScanAttempts: 120 });
-  }
+  let canvas = document.createElement("canvas");
+  let cctx = canvas.getContext("2d", { willReadFrequently: true });
 
   function stopCamera() {
-    try { if (controls) controls.stop(); } catch {}
-    controls = null;
+    scanning = false;
     try {
       let s = video && video.srcObject;
       if (s && s.getTracks) s.getTracks().forEach(t => t.stop());
@@ -1906,8 +1900,7 @@ function showChznScanModal(onSaved) {
 
   async function onDecoded(rawVal) {
     if (busy) return;
-    busy = true;
-    try { if (controls) controls.stop(); } catch {}
+    busy = true; scanning = false;
     statusEl.textContent = "Код распознан, запрашиваю КБЖУ...";
     try {
       let prefill = await ChZ.resolve(rawVal);
@@ -1917,20 +1910,51 @@ function showChznScanModal(onSaved) {
     } catch (e) {
       busy = false;
       statusEl.textContent = (e.notFound ? "Код не найден в Честном знаке. " : "Ошибка: " + e.message + ". ") + "Наведите ещё раз.";
-      if (reader) startCamera(); // resume continuous scan
+      if (stream) { scanning = true; tick(); }
     }
   }
 
+  // Central ROI of the current video frame, upscaled to a square -> ImageData.
+  // Cropping to the guide box removes clutter and makes the code larger, which
+  // is exactly what the locator needs (full-frame search was slow/unreliable).
+  function grabROI() {
+    let vw = video.videoWidth, vh = video.videoHeight;
+    if (!vw || !vh) return null;
+    let side = Math.min(vw, vh) * 0.72;
+    let sx = (vw - side) / 2, sy = (vh - side) / 2;
+    let out = 540;
+    canvas.width = out; canvas.height = out;
+    cctx.drawImage(video, sx, sy, side, side, 0, 0, out, out);
+    return cctx.getImageData(0, 0, out, out);
+  }
+
+  async function tick() {
+    if (busy || !scanning) return;
+    let img = grabROI();
+    if (img) {
+      try {
+        let raw = await ChZ.decodeImageData(img, ["DataMatrix"]);
+        if (raw) { onDecoded(raw); return; }
+      } catch (e) { /* keep scanning */ }
+    }
+    if (scanning) setTimeout(tick, 100);
+  }
+
   async function startCamera() {
-    if (!reader) { statusEl.textContent = "Декодер ZXing не загрузился — пересобери приложение."; return; }
     try {
+      statusEl.textContent = "Загрузка декодера...";
+      await ChZ.zxReady();
+    } catch (e) {
+      statusEl.textContent = "Декодер не загрузился: " + (e.message || e);
+      return;
+    }
+    try {
+      stream = await navigator.mediaDevices.getUserMedia({ video: { facingMode: { ideal: "environment" } } });
+      video.srcObject = stream;
+      await video.play();
+      scanning = true;
       statusEl.textContent = "Наведите на код Честного знака...";
-      controls = await reader.decodeFromConstraints(
-        { video: { facingMode: { ideal: "environment" } } },
-        video,
-        (result) => { if (result && !busy) onDecoded(result.getText()); }
-        // per-frame NotFoundException is normal — ignored
-      );
+      tick();
     } catch (e) {
       statusEl.textContent = "Камера недоступна (" + (e.name || e.message) + "). Выберите фото или введите код.";
     }
@@ -1938,16 +1962,18 @@ function showChznScanModal(onSaved) {
 
   ov.querySelector("#cz-photo").onclick = () => {
     pickFile("image/*", async (buf) => {
-      if (!reader) { statusEl.textContent = "Декодер ZXing не загрузился."; return; }
-      let url = URL.createObjectURL(new Blob([buf]));
       try {
-        let result = await reader.decodeFromImageUrl(url);
-        onDecoded(result.getText());
-      } catch (e) {
-        statusEl.textContent = (e && e.name === "NotFoundException")
-          ? "Код на фото не распознан. Попробуйте более резкое фото."
-          : "Ошибка распознавания: " + (e.message || e);
-      } finally { URL.revokeObjectURL(url); }
+        await ChZ.zxReady();
+        let bmp = await createImageBitmap(new Blob([buf]));
+        let c = document.createElement("canvas");
+        c.width = bmp.width; c.height = bmp.height;
+        let x = c.getContext("2d", { willReadFrequently: true });
+        x.drawImage(bmp, 0, 0);
+        let img = x.getImageData(0, 0, bmp.width, bmp.height);
+        let raw = await ChZ.decodeImageData(img, ["DataMatrix", "QRCode"]);
+        if (raw) onDecoded(raw);
+        else statusEl.textContent = "Код на фото не распознан. Попробуйте более резкое/крупное фото.";
+      } catch (e) { statusEl.textContent = "Ошибка распознавания: " + (e.message || e); }
     }, true);
   };
   ov.querySelector("#cz-manual").onclick = async () => {