add lzw filter support

src/UglyToad.PdfPig/Filters/BitStream.cs

@@ -0,0 +1,65 @@
+namespace UglyToad.PdfPig.Filters
+{
+    using System;
+    using System.Collections.Generic;
+
+    internal class BitStream
+    {
+        private readonly IReadOnlyList<byte> data;
+
+        private int currentWithinByteBitOffset;
+        private int currentByteIndex;
+
+        public BitStream(IReadOnlyList<byte> data)
+        {
+            this.data = data ?? throw new ArgumentNullException(nameof(data));
+        }
+
+        public int Get(int numberOfBits)
+        {
+            var endWithinByteBitOffset = (numberOfBits + currentWithinByteBitOffset) % 8;
+
+            var numberOfBytesToRead = (numberOfBits + currentWithinByteBitOffset) / 8;
+
+            if (endWithinByteBitOffset != 0)
+            {
+                numberOfBytesToRead++;
+            }
+
+            var result = 0;
+            for (var i = 0; i < numberOfBytesToRead; i++)
+            {
+                if (i > 0)
+                {
+                    currentByteIndex++;
+                }
+
+                if (currentByteIndex >= data.Count)
+                {
+                    throw new InvalidOperationException($"Reached the end of the bit stream while trying to read {i} bits.");
+                }
+
+                result <<= 8;
+                result |= data[currentByteIndex];
+            }
+
+            // Trim trailing bits.
+            if (endWithinByteBitOffset > 0)
+            {
+                result >>= 8 - endWithinByteBitOffset;
+            }
+            else
+            {
+                currentByteIndex++;
+            }
+
+            // 'And' out the leading bits.
+            var firstBitOfDataWithinInt = (sizeof(int) * 8) - numberOfBits;
+            result &= (int)(0xffffffff >> firstBitOfDataWithinInt);
+            
+            currentWithinByteBitOffset = endWithinByteBitOffset;
+
+            return result;
+        }
+    }
+}

src/UglyToad.PdfPig/Filters/LzwFilter.cs

@@ -0,0 +1,157 @@
+namespace UglyToad.PdfPig.Filters
+{
+    using System;
+    using System.Collections.Generic;
+    using Tokens;
+    using Util;
+
+    internal class LzwFilter : IFilter
+    {
+        private const int DefaultColors = 1;
+        private const int DefaultBitsPerComponent = 8;
+        private const int DefaultColumns = 1;
+
+        private const int ClearTable = 256;
+        private const int EodMarker = 257;
+
+        private const int NineBitBoundary = 511;
+        private const int TenBitBoundary = 1023;
+        private const int ElevenBitBoundary = 2047;
+
+        private readonly IDecodeParameterResolver decodeParameterResolver;
+        private readonly IPngPredictor pngPredictor;
+
+        public LzwFilter(IDecodeParameterResolver decodeParameterResolver, IPngPredictor pngPredictor)
+        {
+            this.decodeParameterResolver = decodeParameterResolver ?? throw new ArgumentNullException(nameof(decodeParameterResolver));
+            this.pngPredictor = pngPredictor ?? throw new ArgumentNullException(nameof(pngPredictor));
+        }
+
+        public byte[] Decode(IReadOnlyList<byte> input, DictionaryToken streamDictionary, int filterIndex)
+        {
+            var parameters = decodeParameterResolver.GetFilterParameters(streamDictionary, filterIndex);
+
+            var predictor = parameters.GetIntOrDefault(NameToken.Predictor, -1);
+
+            var earlyChange = parameters.GetIntOrDefault(NameToken.EarlyChange, 1);
+
+            if (predictor > 1)
+            {
+                var decompressed = Decode(input, earlyChange == 1);
+
+                var colors = Math.Min(parameters.GetIntOrDefault(NameToken.Colors, DefaultColors), 32);
+                var bitsPerComponent = parameters.GetIntOrDefault(NameToken.BitsPerComponent, DefaultBitsPerComponent);
+                var columns = parameters.GetIntOrDefault(NameToken.Columns, DefaultColumns);
+
+                var result = pngPredictor.Decode(decompressed, predictor, colors, bitsPerComponent, columns);
+
+                return result;
+            }
+
+            var data = Decode(input, earlyChange == 1);
+
+            return data;
+        }
+
+        private static byte[] Decode(IReadOnlyList<byte> input, bool isEarlyChange)
+        {
+            var result = new List<byte>();
+
+            var table = GetDefaultTable();
+
+            var codeBits = 9;
+
+            var data = new BitStream(input);
+
+            var codeOffset = isEarlyChange ? 0 : 1;
+
+            var previous = -1;
+
+            while (true)
+            {
+                var next = data.Get(codeBits);
+
+                if (next == EodMarker)
+                {
+                    break;
+                }
+                
+                if (next == ClearTable)
+                {
+                    table = GetDefaultTable();
+                    previous = -1;
+                    codeBits = 9;
+                    continue;
+                }
+
+                if (table.TryGetValue(next, out var b))
+                {
+                    result.AddRange(b);
+
+                    if (previous >= 0)
+                    {
+                        var lastSequence = table[previous];
+
+                        var newSequence = new byte[lastSequence.Length + 1];
+
+                        Array.Copy(lastSequence, newSequence, lastSequence.Length);
+
+                        newSequence[lastSequence.Length] = b[0];
+
+                        table[table.Count] = newSequence;
+                    }
+                }
+                else
+                {
+                    var lastSequence = table[previous];
+
+                    var newSequence = new byte[lastSequence.Length + 1];
+
+                    Array.Copy(lastSequence, newSequence, lastSequence.Length);
+
+                    newSequence[lastSequence.Length] = lastSequence[0];
+
+                    result.AddRange(newSequence);
+
+                    table[table.Count] = newSequence;
+                }
+                
+                previous = next;
+
+                if (table.Count >= ElevenBitBoundary + codeOffset)
+                {
+                    codeBits = 12;
+                }
+                else if (table.Count >= TenBitBoundary + codeOffset)
+                {
+                    codeBits = 11;
+                }
+                else if (table.Count >= NineBitBoundary + codeOffset)
+                {
+                    codeBits = 10;
+                }
+                else
+                {
+                    codeBits = 9;
+                }
+            }
+
+            return result.ToArray();
+        }
+
+        private static Dictionary<int, byte[]> GetDefaultTable()
+        {
+            var table = new Dictionary<int, byte[]>();
+
+            for (var i = 0; i < 256; i++)
+            {
+                table[i] = new[] { (byte)i };
+            }
+
+            table[ClearTable] = null;
+            table[EodMarker] = null;
+
+            return table;
+        }
+    }
+}

src/UglyToad.PdfPig/Filters/MemoryFilterProvider.cs

@@ -17,6 +17,7 @@
             IFilter AsciiHexFunc() => new AsciiHexDecodeFilter();
             IFilter FlateFunc() => new FlateFilter(decodeParameterResolver, pngPredictor, log);
             IFilter RunLengthFunc() => new RunLengthFilter();
+            IFilter LzwFunc() => new LzwFilter(decodeParameterResolver, pngPredictor);
 
             filterFactories = new Dictionary<string, Func<IFilter>>
             {
@@ -27,7 +28,9 @@
                 {NameToken.FlateDecode.Data, FlateFunc},
                 {NameToken.FlateDecodeAbbreviation.Data, FlateFunc},
                 {NameToken.RunLengthDecode.Data, RunLengthFunc},
-                {NameToken.RunLengthDecodeAbbreviation.Data, RunLengthFunc}
+                {NameToken.RunLengthDecodeAbbreviation.Data, RunLengthFunc},
+                {NameToken.LzwDecode, LzwFunc},
+                {NameToken.LzwDecodeAbbreviation, LzwFunc}
             };
         }