From 42ba372db201ab91ffcdac8e7c0736e6fc93a515 Mon Sep 17 00:00:00 2001
From: Cyrus Najmabadi <cyrus.najmabadi@gmail.com>
Date: Tue, 5 Nov 2024 14:20:20 -0800
Subject: [PATCH] Have the naming style fixer understand the 'I' pattern for
 interfaces better

---
 .../Tests/NamingStyles/NamingStylesTests.cs   | 40 ++++++++++++++-----
 .../Compiler/Core/NamingStyles/NamingStyle.cs | 21 ++++++----
 2 files changed, 45 insertions(+), 16 deletions(-)

diff --git a/src/Analyzers/CSharp/Tests/NamingStyles/NamingStylesTests.cs b/src/Analyzers/CSharp/Tests/NamingStyles/NamingStylesTests.cs
index ee726f9e1485d..c184e762edd05 100644
--- a/src/Analyzers/CSharp/Tests/NamingStyles/NamingStylesTests.cs
+++ b/src/Analyzers/CSharp/Tests/NamingStyles/NamingStylesTests.cs
@@ -2,8 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-#nullable disable
-
 using System.Collections.Immutable;
 using System.Threading;
 using System.Threading.Tasks;
@@ -21,14 +19,10 @@
 namespace Microsoft.CodeAnalysis.Editor.CSharp.UnitTests.Diagnostics.NamingStyles;
 
 [Trait(Traits.Feature, Traits.Features.NamingStyle)]
-public class NamingStylesTests : AbstractCSharpDiagnosticProviderBasedUserDiagnosticTest_NoEditor
+public sealed class NamingStylesTests(ITestOutputHelper logger)
+    : AbstractCSharpDiagnosticProviderBasedUserDiagnosticTest_NoEditor(logger)
 {
-    public NamingStylesTests(ITestOutputHelper logger)
-       : base(logger)
-    {
-    }
-
-    private static readonly NamingStylesTestOptionSets s_options = new NamingStylesTestOptionSets(LanguageNames.CSharp);
+    private static readonly NamingStylesTestOptionSets s_options = new(LanguageNames.CSharp);
 
     internal override (DiagnosticAnalyzer, CodeFixProvider) CreateDiagnosticProviderAndFixer(Workspace workspace)
         => (new CSharpNamingStyleDiagnosticAnalyzer(), new NamingStyleCodeFixProvider());
@@ -1402,6 +1396,34 @@ internal interface
             """, new TestParameters(options: s_options.InterfaceNamesStartWithI));
     }
 
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/17656")]
+    public async Task TestInterfacesStartWithIOnTypeThatAlreadyStartsWithI1()
+    {
+        await TestInRegularAndScript1Async("""
+            interface [|InputStream|] { }
+            """, """
+            interface IInputStream { }
+            """, new TestParameters(options: s_options.InterfaceNamesStartWithI));
+    }
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/17656")]
+    public async Task TestInterfacesStartWithIOnTypeThatAlreadyStartsWithI2()
+    {
+        await TestInRegularAndScript1Async("""
+            interface [|Stream|] { }
+            """, """
+            interface IStream { }
+            """, new TestParameters(options: s_options.InterfaceNamesStartWithI));
+    }
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/17656")]
+    public async Task TestInterfacesStartWithIOnTypeThatAlreadyStartsWithI3()
+    {
+        await TestMissingInRegularAndScriptAsync("""
+            interface [|IInputStream|] { }
+            """, new TestParameters(options: s_options.InterfaceNamesStartWithI));
+    }
+
 #if CODE_STYLE
     [Fact(Skip = "https://github.com/dotnet/roslyn/issues/42218")]
 #else
diff --git a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/NamingStyles/NamingStyle.cs b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/NamingStyles/NamingStyle.cs
index 9e64f37819f53..57d4d16d1a82e 100644
--- a/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/NamingStyles/NamingStyle.cs
+++ b/src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/NamingStyles/NamingStyle.cs
@@ -427,9 +427,7 @@ private string EnsureSuffix(string name)
         for (var i = Suffix.Length; i > 0; i--)
         {
             if (name.EndsWith(Suffix[..i]))
-            {
                 return name + Suffix[i..];
-            }
         }
 
         return name + Suffix;
@@ -437,15 +435,26 @@ private string EnsureSuffix(string name)
 
     private string EnsurePrefix(string name)
     {
+        // Exceptional cases.  If the name is some interface name (like `InputStream`) and the rule is to have a single
+        // character prefix like "Add `I` for interfaces" don't consider the existing 'I' to be a match of the prefix.
+
+        if (Prefix is [var prefixChar] &&
+            char.IsUpper(prefixChar) &&
+            name is [var nameChar1, var nameChar2, ..] &&
+            prefixChar == nameChar1 &&
+            char.IsLower(nameChar2))
+        {
+            // return IInputStream here, even though InputStream already starts with 'I'.
+            return Prefix + name;
+        }
+
         // If the name already starts with any suffix of the Prefix, only prepend the prefix of
         // the Prefix not contained in the longest such Prefix suffix. For example, if the 
         // required prefix is "catdog_" and the name is "dog_test", then only prepend "cat".
         for (var i = 0; i < Prefix.Length; i++)
         {
             if (name.StartsWith(Prefix[i..]))
-            {
                 return Prefix[..i] + name;
-            }
         }
 
         return Prefix + name;
@@ -480,13 +489,11 @@ public void WriteTo(ObjectWriter writer)
     }
 
     public static NamingStyle ReadFrom(ObjectReader reader)
-    {
-        return new NamingStyle(
+        => new(
             reader.ReadGuid(),
             reader.ReadString(),
             reader.ReadString(),
             reader.ReadString(),
             reader.ReadString(),
             (Capitalization)reader.ReadInt32());
-    }
 }