Fix some issues:

- Fix wrong palette code bits init in CalculateBestCacheSize
- Fix wrong slice start index in PrepareMapToPalette
- PixOrCopy: Change len from short to ushort

src/ImageSharp/Formats/WebP/Lossless/BackwardReferenceEncoder.cs

@@ -230,12 +230,10 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
         }
 
         /// <summary>
-        /// Evaluates best possible backward references for specified quality.
-        /// The input cacheBits to 'GetBackwardReferences' sets the maximum cache
-        /// bits to use (passing 0 implies disabling the local color cache).
+        /// Evaluates best possible backward references for specified quality. The input cacheBits to 'GetBackwardReferences'
+        /// sets the maximum cache bits to use (passing 0 implies disabling the local color cache).
         /// The optimal cache bits is evaluated and set for the cacheBits parameter.
-        /// The return value is the pointer to the best of the two backward refs viz,
-        /// refs[0] or refs[1].
+        /// The return value is the pointer to the best of the two backward refs viz, refs[0] or refs[1].
         /// </summary>
         public static Vp8LBackwardRefs GetBackwardReferences(
             int width,
@@ -335,9 +333,9 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
             int pos = 0;
             var colorCache = new ColorCache[WebPConstants.MaxColorCacheBits + 1];
             var histos = new Vp8LHistogram[WebPConstants.MaxColorCacheBits + 1];
-            for (int i = 0; i < WebPConstants.MaxColorCacheBits + 1; i++)
+            for (int i = 0; i <= WebPConstants.MaxColorCacheBits; i++)
             {
-                histos[i] = new Vp8LHistogram(bestCacheBits);
+                histos[i] = new Vp8LHistogram(paletteCodeBits: i);
                 colorCache[i] = new ColorCache();
                 colorCache[i].Init(i);
             }
@@ -369,7 +367,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
                     {
                         if (colorCache[i].Lookup(key) == pix)
                         {
-                            ++histos[i].Literal[WebPConstants.NumLiteralCodes + WebPConstants.CodeLengthCodes + key];
+                            ++histos[i].Literal[WebPConstants.NumLiteralCodes + WebPConstants.NumLengthCodes + key];
                         }
                         else
                         {
@@ -563,7 +561,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
                 if (len != 1)
                 {
                     int offset = hashChain.FindOffset(i);
-                    backwardRefs.Add(PixOrCopy.CreateCopy((uint)offset, (short)len));
+                    backwardRefs.Add(PixOrCopy.CreateCopy((uint)offset, (ushort)len));
 
                     if (useColorCache)
                     {
@@ -689,7 +687,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
                 }
                 else
                 {
-                    refs.Add(PixOrCopy.CreateCopy((uint)offset, (short)len));
+                    refs.Add(PixOrCopy.CreateCopy((uint)offset, (ushort)len));
                     if (useColorCache)
                     {
                         for (j = i; j < i + len; ++j)
@@ -908,7 +906,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
                 int prevRowLen = (i < xSize) ? 0 : FindMatchLength(bgra.Slice(i), bgra.Slice(i - xSize), 0, maxLen);
                 if (rleLen >= prevRowLen && rleLen >= MinLength)
                 {
-                    refs.Add(PixOrCopy.CreateCopy(1, (short)rleLen));
+                    refs.Add(PixOrCopy.CreateCopy(1, (ushort)rleLen));
 
                     // We don't need to update the color cache here since it is always the
                     // same pixel being copied, and that does not change the color cache state.
@@ -916,7 +914,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
                 }
                 else if (prevRowLen >= MinLength)
                 {
-                    refs.Add(PixOrCopy.CreateCopy((uint)xSize, (short)prevRowLen));
+                    refs.Add(PixOrCopy.CreateCopy((uint)xSize, (ushort)prevRowLen));
                     if (useColorCache)
                     {
                         for (int k = 0; k < prevRowLen; k++)
@@ -936,7 +934,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
 
             if (useColorCache)
             {
-                // TODO: VP8LColorCacheClear();
+                // TODO: VP8LColorCacheClear()?
             }
         }
 
@@ -978,7 +976,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
                 }
             }
 
-            // TODO: VP8LColorCacheClear(colorCache);
+            // TODO: VP8LColorCacheClear(colorCache)?
         }
 
         private static void BackwardReferences2DLocality(int xSize, Vp8LBackwardRefs refs)

src/ImageSharp/Formats/WebP/Lossless/HuffmanUtils.cs

@@ -279,13 +279,12 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
             {
                 int value = tree.CodeLengths[i];
                 int k = i + 1;
-                int runs;
                 while (k < depthSize && tree.CodeLengths[k] == value)
                 {
                     k++;
                 }
 
-                runs = k - i;
+                var runs = k - i;
                 if (value == 0)
                 {
                     tokenPos += CodeRepeatedZeros(runs, tokensArray.AsSpan(tokenPos));

src/ImageSharp/Formats/WebP/Lossless/PixOrCopy.cs

@@ -10,7 +10,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
     {
         public PixOrCopyMode Mode { get; set; }
 
-        public short Len { get; set; }
+        public ushort Len { get; set; }
 
         public uint BgraOrDistance { get; set; }
 
@@ -38,7 +38,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
             return retval;
         }
 
-        public static PixOrCopy CreateCopy(uint distance, short len)
+        public static PixOrCopy CreateCopy(uint distance, ushort len)
         {
             var retval = new PixOrCopy()
             {
@@ -60,7 +60,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
             return this.BgraOrDistance;
         }
 
-        public short Length()
+        public ushort Length()
         {
             return this.Len;
         }

src/ImageSharp/Formats/WebP/Lossless/Vp8LEncoder.cs

@@ -427,7 +427,16 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
 
             foreach (CrunchSubConfig subConfig in config.SubConfigs)
             {
-                Vp8LBackwardRefs refsBest = BackwardReferenceEncoder.GetBackwardReferences(width, height, bgra, quality, subConfig.Lz77, ref cacheBits, hashChain, refsArray[0], refsArray[1]); // TODO : Pass do not cache
+                Vp8LBackwardRefs refsBest = BackwardReferenceEncoder.GetBackwardReferences(
+                    width,
+                    height,
+                    bgra,
+                    quality,
+                    subConfig.Lz77,
+                    ref cacheBits,
+                    hashChain,
+                    refsArray[0],
+                    refsArray[1]); // TODO : Pass do not cache
 
                 // Keep the best references aside and use the other element from the first
                 // two as a temporary for later usage.
@@ -473,13 +482,13 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
                 this.bitWriter.PutBits((uint)(writeHistogramImage ? 1 : 0), 1);
                 if (writeHistogramImage)
                 {
-                    using IMemoryOwner<uint> histogramArgbBuffer = this.memoryAllocator.Allocate<uint>(histogramImageXySize);
-                    Span<uint> histogramArgb = histogramArgbBuffer.GetSpan();
+                    using IMemoryOwner<uint> histogramBgraBuffer = this.memoryAllocator.Allocate<uint>(histogramImageXySize);
+                    Span<uint> histogramBgra = histogramBgraBuffer.GetSpan();
                     int maxIndex = 0;
                     for (int i = 0; i < histogramImageXySize; i++)
                     {
                         int symbolIndex = histogramSymbols[i] & 0xffff;
-                        histogramArgb[i] = (uint)(symbolIndex << 8);
+                        histogramBgra[i] = (uint)(symbolIndex << 8);
                         if (symbolIndex >= maxIndex)
                         {
                             maxIndex = symbolIndex + 1;
@@ -487,7 +496,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
                     }
 
                     this.bitWriter.PutBits((uint)(histogramBits - 2), 3);
-                    this.EncodeImageNoHuffman(histogramArgb, hashChain, refsTmp, refsArray[2], LosslessUtils.SubSampleSize(width, histogramBits), LosslessUtils.SubSampleSize(height, histogramBits), quality);
+                    this.EncodeImageNoHuffman(histogramBgra, hashChain, refsTmp, refsArray[2], LosslessUtils.SubSampleSize(width, histogramBits), LosslessUtils.SubSampleSize(height, histogramBits), quality);
                 }
 
                 // Store Huffman codes.
@@ -690,7 +699,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
 
             if (count == 0)
             {
-                // emit minimal tree for empty cases
+                // Emit minimal tree for empty cases.
                 // bits: small tree marker: 1, count-1: 0, large 8-bit code: 0, code: 0
                 this.bitWriter.PutBits(0x01, 4);
             }
@@ -725,10 +734,12 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
             int i;
             var codeLengthBitDepth = new byte[WebPConstants.CodeLengthCodes];
             var codeLengthBitDepthSymbols = new short[WebPConstants.CodeLengthCodes];
-            var huffmanCode = new HuffmanTreeCode();
-            huffmanCode.NumSymbols = WebPConstants.CodeLengthCodes;
-            huffmanCode.CodeLengths = codeLengthBitDepth;
-            huffmanCode.Codes = codeLengthBitDepthSymbols;
+            var huffmanCode = new HuffmanTreeCode
+            {
+                NumSymbols = WebPConstants.CodeLengthCodes,
+                CodeLengths = codeLengthBitDepth,
+                Codes = codeLengthBitDepthSymbols
+            };
 
             this.bitWriter.PutBits(0, 1);
             var numTokens = HuffmanUtils.CreateCompressedHuffmanTree(tree, tokens);
@@ -1313,7 +1324,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
         /// </summary>
         private static void PrepareMapToPalette(Span<uint> palette, int numColors, uint[] sorted, uint[] idxMap)
         {
-            palette.Slice(numColors).CopyTo(sorted);
+            palette.Slice(0, numColors).CopyTo(sorted);
             Array.Sort(sorted, PaletteCompareColorsForSort);
             for (int i = 0; i < numColors; i++)
             {

src/ImageSharp/Formats/WebP/Lossless/Vp8LHistogram.cs

@@ -61,7 +61,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
             this.Alpha = new uint[WebPConstants.NumLiteralCodes + 1];
             this.Distance = new uint[WebPConstants.NumDistanceCodes];
 
-            var literalSize = WebPConstants.NumLiteralCodes + WebPConstants.NumLengthCodes + ((this.PaletteCodeBits > 0) ? (1 << this.PaletteCodeBits) : 0);
+            var literalSize = WebPConstants.NumLiteralCodes + WebPConstants.NumLengthCodes + (1 << WebPConstants.MaxColorCacheBits);
             this.Literal = new uint[literalSize];
 
             // 5 for literal, red, blue, alpha, distance.