refactor: Don't write char for char

Author: linkdotnet

CHANGELOG.md

@@ -6,6 +6,9 @@ All notable changes to **ValueStringBuilder** will be documented in this file. T
 
 ## [Unreleased]
 
+### Changed
+- Refactored `AppendFormat` to be faster especially for longer text.
+
 ## [1.11] - 2023-01-01
 
 ### Added

src/LinkDotNet.StringBuilder/ValueStringBuilder.AppendFormat.cs

@@ -19,6 +19,7 @@ public void AppendFormat<T>(
         T arg)
     {
         var formatIndex = 0;
+        var start = 0;
         while (formatIndex < format.Length)
         {
             var c = format[formatIndex];
@@ -31,19 +32,29 @@ public void AppendFormat<T>(
                     return;
                 }
 
+                if (start != formatIndex)
+                {
+                    Append(format[start..formatIndex]);
+                }
+
                 var placeholder = format.Slice(formatIndex, endIndex + 2);
 
                 GetValidArgumentIndex(placeholder, 0);
 
                 AppendInternal(arg);
                 formatIndex += endIndex + 2;
+                start = formatIndex;
             }
             else
             {
-                Append(c);
                 formatIndex++;
             }
         }
+
+        if (start != formatIndex)
+        {
+            Append(format[start..formatIndex]);
+        }
     }
 
     /// <summary>
@@ -64,6 +75,7 @@ public void AppendFormat<T1, T2>(
         T2 arg2)
     {
         var formatIndex = 0;
+        var start = 0;
         while (formatIndex < format.Length)
         {
             var c = format[formatIndex];
@@ -76,6 +88,11 @@ public void AppendFormat<T1, T2>(
                     return;
                 }
 
+                if (start != formatIndex)
+                {
+                    Append(format[start..formatIndex]);
+                }
+
                 var placeholder = format.Slice(formatIndex, endIndex + 2);
 
                 var index = GetValidArgumentIndex(placeholder, 1);
@@ -91,13 +108,18 @@ public void AppendFormat<T1, T2>(
                 }
 
                 formatIndex += endIndex + 2;
+                start = formatIndex;
             }
             else
             {
-                Append(c);
                 formatIndex++;
             }
         }
+
+        if (start != formatIndex)
+        {
+            Append(format[start..formatIndex]);
+        }
     }
 
     /// <summary>
@@ -121,6 +143,7 @@ public void AppendFormat<T1, T2, T3>(
         T3 arg3)
     {
         var formatIndex = 0;
+        var start = 0;
         while (formatIndex < format.Length)
         {
             var c = format[formatIndex];
@@ -133,6 +156,11 @@ public void AppendFormat<T1, T2, T3>(
                     return;
                 }
 
+                if (start != formatIndex)
+                {
+                    Append(format[start..formatIndex]);
+                }
+
                 var placeholder = format.Slice(formatIndex, endIndex + 2);
 
                 var index = GetValidArgumentIndex(placeholder, 2);
@@ -151,13 +179,18 @@ public void AppendFormat<T1, T2, T3>(
                 }
 
                 formatIndex += endIndex + 2;
+                start = formatIndex;
             }
             else
             {
-                Append(c);
                 formatIndex++;
             }
         }
+
+        if (start != formatIndex)
+        {
+            Append(format[start..formatIndex]);
+        }
     }
 
     private static int GetValidArgumentIndex(ReadOnlySpan<char> placeholder, int allowedRange)