Add single row value delegate with buffer

src/ImageSharp/Advanced/IRowAction{TBuffer}.cs

@@ -0,0 +1,88 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.Buffers;
+using System.Runtime.CompilerServices;
+using SixLabors.ImageSharp.Memory;
+
+namespace SixLabors.ImageSharp.Advanced
+{
+    /// <summary>
+    /// Defines the contract for an action that operates on a row with a temporary buffer.
+    /// </summary>
+    /// <typeparam name="TBuffer">The type of buffer elements.</typeparam>
+    public interface IRowAction<TBuffer>
+        where TBuffer : unmanaged
+    {
+        /// <summary>
+        /// Invokes the method passing the row and a buffer.
+        /// </summary>
+        /// <param name="y">The row y coordinate.</param>
+        /// <param name="span">The contiguous region of memory.</param>
+        void Invoke(int y, Span<TBuffer> span);
+    }
+
+    internal readonly struct WrappingRowAction<T, TBuffer>
+        where T : struct, IRowAction<TBuffer>
+        where TBuffer : unmanaged
+    {
+        public readonly int MinY;
+        public readonly int MaxY;
+        public readonly int StepY;
+        public readonly int MaxX;
+
+        private readonly MemoryAllocator allocator;
+        private readonly T action;
+
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public WrappingRowAction(
+            int minY,
+            int maxY,
+            int stepY,
+            MemoryAllocator allocator,
+            in T action)
+            : this(minY, maxY, stepY, 0, allocator, action)
+        {
+        }
+
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public WrappingRowAction(
+            int minY,
+            int maxY,
+            int stepY,
+            int maxX,
+            MemoryAllocator allocator,
+            in T action)
+        {
+            this.MinY = minY;
+            this.MaxY = maxY;
+            this.StepY = stepY;
+            this.MaxX = maxX;
+            this.allocator = allocator;
+            this.action = action;
+        }
+
+        [MethodImpl(InliningOptions.ShortMethod)]
+        public void Invoke(int i)
+        {
+            int yMin = this.MinY + (i * this.StepY);
+
+            if (yMin >= this.MaxY)
+            {
+                return;
+            }
+
+            int yMax = Math.Min(yMin + this.StepY, this.MaxY);
+
+            using IMemoryOwner<TBuffer> buffer = this.allocator.Allocate<TBuffer>(this.MaxX);
+
+            Span<TBuffer> span = buffer.Memory.Span;
+
+            for (int y = yMin; y < yMax; y++)
+            {
+                Unsafe.AsRef(this.action).Invoke(y, span);
+            }
+        }
+    }
+}

src/ImageSharp/Advanced/ParallelRowIterator.cs

@@ -141,6 +141,72 @@ namespace SixLabors.ImageSharp.Advanced
                 rowAction.Invoke);
         }
 
+        /// <summary>
+        /// Iterate through the rows of a rectangle in optimized batches defined by <see cref="RowInterval"/>-s
+        /// instantiating a temporary buffer for each <paramref name="body"/> invocation.
+        /// </summary>
+        /// <typeparam name="T">The type of row action to perform.</typeparam>
+        /// <typeparam name="TBuffer">The type of buffer elements.</typeparam>
+        /// <param name="rectangle">The <see cref="Rectangle"/>.</param>
+        /// <param name="configuration">The <see cref="Configuration"/> to get the parallel settings from.</param>
+        /// <param name="body">The method body defining the iteration logic on a single <see cref="RowInterval"/>.</param>
+        public static void IterateRows2<T, TBuffer>(Rectangle rectangle, Configuration configuration, in T body)
+            where T : struct, IRowAction<TBuffer>
+            where TBuffer : unmanaged
+        {
+            var parallelSettings = ParallelExecutionSettings.FromConfiguration(configuration);
+            IterateRows2<T, TBuffer>(rectangle, in parallelSettings, in body);
+        }
+
+        /// <summary>
+        /// Iterate through the rows of a rectangle in optimized batches defined by <see cref="RowInterval"/>-s
+        /// instantiating a temporary buffer for each <paramref name="body"/> invocation.
+        /// </summary>
+        internal static void IterateRows2<T, TBuffer>(
+            Rectangle rectangle,
+            in ParallelExecutionSettings parallelSettings,
+            in T body)
+            where T : struct, IRowAction<TBuffer>
+            where TBuffer : unmanaged
+        {
+            ValidateRectangle(rectangle);
+
+            int top = rectangle.Top;
+            int bottom = rectangle.Bottom;
+            int width = rectangle.Width;
+            int height = rectangle.Height;
+
+            int maxSteps = DivideCeil(width * height, parallelSettings.MinimumPixelsProcessedPerTask);
+            int numOfSteps = Math.Min(parallelSettings.MaxDegreeOfParallelism, maxSteps);
+            MemoryAllocator allocator = parallelSettings.MemoryAllocator;
+
+            // Avoid TPL overhead in this trivial case:
+            if (numOfSteps == 1)
+            {
+                using (IMemoryOwner<TBuffer> buffer = allocator.Allocate<TBuffer>(width))
+                {
+                    Span<TBuffer> span = buffer.Memory.Span;
+
+                    for (int y = top; y < bottom; y++)
+                    {
+                        Unsafe.AsRef(body).Invoke(y, span);
+                    }
+                }
+
+                return;
+            }
+
+            int verticalStep = DivideCeil(height, numOfSteps);
+            var parallelOptions = new ParallelOptions { MaxDegreeOfParallelism = numOfSteps };
+            var rowAction = new WrappingRowAction<T, TBuffer>(top, bottom, verticalStep, width, allocator, in body);
+
+            Parallel.For(
+                0,
+                numOfSteps,
+                parallelOptions,
+                rowAction.Invoke);
+        }
+
         /// <summary>
         /// Iterate through the rows of a rectangle in optimized batches defined by <see cref="RowInterval"/>-s.
         /// </summary>