Add insert and preserve links example

insert-pages-and-preserve-links/.gitignore

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+node_modules
+output.pdf

insert-pages-and-preserve-links/README.md

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+Example of inserting one PDF into another, preserving the links in the first PDF.
+
+See [this issue](https://github.com/galkahana/HummusJS/issues/437) for more explanation.

insert-pages-and-preserve-links/index.js

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+const inserter = require('./inserter');
+
+inserter.insert(
+    './insert.pdf',
+    './pdf-with-links.pdf', 
+    2, 
+    './output.pdf'
+);

insert-pages-and-preserve-links/inserter.js

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+const fs = require('fs');
+const hummus = require('hummus');
+
+function insert(insertPDFPath, mainPDFPath, insertPageNumber, savePath){
+    if (!fs.existsSync(insertPDFPath)){
+        throw `No file exists at insertPDFPath ${insertPDFPath}.`;
+    }
+    if (!fs.existsSync(mainPDFPath)){
+        throw `No file exists at mainPDFPath ${mainPDFPath}.`;
+    }
+
+    let insertPage = Number.parseInt(insertPageNumber);
+    if (isNaN(insertPage) || insertPage < 1){
+        throw `insertPageNumber was ${insertPageNumber}; it must be a positive integer.`;
+    }
+
+    let mainReader = hummus.createReader(mainPDFPath);
+    let insertPDFReader = hummus.createReader(insertPDFPath);
+    let insertPDFLength = insertPDFReader.getPagesCount();
+    let outputWriter = hummus.createWriter(savePath);
+
+    //First pages main.pdf, up to insert point
+    appendPDFPageFromPDFWithAnnotations(outputWriter, mainPDFPath, 0, insertPage - 1);
+
+    //insert.pdf
+    outputWriter.appendPDFPagesFromPDF(insertPDFPath);
+
+    //rest of main.pdf
+    appendPDFPageFromPDFWithAnnotations(outputWriter, mainPDFPath, insertPage - 1, mainReader.getPagesCount());
+
+    outputWriter.end();
+    console.log(`${insertPDFPath} inserted.`);
+
+    updateLinkDestinations(savePath, insertPDFLength);
+    console.log(`Links updated.`);
+    console.log(`Saved output to ${savePath}`);
+}
+
+//see https://github.com/galkahana/HummusJSSamples/blob/master/appending-pages-with-comments/appendWithComments.js
+function appendPDFPageFromPDFWithAnnotations(pdfWriter,sourcePDFPath, startPage, endPage) {
+    let cpyCxt = pdfWriter.createPDFCopyingContext(sourcePDFPath);
+    let cpyCxtParser = cpyCxt.getSourceDocumentParser();
+
+    for (let i=startPage; i < endPage; ++i) {
+      let pageDictionary = cpyCxtParser.parsePageDictionary(i);
+      if(!pageDictionary.exists('Annots')) {
+          cpyCxt.appendPDFPageFromPDF(i);            
+      }
+      else {
+        let reffedObjects;
+        pdfWriter.getEvents().once('OnPageWrite',function(params) {
+            params.pageDictionaryContext.writeKey('Annots');
+            reffedObjects = cpyCxt.copyDirectObjectWithDeepCopy(pageDictionary.queryObject('Annots'))
+        })   
+        cpyCxt.appendPDFPageFromPDF(i);
+        if(reffedObjects && reffedObjects.length > 0)
+            cpyCxt.copyNewObjectsForDirectObject(reffedObjects)
+      }        
+    }
+}
+
+function updateLinkDestinations(savePath, insertPDFLength) {
+    let writer = hummus.createWriterToModify(savePath);
+    let reader = writer.getModifiedFileParser(savePath);
+    let copyingContext = writer.createPDFCopyingContextForModifiedFile();
+    let pageIDs = getPageIDs(reader);
+
+    for (let i = 0; i < reader.getPagesCount(); i++) {
+      let pageDictionary = reader.parsePageDictionary(i);
+      if (pageDictionary.exists("Annots")) {
+        let parsedPageDictionary = reader.parsePageDictionary(i);
+        let annots = reader.queryDictionaryObject(parsedPageDictionary, "Annots");
+
+        for (let j = 0; j < annots.getLength(); j++) {
+          let annotationIndirectReference = annots.queryObject(j);
+          let annotation = reader.queryArrayObject(annots, j);
+          let annotationObject = annotation.toJSObject();
+          let destPDFArray = reader.queryDictionaryObject(annotation, "Dest");
+          console.log(destPDFArray);
+          let destArrayObject = destPDFArray.toJSArray();
+          let oldDestPageID = destArrayObject[0].getObjectID();
+          let oldDestPageIndex = getOldPageIDIndexInOldPDF(reader, oldDestPageID);
+          let newDestPageID = pageIDs[oldDestPageIndex + insertPDFLength];
+
+          let objectContext = writer.getObjectsContext();
+          objectContext.startModifiedIndirectObject(
+            annotationIndirectReference.getObjectID()
+          );
+          let modifiedAnnotation = writer.getObjectsContext().startDictionary();
+
+          //copy all keys except Dest to the modified annotation
+          Object.getOwnPropertyNames(annotationObject).forEach(
+            (element, index, array) => {
+              if (element != "Dest") {
+                modifiedAnnotation.writeKey(element);
+                copyingContext.copyDirectObjectAsIs(annotationObject[element]);
+              }
+            }
+          );
+
+          //Add the Dest key and make it an array with the first element being the new target page
+          modifiedAnnotation.writeKey("Dest");
+          objectContext.startArray().writeIndirectObjectReference(newDestPageID);
+
+          //copy other elements of the old Dest array
+          for (let k = 1; k < destArrayObject.length; k++) {
+            copyingContext.copyDirectObjectAsIs(destArrayObject[k]); 
+          }
+
+          objectContext
+            .endArray()
+            .endLine()
+            .endDictionary(modifiedAnnotation)
+            .endIndirectObject();
+        }
+      }
+    }
+
+    writer.end();
+  }
+
+  function getPageIDs(reader){
+    let IDs = [];        
+    for (let i = 0; i < reader.getPagesCount(); i++){
+        IDs.push(reader.getPageObjectID(i));
+    }
+    return IDs;
+  }
+
+  function getOldPageIDIndexInOldPDF(reader, oldPageID){
+    let oldPageDict = reader.parseNewObject(oldPageID).toPDFDictionary();
+    let parent = reader.queryDictionaryObject(oldPageDict, 'Parent').toJSObject();
+    let oldPageIDs = parent.Kids.toJSArray().map(e => e.getObjectID());
+    return oldPageIDs.indexOf(oldPageID);
+  }
+
+module.exports =  { insert };