tools/add_lego.js: set cover from largest image and omit it from body

tools/add_lego.js

@@ -132,6 +132,58 @@ function escapeMarkdownAlt(text) {
     .replace(/\]/g, '\\]');
 }
 
+// Lightweight dimensions readers for PNG/JPEG/GIF. Fallback to file size.
+async function getImageDimensions(filePath) {
+  const fd = await fs.open(filePath, 'r');
+  try {
+    const stat = await fd.stat();
+    const size = stat.size || 0;
+    const hdr = Buffer.alloc(64);
+    const { bytesRead } = await fd.read(hdr, 0, hdr.length, 0);
+    const b = hdr.slice(0, bytesRead);
+    // PNG
+    if (b.length >= 24 && b[0] === 0x89 && b[1] === 0x50 && b[2] === 0x4E && b[3] === 0x47) {
+      const width = b.readUInt32BE(16);
+      const height = b.readUInt32BE(20);
+      return { width, height, area: width * height, size };
+    }
+    // GIF
+    if (b.length >= 10 && b[0] === 0x47 && b[1] === 0x49 && b[2] === 0x46) {
+      const width = b.readUInt16LE(6);
+      const height = b.readUInt16LE(8);
+      return { width, height, area: width * height, size };
+    }
+    // JPEG: scan segments for SOF markers
+    if (b.length >= 4 && b[0] === 0xFF && b[1] === 0xD8) {
+      let pos = 2;
+      while (pos + 3 < size) {
+        const markerBuf = Buffer.alloc(4);
+        await fd.read(markerBuf, 0, 4, pos);
+        if (markerBuf[0] !== 0xFF) { pos += 1; continue; }
+        const marker = markerBuf[1];
+        const hasLen = marker !== 0xD8 && marker !== 0xD9;
+        let segLen = 0;
+        if (hasLen) segLen = markerBuf.readUInt16BE(2);
+        // SOF0..SOF15 except DHT(0xC4), JPG(0xC8), DAC(0xCC)
+        if (marker >= 0xC0 && marker <= 0xCF && ![0xC4, 0xC8, 0xCC].includes(marker)) {
+          const segHdr = Buffer.alloc(7);
+          await fd.read(segHdr, 0, segHdr.length, pos + 4);
+          const height = segHdr.readUInt16BE(1);
+          const width = segHdr.readUInt16BE(3);
+          return { width, height, area: width * height, size };
+        }
+        if (!hasLen || segLen < 2) { pos += 2; continue; }
+        pos += 2 + segLen;
+      }
+    }
+    return { width: 0, height: 0, area: 0, size };
+  } catch (_) {
+    return { width: 0, height: 0, area: 0, size: 0 };
+  } finally {
+    await fd.close();
+  }
+}
+
 async function readYamlTitle(yamlPath) {
   try {
     const content = await fs.readFile(yamlPath, 'utf8');
@@ -327,6 +379,23 @@ async function main() {
       imageFiles = Array.from(downloadedImages);
     }
 
+    // Choose cover: prefer downloaded this run; fallback to all images
+    let coverFile = null;
+    const coverCandidates = (downloadedImages.size > 0)
+      ? Array.from(downloadedImages).filter((b) => imageFiles.includes(b))
+      : imageFiles.slice();
+    if (coverCandidates.length > 0) {
+      let best = null;
+      let bestScore = -1;
+      for (const base of coverCandidates) {
+        const full = path.join(imagesDir, base);
+        const dim = await getImageDimensions(full);
+        const score = (dim.area && dim.area > 0) ? dim.area : (dim.size || 0);
+        if (score > bestScore) { bestScore = score; best = base; }
+      }
+      coverFile = best;
+    }
+
     let body = '';
     if (imageFiles.length > 0) {
       const ordered = [
@@ -335,6 +404,7 @@ async function main() {
       ];
       const lines = [];
       for (const base of ordered) {
+        if (coverFile && base === coverFile) continue;
         const nameNoExt = base.replace(/\.[^.]+$/, '');
         const yamlPath = path.join(dataImagesDir, `${nameNoExt}.yaml`);
         const altRaw = (await readYamlTitle(yamlPath)) || `${pageTitle}`;
@@ -345,13 +415,16 @@ async function main() {
       body = lines.join('\n') + '\n';
     }
 
+    const coverLine = coverFile ? `cover: images/${coverFile}\n` : '';
     const md = `---\n` +
                `title: "${pageTitle.replace(/"/g, '\\"')}"\n` +
                `date: ${date}\n` +
+               coverLine +
                `---\n\n` +
                body;
     await fs.writeFile(indexPath, md, 'utf8');
-    console.log(`[create] ${path.relative(ROOT, indexPath)} with ${imageFiles.length} image reference(s)`);
+    const coverInfo = coverFile ? `, cover=images/${coverFile}` : '';
+    console.log(`[create] ${path.relative(ROOT, indexPath)} with ${imageFiles.length} image reference(s)${coverInfo}`);
   }
 
   // Report downloaded images that are not referenced in an existing index.md