Implement Compressed XREF

 # Changelog
 
-## 3.1.1
+## 3.2.0
 
 - Fix documentation
 - Add Positioned.fill()
 - Improve GraphicState
 - Add SVG Color filter
+- Implement Compressed XREF
 
 ## 3.1.0
 

@@ -57,7 +57,7 @@ class PdfCatalog extends PdfObjectDict {
     super.prepare();
 
     /// the PDF specification version, overrides the header version starting from 1.4
-    params['/Version'] = PdfName('/${pdfDocument.version}');
+    params['/Version'] = PdfName('/${pdfDocument.versionString}');
 
     params['/Pages'] = pdfPageList.ref();
 

@@ -600,6 +600,7 @@ class PdfDictStream extends PdfDict<PdfDataType> {
     Map<String, PdfDataType> values = const <String, PdfDataType>{},
     required this.data,
     this.isBinary = false,
+    this.encrypt = true,
   }) : super.values(values);
 
   final Uint8List data;
@@ -608,6 +609,8 @@ class PdfDictStream extends PdfDict<PdfDataType> {
 
   final bool isBinary;
 
+  final bool encrypt;
+
   @override
   void output(PdfStream s) {
     final _values = PdfDict(values);
@@ -638,7 +641,7 @@ class PdfDictStream extends PdfDict<PdfDataType> {
       }
     }
 
-    if (object.pdfDocument.encryption != null) {
+    if (encrypt && object.pdfDocument.encryption != null) {
       _data = object.pdfDocument.encryption!.encrypt(_data, object);
     }
 

@@ -35,6 +35,15 @@ import 'page_list.dart';
 import 'signature.dart';
 import 'stream.dart';
 
+/// PDF version to generate
+enum PdfVersion {
+  /// PDF 1.4
+  pdf_1_4,
+
+  /// PDF 1.5 to 1.7
+  pdf_1_5,
+}
+
 /// Display hint for the PDF viewer
 enum PdfPageMode {
   /// This page mode indicates that the document
@@ -69,6 +78,7 @@ class PdfDocument {
     PdfPageMode pageMode = PdfPageMode.none,
     DeflateCallback? deflate,
     bool compress = true,
+    this.version = PdfVersion.pdf_1_4,
   })  : deflate = compress ? (deflate ?? defaultDeflate) : null,
         prev = null,
         _objser = 1 {
@@ -84,7 +94,8 @@ class PdfDocument {
     DeflateCallback? deflate,
     bool compress = true,
   })  : deflate = compress ? (deflate ?? defaultDeflate) : null,
-        _objser = prev!.size {
+        _objser = prev!.size,
+        version = prev.version {
     // Now create some standard objects
     pdfPageList = PdfPageList(this);
     pdfNames = PdfNames(this);
@@ -107,6 +118,9 @@ class PdfDocument {
   /// This is the Catalog object, which is required by each Pdf Document
   late PdfCatalog catalog;
 
+  /// PDF version to generate
+  final PdfVersion version;
+
   /// This is the info object. Although this is an optional object, we
   /// include it.
   PdfInfo? info;
@@ -139,7 +153,7 @@ class PdfDocument {
   PdfGraphicStates? _graphicStates;
 
   /// The PDF specification version
-  final String version = '1.7';
+  final String versionString = '1.7';
 
   /// This holds the current fonts
   final Set<PdfFont> fonts = <PdfFont>{};
@@ -188,7 +202,7 @@ class PdfDocument {
 
   /// This writes the document to an OutputStream.
   Future<void> _write(PdfStream os) async {
-    final pos = PdfOutput(os);
+    final pos = PdfOutput(os, version);
 
     // Write each object to the [PdfStream]. We call via the output
     // as that builds the xref table

@@ -32,6 +32,8 @@ abstract class PdfDocumentParserBase {
   /// The offset of the previous cross reference table
   int get xrefOffset;
 
+  PdfVersion get version => PdfVersion.pdf_1_4;
+
   /// Import the existing objects into the new PDF document
   void mergeDocument(PdfDocument pdfDocument);
 }

@@ -16,6 +16,7 @@
 
 import 'catalog.dart';
 import 'data_types.dart';
+import 'document.dart';
 import 'encryption.dart';
 import 'info.dart';
 import 'object.dart';
@@ -26,11 +27,24 @@ import 'xref.dart';
 /// PDF document writer
 class PdfOutput {
   /// This creates a Pdf [PdfStream]
-  PdfOutput(this.os) {
-    os.putString('%PDF-1.4\n');
+  PdfOutput(this.os, this.version) {
+    String v;
+    switch (version) {
+      case PdfVersion.pdf_1_4:
+        v = '1.4';
+        break;
+      case PdfVersion.pdf_1_5:
+        v = '1.5';
+        break;
+    }
+
+    os.putString('%PDF-$v\n');
     os.putBytes(const <int>[0x25, 0xC2, 0xA5, 0xC2, 0xB1, 0xC3, 0xAB, 0x0A]);
   }
 
+  /// Pdf version to output
+  final PdfVersion version;
+
   /// This is the actual [PdfStream] used to write to.
   final PdfStream os;
 
@@ -49,6 +63,9 @@ class PdfOutput {
   /// This is used to track the /Sign object (signature)
   PdfSignature? signatureID;
 
+  /// Generate a compressed cross reference table
+  bool get isCompressed => version.index > PdfVersion.pdf_1_4.index;
+
   /// This method writes a [PdfObject] to the stream.
   void write(PdfObject ob) {
     // Check the object to see if it's one that is needed later
@@ -73,12 +90,6 @@ class PdfOutput {
       throw Exception('Root object is not present in document');
     }
 
-    final _xref = os.offset;
-    xref.output(os);
-
-    // now the trailer object
-    os.putString('trailer\n');
-
     final params = PdfDict();
 
     // the number of entries (REQUIRED)
@@ -104,9 +115,22 @@ class PdfOutput {
       params['/Prev'] = PdfNum(rootID!.pdfDocument.prev!.xrefOffset);
     }
 
-    // end the trailer object
-    params.output(os);
-    os.putString('\nstartxref\n$_xref\n%%EOF\n');
+    final _xref = os.offset;
+    if (isCompressed) {
+      xref.outputCompressed(rootID!, os, params);
+    } else {
+      xref.output(os);
+    }
+
+    if (!isCompressed) {
+      // the trailer object
+      os.putString('trailer\n');
+      params.output(os);
+      os.putByte(0x0a);
+    }
+
+    // the reference to the xref object
+    os.putString('startxref\n$_xref\n%%EOF\n');
 
     if (signatureID != null) {
       await signatureID!.writeSignature(os);

@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+import 'dart:typed_data';
+
 import 'package:pdf/src/pdf/stream.dart';
 
 import 'data_types.dart';
@@ -45,6 +47,24 @@ class PdfXref {
     return rs + ' n ';
   }
 
+  /// The xref in the format of the compressed xref section in the Pdf file
+  int cref(ByteData o, int ofs, List<int> w) {
+    assert(w.length >= 3);
+
+    void setVal(int l, int v) {
+      for (var n = 0; n < l; n++) {
+        o.setUint8(ofs, (v >> (l - n - 1) * 8) & 0xff);
+        ofs++;
+      }
+    }
+
+    setVal(w[0], 1);
+    setVal(w[1], offset);
+    setVal(w[2], generation);
+
+    return ofs;
+  }
+
   @override
   bool operator ==(Object other) {
     if (other is PdfXref) {
@@ -113,4 +133,70 @@ class PdfXrefTable extends PdfDataType {
     // now write the last block
     _writeblock(s, firstid, block);
   }
+
+  /// Output a compressed cross-reference table
+  void outputCompressed(PdfObject object, PdfStream s, PdfDict params) {
+    // Write this object too
+    final id = offsets.last.id + 1;
+    final offset = s.offset;
+    offsets.add(PdfXref(id, offset));
+
+    // Sort all references
+    offsets.sort((a, b) => a.id.compareTo(b.id));
+
+    s.putString('$id 0 obj\n');
+
+    params['/Type'] = const PdfName('/XRef');
+    params['/Size'] = PdfNum(id + 1);
+
+    var firstid = 0; // First id in block
+    var lastid = 0; // The last id used
+    final blocks = <int>[]; // xrefs in this block first, count
+
+    // We need block 0 to exist
+    blocks.add(firstid);
+
+    for (var x in offsets) {
+      // check to see if block is in range
+      if (x.id != (lastid + 1)) {
+        // no, so store this block, and reset
+        blocks.add(lastid - firstid + 1);
+        firstid = x.id;
+        blocks.add(firstid);
+      }
+      lastid = x.id;
+    }
+    blocks.add(lastid - firstid + 1);
+
+    if (!(blocks.length == 2 && blocks[0] == 0 && blocks[1] == id + 1)) {
+      params['/Index'] = PdfArray.fromNum(blocks);
+    }
+
+    var bytes = 2; // A pdf less than 256 bytes is unlikely
+    while (1 << (bytes * 8) < offset) {
+      bytes++;
+    }
+
+    final w = [1, bytes, 1];
+    params['/W'] = PdfArray.fromNum(w);
+    final wl = w.reduce((a, b) => a + b);
+
+    final o = ByteData((offsets.length + 2) * wl);
+    var ofs = 0;
+    // Write offset zero, all zeros
+    ofs += wl;
+
+    for (var x in offsets) {
+      ofs = x.cref(o, ofs, w);
+    }
+
+    // Write the object
+    PdfDictStream(
+      object: object,
+      data: o.buffer.asUint8List(),
+      isBinary: true,
+      encrypt: false,
+      values: params.values,
+    ).output(s);
+  }
 }

@@ -26,6 +26,7 @@ class Document {
     PdfPageMode pageMode = PdfPageMode.none,
     DeflateCallback? deflate,
     bool compress = true,
+    PdfVersion version = PdfVersion.pdf_1_4,
     this.theme,
     String? title,
     String? author,
@@ -37,6 +38,7 @@ class Document {
           pageMode: pageMode,
           deflate: deflate,
           compress: compress,
+          version: version,
         ) {
     if (title != null ||
         author != null ||

@@ -4,7 +4,7 @@ description: A pdf producer for Dart. It can create pdf files for both web or fl
 homepage: https://github.com/DavBfr/dart_pdf/tree/master/pdf
 repository: https://github.com/DavBfr/dart_pdf
 issue_tracker: https://github.com/DavBfr/dart_pdf/issues
-version: 3.1.1
+version: 3.2.0
 
 environment:
   sdk: ">=2.12.0-0 <3.0.0"