do not split surrogates in ParameterInstance.ToDisplayText() (#1705)

Author: novak-oleksandr

src/BenchmarkDotNet/Parameters/ParameterInstance.cs

@@ -71,9 +71,33 @@ private static string Trim(string value, int maxDisplayTextInnerLength)
 
             var postfix = $" [{value.Length}]";
             const string dots = "(...)";
-            int take = (maxDisplayTextInnerLength - postfix.Length - dots.Length) / 2;
 
-            return value.Substring(0, take) + dots + value.Substring(value.Length - take, take) + postfix;
+            var takeFromStart = (maxDisplayTextInnerLength - postfix.Length - dots.Length) / 2;
+            var takeFromEnd = takeFromStart;
+
+            if (IsFirstCharInSurrogatePair(value[takeFromStart-1]))
+            {
+                takeFromStart = Math.Max(0, takeFromStart - 1);
+            }
+
+            if (IsSecondCharInSurrogatePair(value[value.Length - takeFromEnd]))
+            {
+                takeFromEnd = Math.Max(0, takeFromEnd - 1);
+            }
+
+            var result = value.Substring(0, takeFromStart) + dots + value.Substring(value.Length - takeFromEnd, takeFromEnd) + postfix;
+
+            return result;
+        }
+
+        private static bool IsFirstCharInSurrogatePair(char c)
+        {
+            return BitConverter.IsLittleEndian ? char.IsHighSurrogate(c) : char.IsLowSurrogate(c);
+        }
+
+        private static bool IsSecondCharInSurrogatePair(char c)
+        {
+            return BitConverter.IsLittleEndian ? char.IsLowSurrogate(c) : char.IsHighSurrogate(c);
         }
     }
 }

tests/BenchmarkDotNet.IntegrationTests/ParamSourceTests.cs

@@ -0,0 +1,39 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using BenchmarkDotNet.Attributes;
+using Xunit;
+
+namespace BenchmarkDotNet.IntegrationTests
+{
+    public class ParamSourceTests: BenchmarkTestExecutor
+    {
+        [Fact]
+        public void ParamSourceCanHandleStringWithSurrogates()
+        {
+            CanExecute<ParamSourceIsStringWithSurrogates>(CreateSimpleConfig());
+        }
+
+        public class ParamSourceIsStringWithSurrogates
+        {
+            public IEnumerable<string> StringValues
+            {
+                get
+                {
+                    yield return "a" + string.Join("", Enumerable.Repeat("😀", 40)) + "a";
+                    yield return "a" + string.Join("", Enumerable.Repeat("😀", 40));
+                    yield return string.Join("", Enumerable.Repeat("😀", 40)) + "a";
+                    yield return string.Join("", Enumerable.Repeat("😀", 40));
+                }
+            }
+
+            [ParamsSource(nameof(StringValues))]
+            public string _ { get; set; }
+
+            [Benchmark]
+            public void Method() { }
+        }
+    }
+}