fix: Reduce string allocations during SimpleJson.ParseString (#2977)

Noticed this allocation while looking at a profile of solution load in visual studio. These StringBuilder allocations were showing up as 0.5% of total allocations. I took a peek at the code, and reaslized the StringBuilder need not be created unless there is a '\' in the string value being parsed. In that case, just copy already seen characters into a StringBuilder and continue as previously.

Co-authored-by: Nick Floyd <139819+nickfloyd@users.noreply.github.com>

Octokit.Tests/SimpleJsonTests.cs

@@ -0,0 +1,36 @@
+using Octokit;
+using System.Threading.Tasks;
+using Xunit;
+
+public class SimpleJsonTests
+{
+    [Theory]
+    [InlineData("\"abc\"", "abc")]
+    [InlineData(" \"abc\" ", "abc")]
+    [InlineData("\" abc \" ", " abc ")]
+    [InlineData("\"abc\\\"def\"", "abc\"def")]
+    [InlineData("\"abc\\r\\ndef\"", "abc\r\ndef")]
+    public async Task ParseStringSuccess(string input, string expected)
+    {
+        int index = 0;
+        bool success = true;
+
+        string actual = SimpleJson.ParseString(input.ToCharArray(), ref index, ref success);
+
+        Assert.True(success);
+        Assert.Equal(expected, actual);
+    }
+
+    [Theory]
+    [InlineData("\"abc")]
+    public async Task ParseStringIncomplete(string input)
+    {
+        int index = 0;
+        bool success = true;
+
+        string actual = SimpleJson.ParseString(input.ToCharArray(), ref index, ref success);
+
+        Assert.False(success);
+        Assert.Null(actual);
+    }
+}

Octokit/SimpleJson.cs

@@ -792,15 +792,18 @@ static object ParseValue(char[] json, ref int index, ref bool success)
             return null;
         }
 
-        static string ParseString(char[] json, ref int index, ref bool success)
+        internal static string ParseString(char[] json, ref int index, ref bool success)
         {
-            StringBuilder s = new StringBuilder(BUILDER_CAPACITY);
+            // Avoid allocating this StringBuilder unless a backslash is encountered in the json
+            StringBuilder s = null;
             char c;
 
             EatWhitespace(json, ref index);
 
             // "
             c = json[index++];
+
+            int startIndex = index;
             bool complete = false;
             while (!complete)
             {
@@ -815,6 +818,13 @@ static string ParseString(char[] json, ref int index, ref bool success)
                 }
                 else if (c == '\\')
                 {
+                    if (s == null)
+                    {
+                        s = new StringBuilder(BUILDER_CAPACITY);
+                        for (int i = startIndex; i < index - 1; i++)
+                            s.Append(json[i]);
+                    }
+
                     if (index == json.Length)
                         break;
                     c = json[index++];
@@ -875,14 +885,21 @@ static string ParseString(char[] json, ref int index, ref bool success)
                     }
                 }
                 else
-                    s.Append(c);
+                {
+                    if (s != null)
+                        s.Append(c);
+                }
             }
             if (!complete)
             {
                 success = false;
                 return null;
             }
-            return s.ToString();
+
+            if (s != null)
+                return s.ToString();
+
+            return new string(json, startIndex, index - startIndex - 1);
         }
 
         private static string ConvertFromUtf32(int utf32)