Optimize the string-like Span APIs for OrdinalIgnoreCase (portable Sp…

…an) (#28239)

* Optimize the string-like Span APIs for OrdinalIgnoreCase (portable Span)

* Addressing PR feedback

src/System.Memory/src/System/MemoryExtensions.Portable.cs

@@ -5,6 +5,7 @@
 using System.Diagnostics;
 using System.Globalization;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 
 namespace System
 {
@@ -40,10 +41,25 @@ public static bool Equals(this ReadOnlySpan<char> span, ReadOnlySpan<char> other
             {
                 return span.SequenceEqual<char>(other);
             }
+            else if (comparisonType == StringComparison.OrdinalIgnoreCase)
+            {
+                if (span.Length != value.Length)
+                    return false;
+                return EqualsOrdinalIgnoreCase(span, value);
+            }
 
             return span.ToString().Equals(other.ToString(), comparisonType);
         }
 
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        private static bool EqualsOrdinalIgnoreCase(ReadOnlySpan<char> span, ReadOnlySpan<char> value)
+        {
+            Debug.Assert(span.Length == value.Length);
+            if (value.Length == 0)  // span.Length == value.Length == 0
+                return true;
+            return (CompareToOrdinalIgnoreCase(span, value) == 0);
+        }
+
         /// <summary>
         /// Compares the specified <paramref name="span"/> and <paramref name="other"/> using the specified <paramref name="comparisonType"/>,
         /// and returns an integer that indicates their relative position in the sort order.
@@ -57,10 +73,60 @@ public static int CompareTo(this ReadOnlySpan<char> span, ReadOnlySpan<char> oth
             {
                 return span.SequenceCompareTo(other);
             }
+            else if (comparisonType == StringComparison.OrdinalIgnoreCase)
+            {
+                return CompareToOrdinalIgnoreCase(span, value);
+            }
 
             return string.Compare(span.ToString(), other.ToString(), comparisonType);
         }
 
+        // Borrowed from https://github.com/dotnet/coreclr/blob/master/src/mscorlib/shared/System/Globalization/CompareInfo.cs#L539
+        private static unsafe int CompareToOrdinalIgnoreCase(ReadOnlySpan<char> strA, ReadOnlySpan<char> strB)
+        {
+            int length = Math.Min(strA.Length, strB.Length);
+            int range = length;
+
+            fixed (char* ap = &MemoryMarshal.GetReference(strA))
+            fixed (char* bp = &MemoryMarshal.GetReference(strB))
+            {
+                char* a = ap;
+                char* b = bp;
+
+                while (length != 0 && (*a <= 0x7F) && (*b <= 0x7F))
+                {
+                    int charA = *a;
+                    int charB = *b;
+
+                    if (charA == charB)
+                    {
+                        a++; b++;
+                        length--;
+                        continue;
+                    }
+
+                    // uppercase both chars - notice that we need just one compare per char
+                    if ((uint)(charA - 'a') <= 'z' - 'a') charA -= 0x20;
+                    if ((uint)(charB - 'a') <= 'z' - 'a') charB -= 0x20;
+
+                    // Return the (case-insensitive) difference between them.
+                    if (charA != charB)
+                        return charA - charB;
+
+                    // Next char
+                    a++; b++;
+                    length--;
+                }
+
+                if (length == 0)
+                    return strA.Length - strB.Length;
+
+                range -= length;
+
+                return string.Compare(strA.Slice(range).ToString(), strB.Slice(range).ToString(), StringComparison.OrdinalIgnoreCase);
+            }
+        }
+
         /// <summary>
         /// Reports the zero-based index of the first occurrence of the specified <paramref name="value"/> in the current <paramref name="span"/>.
         /// <param name="span">The source span.</param>
@@ -175,6 +241,10 @@ public static bool EndsWith(this ReadOnlySpan<char> span, ReadOnlySpan<char> oth
             {
                 return span.EndsWith<char>(other);
             }
+            else if (comparisonType == StringComparison.OrdinalIgnoreCase)
+            {
+                return value.Length <= span.Length && EqualsOrdinalIgnoreCase(span.Slice(span.Length - value.Length), value);
+            }
 
             string sourceString = span.ToString();
             string valueString = other.ToString();
@@ -193,6 +263,10 @@ public static bool StartsWith(this ReadOnlySpan<char> span, ReadOnlySpan<char> o
             {
                 return span.StartsWith<char>(other);
             }
+            else if (comparisonType == StringComparison.OrdinalIgnoreCase)
+            {
+                return value.Length <= span.Length && EqualsOrdinalIgnoreCase(span.Slice(0, value.Length), value);
+            }
 
             string sourceString = span.ToString();
             string valueString = other.ToString();

src/System.Memory/tests/ReadOnlySpan/CompareTo.cs

@@ -115,9 +115,12 @@ public static void CompareToNoMatch_StringComparison()
                     var secondSpan = new ReadOnlySpan<char>(second);
                     Assert.True(0 > firstSpan.CompareTo(secondSpan, StringComparison.Ordinal));
 
+                    // Due to differences in the implementation, the exact result of CompareTo will not necessarily match with string.Compare.
+                    // However, the sign will match, which is what defines correctness.
                     Assert.Equal(
-                        string.Compare(firstSpan.ToString(), secondSpan.ToString(), StringComparison.OrdinalIgnoreCase),
-                        firstSpan.CompareTo(secondSpan, StringComparison.OrdinalIgnoreCase));
+                        Math.Sign(string.Compare(firstSpan.ToString(), secondSpan.ToString(), StringComparison.OrdinalIgnoreCase)),
+                        Math.Sign(firstSpan.CompareTo(secondSpan, StringComparison.OrdinalIgnoreCase)));
+
                     Assert.Equal(
                         string.Compare(firstSpan.ToString(), secondSpan.ToString(), StringComparison.CurrentCulture),
                         firstSpan.CompareTo(secondSpan, StringComparison.CurrentCulture));