From 9cf0013afa12e6e7f4648b29965c41d49e1cf73f Mon Sep 17 00:00:00 2001
From: Andreas Gey <a.gey@fis-gmbh.de>
Date: Fri, 30 Jan 2026 14:47:48 +0100
Subject: [PATCH] Add support for PDF RunLengthDecode filter

Implemented RunLengthDecode filter for PDF streams per PDF 1.7 spec. Added a new RunLengthDecode class with a Decode method, registered it in Filtering.cs, and updated GetFilter to return it. This enables decompression of streams using the RunLengthDecode filter, improving PDF compatibility.
---
 .../src/PdfSharp/Pdf.Filters/Filtering.cs     |  4 ++
 .../PdfSharp/Pdf.Filters/RunLengthDecode.cs   | 69 +++++++++++++++++++
 2 files changed, 73 insertions(+)
 create mode 100644 src/foundation/src/PDFsharp/src/PdfSharp/Pdf.Filters/RunLengthDecode.cs

diff --git a/src/foundation/src/PDFsharp/src/PdfSharp/Pdf.Filters/Filtering.cs b/src/foundation/src/PDFsharp/src/PdfSharp/Pdf.Filters/Filtering.cs
index 1f48f8f8..2244982b 100644
--- a/src/foundation/src/PDFsharp/src/PdfSharp/Pdf.Filters/Filtering.cs
+++ b/src/foundation/src/PDFsharp/src/PdfSharp/Pdf.Filters/Filtering.cs
@@ -170,6 +170,8 @@ public static Filter GetFilter(string filterName)
 
                 
                 case PdfFilterNames.RunLengthDecode:
+                    return _runLengthDecode ??= new RunLengthDecode();
+
                 case PdfFilterNames.CcittFaxDecode:
                 case PdfFilterNames.Jbig2Decode:
                 case PdfFilterNames.JpxDecode:
@@ -220,6 +222,8 @@ public static AsciiHexDecode AsciiHexDecode
 
         static DctDecode? _dctDecode;
 
+        static RunLengthDecode? _runLengthDecode;
+
         //jpxDecode
         //crypt
 
diff --git a/src/foundation/src/PDFsharp/src/PdfSharp/Pdf.Filters/RunLengthDecode.cs b/src/foundation/src/PDFsharp/src/PdfSharp/Pdf.Filters/RunLengthDecode.cs
new file mode 100644
index 00000000..1a1c3f1e
--- /dev/null
+++ b/src/foundation/src/PDFsharp/src/PdfSharp/Pdf.Filters/RunLengthDecode.cs
@@ -0,0 +1,69 @@
+namespace PdfSharp.Pdf.Filters
+{
+    /// <summary>
+    /// https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/pdfreference1.7old.pdf
+    /// The RunLengthDecode filter decodes data that has been encoded in a simple
+    /// byte-oriented format based on run length.The encoded data is a sequence of
+    /// runs, where each run consists of a length byte followed by 1 to 128 bytes of data.If
+    /// the length byte is in the range 0 to 127, the following length + 1 (1 to 128) bytes
+    /// are copied literally during decompression.If length is in the range 129 to 255, the
+    /// following single byte is to be copied 257 − length (2 to 128) times during
+    /// decompression.A length value of 128 denotes EOD.
+    /// The compression achieved by run-length encoding depends on the input data.In
+    /// the best case (all zeros), a compression of approximately 64:1 is achieved for long
+    /// files. The worst case (the hexadecimal sequence 00 alternating with FF) results in
+    /// an expansion of 127:128. 
+    /// </summary>
+    internal class RunLengthDecode : Filter
+    {
+        public override Byte[] Encode(byte[] data)
+        {
+            throw new NotImplementedException();
+        }
+
+        public override Byte[] Decode(byte[] compressedData, FilterParms? parms)
+        {
+            var decompressedData = new List<byte>();
+            var i = 0;
+
+            // Iterate through the compressed stream and decode the RLE data
+            while (i < compressedData.Length)
+            {
+                var lengthByte = compressedData[i];
+                i++;
+
+                // If lengthByte is 128, it means EOD (End of Data)
+                if (lengthByte == 128)
+                {
+                    break;
+                }
+
+                if (lengthByte <= 127)
+                {
+                    // Copy (lengthByte + 1) bytes literally
+                    var length = lengthByte + 1;
+                    for (var j = 0; j < length; j++)
+                    {
+                        decompressedData.Add(compressedData[i]);
+                        i++;
+                    }
+                }
+                else
+                {
+                    // Repeat the next byte (257 - lengthByte) times
+                    var repeatCount = 257 - lengthByte;
+                    var value = compressedData[i];
+                    i++;
+
+                    for (var j = 0; j < repeatCount; j++)
+                    {
+                        decompressedData.Add(value);
+                    }
+                }
+            }
+
+            // Return the decompressed byte array
+            return decompressedData.ToArray();
+        }
+    }
+}
\ No newline at end of file