Add missing null checks

src/ImageSharp/Formats/WebP/BitWriter/Vp8LBitWriter.cs

@@ -112,7 +112,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.BitWriter
         }
 
         /// <summary>
-        /// Writes the encoded bytes of the image to the stream. Call BitWriterFinish() before this. 
+        /// Writes the encoded bytes of the image to the stream. Call BitWriterFinish() before this.
         /// </summary>
         /// <param name="stream">The stream to write to.</param>
         public void WriteToStream(Stream stream)

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

@@ -26,7 +26,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
 
         private const uint NonTrivialSym = 0xffffffff;
 
-        private const short InvalidHistogramSymbol = Int16.MaxValue;
+        private const short InvalidHistogramSymbol = short.MaxValue;
 
         public static void GetHistoImageSymbols(int xSize, int ySize, Vp8LBackwardRefs refs, int quality, int histoBits, int cacheBits, List<Vp8LHistogram> imageHisto, Vp8LHistogram tmpHisto, short[] histogramSymbols)
         {
@@ -67,7 +67,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
 
             // Cubic ramp between 1 and MaxHistoGreedy:
             int thresholdSize = (int)(1 + (x * x * x * (MaxHistoGreedy - 1)));
-            bool doGreedy = HistogramCombineStochastic(imageHisto, ref numUsed, thresholdSize);
+            bool doGreedy = HistogramCombineStochastic(imageHisto, thresholdSize);
             if (doGreedy)
             {
                 HistogramCombineGreedy(imageHisto);
@@ -303,14 +303,15 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
         /// Perform histogram aggregation using a stochastic approach.
         /// </summary>
         /// <returns>true if a greedy approach needs to be performed afterwards, false otherwise.</returns>
-        private static bool HistogramCombineStochastic(List<Vp8LHistogram> histograms, ref int numUsed, int minClusterSize)
+        private static bool HistogramCombineStochastic(List<Vp8LHistogram> histograms, int minClusterSize)
         {
             var rand = new Random();
             int triesWithNoSuccess = 0;
+            var numUsed = histograms.Count(h => h != null);
             int outerIters = numUsed;
             int numTriesNoSuccess = outerIters / 2;
 
-            if (histograms.Count < minClusterSize)
+            if (numUsed < minClusterSize)
             {
                 return true;
             }
@@ -333,7 +334,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
                 double bestCost = (histoPriorityList.Count == 0) ? 0.0d : histoPriorityList[0].CostDiff;
                 int bestIdx1 = -1;
                 int bestIdx2 = 1;
-                int numTries = numUsed / 2; // TODO: should that be histogram.Count/2?
+                int numTries = numUsed / 2;
                 uint randRange = (uint)((numUsed - 1) * numUsed);
 
                 // Pick random samples.
@@ -454,16 +455,26 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
 
         private static void HistogramCombineGreedy(List<Vp8LHistogram> histograms)
         {
-            int histoSize = histograms.Count;
+            int histoSize = histograms.Count(h => h != null);
 
             // Priority list of histogram pairs.
             var histoPriorityList = new List<HistogramPair>();
             int maxSize = histoSize * histoSize;
 
-            for (int i = 0; i < histograms.Count; i++)
+            for (int i = 0; i < histoSize; i++)
             {
-                for (int j = i + 1; j < histograms.Count; j++)
+                if (histograms[i] == null)
+                {
+                    continue;
+                }
+
+                for (int j = i + 1; j < histoSize; j++)
                 {
+                    if (histograms[j] == null)
+                    {
+                        continue;
+                    }
+
                     HistoPriorityListPush(histoPriorityList, maxSize, histograms, i, j, 0.0d);
                 }
             }
@@ -496,7 +507,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
                 }
 
                 // Push new pairs formed with combined histogram to the list.
-                for (int i = 0; i < histograms.Count; i++)
+                for (int i = 0; i < histoSize; i++)
                 {
                     if (i == idx1 || histograms[i] == null)
                     {

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

@@ -7,7 +7,6 @@ using System.Buffers.Binary;
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
-using System.Numerics;
 using System.Runtime.CompilerServices;
 
 using SixLabors.ImageSharp.Formats.WebP.BitWriter;
@@ -34,13 +33,13 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
         /// <summary>
         /// The <see cref="MemoryAllocator"/> to use for buffer allocations.
         /// </summary>
-        private MemoryAllocator memoryAllocator;
+        private readonly MemoryAllocator memoryAllocator;
 
         /// <summary>
         /// A bit writer for writing lossless webp streams.
         /// </summary>
         private Vp8LBitWriter bitWriter;
-        
+
         private const int ApplyPaletteGreedyMax = 4;
 
         private const int PaletteInvSizeBits = 11;
@@ -309,9 +308,20 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
                 this.bitWriter.PutBits(0, 1); // No more transforms.
 
                 // Encode and write the transformed image.
-                this.EncodeImage(bgra, this.HashChain, this.Refs, this.CurrentWidth, height, quality, useCache, crunchConfig,
-                    this.CacheBits, this.HistoBits, bytePosition);
+                this.EncodeImage(
+                    bgra,
+                    this.HashChain,
+                    this.Refs,
+                    this.CurrentWidth,
+                    height,
+                    quality,
+                    useCache,
+                    crunchConfig,
+                    this.CacheBits,
+                    this.HistoBits,
+                    bytePosition);
             }
+
             // TODO: Comparison and picking of best (smallest) encoding
         }
 
@@ -336,7 +346,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
             // Try out multiple LZ77 on images with few colors.
             var nlz77s = (this.PaletteSize > 0 && this.PaletteSize <= 16) ? 2 : 1;
             EntropyIx entropyIdx = this.AnalyzeEntropy(image, usePalette, this.PaletteSize, this.TransformBits, out redAndBlueAlwaysZero);
-            
+
             bool doNotCache = false;
             var crunchConfigs = new List<CrunchConfig>();
 
@@ -358,7 +368,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
             else
             {
                 // Only choose the guessed best transform.
-                crunchConfigs.Add(new CrunchConfig {EntropyIdx = entropyIdx});
+                crunchConfigs.Add(new CrunchConfig { EntropyIdx = entropyIdx });
                 if (configQuality >= 75 && method == 5)
                 {
                     // Test with and without color cache.
@@ -367,7 +377,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
                     // If we have a palette, also check in combination with spatial.
                     if (entropyIdx == EntropyIx.Palette)
                     {
-                        crunchConfigs.Add(new CrunchConfig { EntropyIdx = EntropyIx.PaletteAndSpatial});
+                        crunchConfigs.Add(new CrunchConfig { EntropyIdx = EntropyIx.PaletteAndSpatial });
                     }
                 }
             }
@@ -1584,8 +1594,6 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
 
         public void AllocateTransformBuffer(int width, int height)
         {
-            int imageSize = width * height;
-
             // VP8LResidualImage needs room for 2 scanlines of uint32 pixels with an extra
             // pixel in each, plus 2 regular scanlines of bytes.
             int argbScratchSize = this.UsePredictorTransform ? ((width + 1) * 2) + (((width * 2) + 4 - 1) / 4) : 0;
@@ -1620,6 +1628,7 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
         private class CrunchConfig
         {
             public EntropyIx EntropyIdx { get; set; }
+
             public List<CrunchSubConfig> SubConfigs { get; } = new List<CrunchSubConfig>();
         }
 
@@ -1627,9 +1636,8 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossless
         private class CrunchSubConfig
         {
             public int Lz77 { get; set; }
+
             public bool DoNotCache { get; set; }
         }
     }
-
-   
 }