Feature/add indexer

src/ZString.Unity/Assets/Scripts/ZString/Utf16ValueStringBuilder.cs

@@ -46,6 +46,12 @@ static Utf16ValueStringBuilder()
 
         /// <summary>Length of written buffer.</summary>
         public int Length => index;
+
+        /// <summary>Gets a <see cref="char"/> stored under <paramref name="index"/> </summary>
+        [IndexerName("Chars")]
+        // ReSharper disable once ParameterHidesMember - follow naming from StringBuilder to ease migration.
+        public char this[int index] => buffer![index];
+
         /// <summary>Get the written buffer data.</summary>
         public ReadOnlySpan<char> AsSpan() => buffer.AsSpan(0, index);
         /// <summary>Get the written buffer data.</summary>

src/ZString/Utf16ValueStringBuilder.cs

@@ -46,6 +46,12 @@ static Utf16ValueStringBuilder()
 
         /// <summary>Length of written buffer.</summary>
         public int Length => index;
+
+        /// <summary>Gets a <see cref="char"/> stored under <paramref name="index"/> </summary>
+        [IndexerName("Chars")]
+        // ReSharper disable once ParameterHidesMember - follow naming from StringBuilder to ease migration.
+        public char this[int index] => buffer![index];
+
         /// <summary>Get the written buffer data.</summary>
         public ReadOnlySpan<char> AsSpan() => buffer.AsSpan(0, index);
         /// <summary>Get the written buffer data.</summary>

tests/ZString.Tests/StringBuilderTest.cs

@@ -1,6 +1,6 @@
+using System;
 using Cysharp.Text;
 using FluentAssertions;
-using System.Text;
 using Xunit;
 
 namespace ZStringTests
@@ -22,5 +22,106 @@ public void Utf8DisposeTest()
             sb.Dispose();
             sb.Dispose(); // call more than once
         }
+
+        [Theory]
+        [InlineData("cucu", "mber")]
+        [InlineData("Choco", "pie")]
+        public void Append_WhenCalled_IsCorrect(string a, string b)
+        {
+            using var sut = ZString.CreateStringBuilder();
+            sut.Append(a);
+            sut.Append(b);
+
+            var result = sut.ToString();
+            result.Should().Be($"{a}{b}");
+        }
+
+        [Theory]
+        [InlineData("cucu", "mber")]
+        [InlineData("Choco", "pie")]
+        public void Append_WhenChars_IsCorrect(string species, string b)
+        {
+            using var sut = ZString.CreateStringBuilder();
+            sut.Append(species);
+            var chars = new ReadOnlySpan<char>(b.ToCharArray());
+            sut.Append(chars);
+
+            var result = sut.ToString();
+            result.Should().Be($"{species}{b}");
+        }
+
+        [Theory, InlineData("choco", "apple", "pie")]
+        public void Replace_WhenCalled_IsCorrect(string species, string alternative, string mainPart)
+        {
+            using var sut = ZString.CreateStringBuilder();
+            sut.Append(species);
+            sut.Append(mainPart);
+            sut.Replace(species, alternative);
+
+            var result = sut.ToString();
+            result.Should().Be($"{alternative}{mainPart}");
+        }
+
+        [Theory, InlineData("choco", "apple", "pie")]
+        public void Insert_WhenCalled_IsCorrect(string species, string alternative, string mainPart)
+        {
+            using var sut = ZString.CreateStringBuilder();
+            sut.Append(species);
+            sut.Append(mainPart);
+            sut.Insert(species.Length, alternative);
+
+            var result = sut.ToString();
+            result.Should().Be($"{species}{alternative}{mainPart}");
+        }
+
+        [Theory]
+        [InlineData("cat")]
+        [InlineData("dog")]
+        public void IndexerAlike_WhenCalled_IsCorrect(string body)
+        {
+            using var sut = ZString.CreateStringBuilder();
+            sut.Append(body);
+
+            var letter = sut.AsSpan()[1];
+            letter.Should().Be(body[1]);
+        }
+
+        [Theory]
+        [InlineData("cat")]
+        [InlineData("dog")]
+        public void IndexerAlike_WhenCalledOnLast_IsCorrect(string body)
+        {
+            using var sut = ZString.CreateStringBuilder();
+            sut.Append(body);
+
+            var index = body.Length - 1;
+            var letter = sut.AsSpan()[index];
+            letter.Should().Be(body[index]);
+        }
+
+        [Theory]
+        [InlineData("cat")]
+        [InlineData("dog")]
+        public void Indexer_WhenCalled_IsCorrect(string body)
+        {
+            using var sut = ZString.CreateStringBuilder();
+            sut.Append(body);
+
+            var letter = sut[1];
+            letter.Should().Be(body[1]);
+        }
+
+        [Theory]
+        [InlineData("cat")]
+        [InlineData("dog")]
+        public void Indexer_WhenCalledOnLast_IsCorrect(string body)
+        {
+            using var sut = ZString.CreateStringBuilder();
+            sut.Append(body);
+
+            var index = body.Length - 1;
+            var letter = sut[index];
+            letter.Should().Be(body[index]);
+        }        
     }
 }