Fix some issues in Vp8BitReader

src/ImageSharp/Formats/WebP/LoopFilter.cs

@@ -5,7 +5,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
 {
     internal enum LoopFilter
     {
-        Normal,
+        Complex,
         Simple,
         None
     }

src/ImageSharp/Formats/WebP/Vp8BitReader.cs

@@ -36,16 +36,6 @@ namespace SixLabors.ImageSharp.Formats.WebP
         /// </summary>
         private byte buf;
 
-        /// <summary>
-        /// End of read buffer.
-        /// </summary>
-        // private byte bufEnd;
-
-        /// <summary>
-        /// Max packed-read position on buffer.
-        /// </summary>
-        // private byte bufMax;
-
         /// <summary>
         /// True if input is exhausted.
         /// </summary>
@@ -68,7 +58,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
 
             this.range = 255 - 1;
             this.value = 0;
-            this.bits = -8; // to load the very first 8bits;
+            this.bits = -8; // to load the very first 8bits.
             this.eof = false;
             this.pos = 0;
 
@@ -87,7 +77,8 @@ namespace SixLabors.ImageSharp.Formats.WebP
 
             int pos = this.bits;
             uint split = (uint)((range * prob) >> 8);
-            bool bit = this.value > split;
+            ulong value = this.value >> pos;
+            bool bit = value > split;
             if (bit)
             {
                 range -= split;
@@ -117,9 +108,9 @@ namespace SixLabors.ImageSharp.Formats.WebP
             Guard.MustBeGreaterThan(nBits, 0, nameof(nBits));
 
             uint v = 0;
-            while (this.bits-- > 0)
+            while (nBits-- > 0)
             {
-                v |= (uint)this.GetBit(0x80) << this.bits;
+                v |= (uint)this.GetBit(0x80) << nBits;
             }
 
             return v;
@@ -140,7 +131,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
                 ulong bits;
                 ulong inBits = BinaryPrimitives.ReadUInt64LittleEndian(this.Data.AsSpan().Slice((int)this.pos, 8));
                 this.pos += BitsCount >> 3;
-                this.buf = this.Data[BitsCount >> 3];
+                this.buf = this.Data[this.pos];
                 bits = this.ByteSwap64(inBits);
                 bits >>= 64 - BitsCount;
                 this.value = bits | (this.value << BitsCount);
@@ -182,9 +173,20 @@ namespace SixLabors.ImageSharp.Formats.WebP
 
         private int BitsLog2Floor(uint n)
         {
-            long firstSetBit;
-            throw new NotImplementedException();
-            // BitScanReverse(firstSetBit, n);
+            // Search the mask data from most significant bit (MSB) to least significant bit (LSB) for a set bit (1).
+            // https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanreverse-bitscanreverse64?view=vs-2019
+            uint mask = 1;
+            for (int y = 0; y < 32; y++)
+            {
+                if ((mask & n) == mask)
+                {
+                    return y;
+                }
+
+                mask <<= 1;
+            }
+
+            return 0;
         }
     }
 }

src/ImageSharp/Formats/WebP/WebPDecoderCore.cs

@@ -284,7 +284,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
         {
             this.webpMetadata.Format = WebPFormatType.Lossy;
 
-            // VP8 data size.
+            // VP8 data size (not including this 4 bytes).
             this.currentStream.Read(this.buffer, 0, 4);
             uint dataSize = BinaryPrimitives.ReadUInt32LittleEndian(this.buffer);
 
@@ -339,6 +339,13 @@ namespace SixLabors.ImageSharp.Formats.WebP
                 WebPThrowHelper.ThrowImageFormatException("width or height can not be zero");
             }
 
+            // The size of the encoded image data payload.
+            uint imageDataSize = dataSize - 10; // we have read 10 bytes already.
+            if (partitionLength > imageDataSize)
+            {
+                WebPThrowHelper.ThrowImageFormatException("bad partition length");
+            }
+
             var vp8FrameHeader = new Vp8FrameHeader()
                                  {
                                      KeyFrame = true,
@@ -346,7 +353,10 @@ namespace SixLabors.ImageSharp.Formats.WebP
                                      PartitionLength = partitionLength
                                  };
 
-            var bitReader = new Vp8BitReader(this.currentStream, dataSize - 10, this.memoryAllocator);
+            var bitReader = new Vp8BitReader(
+                this.currentStream,
+                imageDataSize,
+                this.memoryAllocator);
 
             // Paragraph 9.2: color space and clamp type follow
             sbyte colorSpace = (sbyte)bitReader.ReadValue(1);
@@ -394,10 +404,14 @@ namespace SixLabors.ImageSharp.Formats.WebP
 
             // Paragraph 9.4: Parse the filter specs.
             var vp8FilterHeader = new Vp8FilterHeader();
-            vp8FilterHeader.LoopFilter = bitReader.ReadBool() ? LoopFilter.Simple : LoopFilter.Normal;
+            vp8FilterHeader.LoopFilter = bitReader.ReadBool() ? LoopFilter.Simple : LoopFilter.Complex;
             vp8FilterHeader.Level = (int)bitReader.ReadValue(6);
             vp8FilterHeader.Sharpness = (int)bitReader.ReadValue(3);
             vp8FilterHeader.UseLfDelta = bitReader.ReadBool();
+
+            // TODO: use enum here?
+            // 0 = 0ff, 1 = simple, 2 = complex
+            int filterType = (vp8FilterHeader.Level is 0) ? 0 : vp8FilterHeader.LoopFilter is LoopFilter.Simple ? 1 : 2;
             if (vp8FilterHeader.UseLfDelta)
             {
                 // Update lf-delta?
@@ -423,7 +437,12 @@ namespace SixLabors.ImageSharp.Formats.WebP
                 }
             }
 
-            // TODO: ParsePartitions
+            // Paragraph 9.5: ParsePartitions.
+            int numPartsMinusOne = (1 << (int)bitReader.ReadValue(2)) - 1;
+            int lastPart = numPartsMinusOne;
+            // TODO: check if we have enough data available here, throw exception if not
+
+
 
             return new WebPImageInfo()
                    {

src/ImageSharp/Formats/WebP/WebPLossyDecoder.cs

@@ -43,7 +43,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
             switch (version)
             {
                 case 0:
-                    return new Vp8Profile { ReconstructionFilter = ReconstructionFilter.Bicubic, LoopFilter = LoopFilter.Normal };
+                    return new Vp8Profile { ReconstructionFilter = ReconstructionFilter.Bicubic, LoopFilter = LoopFilter.Complex };
                 case 1:
                     return new Vp8Profile { ReconstructionFilter = ReconstructionFilter.Bilinear, LoopFilter = LoopFilter.Simple };
                 case 2: