UI improvements, merge

src/pdf/pdfService.ts

@@ -18,6 +18,61 @@ export async function loadPdfFromFile(file: File): Promise<PdfFile> {
   };
 }
 
+export async function mergePdfFiles(
+  basePdf: PdfFile,
+  newPdf: PdfFile,
+  insertAt: number
+): Promise<PdfFile> {
+  const baseDoc = await PDFDocument.load(basePdf.arrayBuffer);
+  const newDoc = await PDFDocument.load(newPdf.arrayBuffer);
+
+  const mergedDoc = await PDFDocument.create();
+
+  const basePageCount = baseDoc.getPageCount();
+  const newPageCount = newDoc.getPageCount();
+
+  const clampedInsertAt = Math.min(Math.max(insertAt, 0), basePageCount);
+
+  const basePages = await mergedDoc.copyPages(
+    baseDoc,
+    Array.from({ length: basePageCount }, (_, i) => i)
+  );
+  const newPages = await mergedDoc.copyPages(
+    newDoc,
+    Array.from({ length: newPageCount }, (_, i) => i)
+  );
+
+  // base pages before insertion
+  for (let i = 0; i < clampedInsertAt; i += 1) {
+    mergedDoc.addPage(basePages[i]);
+  }
+
+  // inserted new pages
+  for (let i = 0; i < newPages.length; i += 1) {
+    mergedDoc.addPage(newPages[i]);
+  }
+
+  // remaining base pages
+  for (let i = clampedInsertAt; i < basePages.length; i += 1) {
+    mergedDoc.addPage(basePages[i]);
+  }
+
+  const bytes = await mergedDoc.save();
+  const buffer = bytes.buffer.slice(
+    bytes.byteOffset,
+    bytes.byteOffset + bytes.byteLength
+  );
+
+  const baseName = basePdf.name.replace(/\.pdf$/i, '');
+  const newName = newPdf.name.replace(/\.pdf$/i, '');
+
+  return {
+    name: `${baseName}_plus_${newName}.pdf`,
+    arrayBuffer: buffer,
+    pageCount: basePageCount + newPageCount,
+  };
+}
+
 export async function splitIntoSinglePages(
   pdf: PdfFile
 ): Promise<SplitResult[]> {