Replace indexed span iteration with foreach (+performance)

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

@@ -149,9 +149,8 @@ internal static class BackwardReferenceEncoder
         }
 
         // Find the cacheBits giving the lowest entropy.
-        for (int idx = 0; idx < refs.Count; idx++)
+        foreach (PixOrCopy v in refs)
         {
-            PixOrCopy v = refs[idx];
             if (v.IsLiteral())
             {
                 uint pix = bgra[pos++];
@@ -780,9 +779,8 @@ internal static class BackwardReferenceEncoder
     {
         int pixelIndex = 0;
         ColorCache colorCache = new(cacheBits);
-        for (int idx = 0; idx < refs.Count; idx++)
+        foreach (ref PixOrCopy v in refs)
         {
-            PixOrCopy v = refs[idx];
             if (v.IsLiteral())
             {
                 uint bgraLiteral = v.BgraOrDistance;
@@ -790,7 +788,7 @@ internal static class BackwardReferenceEncoder
                 if (ix >= 0)
                 {
                     // Color cache contains bgraLiteral
-                    refs[idx] = PixOrCopy.CreateCacheIdx(ix);
+                    v = PixOrCopy.CreateCacheIdx(ix);
                 }
                 else
                 {
@@ -812,15 +810,13 @@ internal static class BackwardReferenceEncoder
 
     private static void BackwardReferences2DLocality(int xSize, Vp8LBackwardRefs refs)
     {
-        for (int idx = 0; idx < refs.Count; idx++)
+        foreach (ref PixOrCopy v in refs)
         {
-            PixOrCopy v = refs[idx];
-
             if (v.IsCopy())
             {
                 int dist = (int)v.BgraOrDistance;
                 int transformedDist = DistanceToPlaneCode(xSize, dist);
-                refs[idx] = PixOrCopy.CreateCopy((uint)transformedDist, v.Len);
+                v = PixOrCopy.CreateCopy((uint)transformedDist, v.Len);
             }
         }
     }

src/ImageSharp/Formats/Webp/Lossless/CostModel.cs

@@ -40,9 +40,9 @@ internal class CostModel
         using OwnedVp8LHistogram histogram = OwnedVp8LHistogram.Create(this.memoryAllocator, cacheBits);
 
         // The following code is similar to HistogramCreate but converts the distance to plane code.
-        for (int i = 0; i < backwardRefs.Count; i++)
+        foreach (PixOrCopy v in backwardRefs)
         {
-            histogram.AddSinglePixOrCopy(backwardRefs[i], true, xSize);
+            histogram.AddSinglePixOrCopy(v, true, xSize);
         }
 
         ConvertPopulationCountTableToBitEstimates(histogram.NumCodes(), histogram.Literal, this.Literal);

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

@@ -110,9 +110,8 @@ internal static class HistogramEncoder
         int x = 0, y = 0;
         int histoXSize = LosslessUtils.SubSampleSize(xSize, histoBits);
 
-        for (int i = 0; i < backwardRefs.Count; i++)
+        foreach (PixOrCopy v in backwardRefs)
         {
-            PixOrCopy v = backwardRefs[i];
             int ix = ((y >> histoBits) * histoXSize) + (x >> histoBits);
             histograms[ix].AddSinglePixOrCopy(v, false);
             x += v.Len;
@@ -217,7 +216,7 @@ internal static class HistogramEncoder
             clusterMappings[idx] = (ushort)idx;
         }
 
-        List<int> indicesToRemove = new();
+        List<int> indicesToRemove = [];
         Vp8LStreaks stats = new();
         Vp8LBitEntropy bitsEntropy = new();
         for (int idx = 0; idx < histograms.Count; idx++)
@@ -345,7 +344,7 @@ internal static class HistogramEncoder
 
         // Priority list of histogram pairs. Its size impacts the quality of the compression and the speed:
         // the smaller the faster but the worse for the compression.
-        List<HistogramPair> histoPriorityList = new();
+        List<HistogramPair> histoPriorityList = [];
         const int maxSize = 9;
 
         // Fill the initial mapping.
@@ -480,7 +479,7 @@ internal static class HistogramEncoder
         int histoSize = histograms.Count(h => h != null);
 
         // Priority list of histogram pairs.
-        List<HistogramPair> histoPriorityList = new();
+        List<HistogramPair> histoPriorityList = [];
         int maxSize = histoSize * histoSize;
         Vp8LStreaks stats = new();
         Vp8LBitEntropy bitsEntropy = new();

src/ImageSharp/Formats/Webp/Lossless/Vp8LBackwardRefs.cs

@@ -9,20 +9,19 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossless;
 internal class Vp8LBackwardRefs : IDisposable
 {
     private readonly IMemoryOwner<PixOrCopy> refs;
+    private int count;
 
     public Vp8LBackwardRefs(MemoryAllocator memoryAllocator, int pixels)
     {
         this.refs = memoryAllocator.Allocate<PixOrCopy>(pixels);
-        this.Count = 0;
+        this.count = 0;
     }
 
-    public int Count { get; private set; }
+    public void Add(PixOrCopy pixOrCopy) => this.refs.Memory.Span[this.count++] = pixOrCopy;
 
-    public ref PixOrCopy this[int index] => ref this.refs.Memory.Span[index];
+    public void Clear() => this.count = 0;
 
-    public void Add(PixOrCopy pixOrCopy) => this.refs.Memory.Span[this.Count++] = pixOrCopy;
+    public Span<PixOrCopy>.Enumerator GetEnumerator() => this.refs.Slice(0, this.count).GetEnumerator();
-
-    public void Clear() => this.Count = 0;
 
     /// <inheritdoc/>
     public void Dispose() => this.refs.Dispose();

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

@@ -1058,9 +1058,8 @@ internal class Vp8LEncoder : IDisposable
         int histogramIx = histogramSymbols[0];
         Span<HuffmanTreeCode> codes = huffmanCodes.AsSpan(5 * histogramIx);
 
-        for (int i = 0; i < backwardRefs.Count; i++)
+        foreach (PixOrCopy v in backwardRefs)
         {
-            PixOrCopy v = backwardRefs[i];
             if (tileX != (x & tileMask) || tileY != (y & tileMask))
             {
                 tileX = x & tileMask;

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

@@ -138,9 +138,9 @@ internal abstract unsafe class Vp8LHistogram
     /// <param name="refs">The backward references.</param>
     public void StoreRefs(Vp8LBackwardRefs refs)
     {
-        for (int i = 0; i < refs.Count; i++)
+        foreach (PixOrCopy v in refs)
         {
-            this.AddSinglePixOrCopy(refs[i], false);
+            this.AddSinglePixOrCopy(v, false);
         }
     }