fix build errors against old targets

Author: antonfirsov

src/ImageSharp.Drawing/Common/Extensions/NumericExtensions.cs

@@ -0,0 +1,52 @@
+// Copyright (c) Six Labors.
+// Licensed under the Apache License, Version 2.0.
+
+using System.Runtime.CompilerServices;
+
+namespace SixLabors.ImageSharp.Drawing
+{
+    internal static class NumericExtensions
+    {
+        // https://apisof.net/catalog/System.Numerics.BitOperations.Log2(UInt32)
+        // BitOperations.Log2() has been introduced in .NET Core 3.0,
+        // since we do target only 3.1+, we can detect it's presence by using SUPPORTS_RUNTIME_INTRINSICS
+        // TODO: Ideally this should have a separate definition in Build.props, but that adaption should be done cross-repo. Using a workaround until then.
+#if SUPPORTS_RUNTIME_INTRINSICS
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int Log2(uint value) => System.Numerics.BitOperations.Log2(value);
+#else
+        private static System.ReadOnlySpan<byte> Log2DeBruijn => new byte[32]
+        {
+            00, 09, 01, 10, 13, 21, 02, 29,
+            11, 14, 16, 18, 22, 25, 03, 30,
+            08, 12, 20, 28, 15, 17, 24, 07,
+            19, 27, 23, 06, 26, 05, 04, 31
+        };
+
+#pragma warning disable SA1515, SA1414, SA1114
+        // Adapted from:
+        // https://github.com/dotnet/runtime/blob/5c65d891f203618245184fa54397ced0a8ca806c/src/libraries/System.Private.CoreLib/src/System/Numerics/BitOperations.cs#L205-L223
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public static int Log2(uint value)
+        {
+            // No AggressiveInlining due to large method size
+            // Has conventional contract 0->0 (Log(0) is undefined)
+
+            // Fill trailing zeros with ones, eg 00010010 becomes 00011111
+            value |= value >> 01;
+            value |= value >> 02;
+            value |= value >> 04;
+            value |= value >> 08;
+            value |= value >> 16;
+
+            // uint.MaxValue >> 27 is always in range [0 - 31] so we use Unsafe.AddByteOffset to avoid bounds check
+            return Unsafe.AddByteOffset(
+                // Using deBruijn sequence, k=2, n=5 (2^5=32) : 0b_0000_0111_1100_0100_1010_1100_1101_1101u
+                ref System.Runtime.InteropServices.MemoryMarshal.GetReference(Log2DeBruijn),
+                // uint|long -> IntPtr cast on 32-bit platforms does expensive overflow checks not needed here
+                (System.IntPtr)(int)((value * 0x07C4ACDDu) >> 27));
+        }
+#pragma warning restore
+#endif
+    }
+}

src/ImageSharp.Drawing/Shapes/Helpers/TopologyUtilities.cs

@@ -46,7 +46,7 @@ private static int GetPolygonOrientation(ReadOnlySpan<PointF> polygon)
 
             // Normally, this should be a tolerant comparison, we don't have a special path for zero-area
             // (or for self-intersecting, semi-zero-area) polygons in edge scanning.
-            return MathF.Sign(sum);
+            return Math.Sign(sum);
         }
     }
 }

src/ImageSharp.Drawing/Utilities/SortUtility.KeyValueSort.cs

@@ -49,7 +49,7 @@ private static void IntrospectiveSort(Span<float> keys, Span<TValue> values)
             {
                 if (keys.Length > 1)
                 {
-                    IntroSort(keys, values, 2 * (BitOperations.Log2((uint)keys.Length) + 1));
+                    IntroSort(keys, values, 2 * (NumericExtensions.Log2((uint)keys.Length) + 1));
                 }
             }
 
@@ -89,7 +89,11 @@ private static void IntroSort(Span<float> keys, Span<TValue> values, int depthLi
                     int p = PickPivotAndPartition(keys.Slice(0, partitionSize), values.Slice(0, partitionSize));
 
                     // Note we've already partitioned around the pivot and do not have to move the pivot again.
-                    IntroSort(keys[(p + 1) .. partitionSize], values[(p + 1) .. partitionSize], depthLimit);
+                    int s = p + 1;
+                    int l = partitionSize - s;
+
+                    // IntroSort(keys[(p + 1) .. partitionSize], values[(p + 1) .. partitionSize], depthLimit);
+                    IntroSort(keys.Slice(s, l), values.Slice(s, l), depthLimit);
                     partitionSize = p;
                 }
             }

tests/ImageSharp.Drawing.Tests/Drawing/DrawingRobustnessTests.cs

@@ -124,7 +124,6 @@ private Image<Rgba32> FillGeoJsonPolygons(TestImageProvider<Rgba32> provider, st
                 GraphicsOptions = new GraphicsOptions() {Antialias = aa > 0, AntialiasSubpixelDepth = aa},
             };
             var rnd = new Random(42);
-            int i = 0;
             byte[] rgb = new byte[3];
             foreach (PointF[] loop in points)
             {