Rework SnakeCase/KebabCase naming policies to closer match Json.NET's

src/libraries/System.Text.Json/Common/JsonSeparatorNamingPolicy.cs

@@ -2,7 +2,9 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Buffers;
+using System.Diagnostics;
 using System.Globalization;
+using System.Runtime.CompilerServices;
 
 namespace System.Text.Json
 {
@@ -11,8 +13,13 @@ internal abstract class JsonSeparatorNamingPolicy : JsonNamingPolicy
         private readonly bool _lowercase;
         private readonly char _separator;
 
-        internal JsonSeparatorNamingPolicy(bool lowercase, char separator) =>
-            (_lowercase, _separator) = (lowercase, separator);
+        internal JsonSeparatorNamingPolicy(bool lowercase, char separator)
+        {
+            Debug.Assert(char.IsPunctuation(separator));
+
+            _lowercase = lowercase;
+            _separator = separator;
+        }
 
         public sealed override string ConvertName(string name)
         {
@@ -21,149 +28,151 @@ public sealed override string ConvertName(string name)
                 ThrowHelper.ThrowArgumentNullException(nameof(name));
             }
 
-            // Rented buffer 20% longer that the input.
-            int rentedBufferLength = (12 * name.Length) / 10;
-            char[]? rentedBuffer = rentedBufferLength > JsonConstants.StackallocCharThreshold
-                ? ArrayPool<char>.Shared.Rent(rentedBufferLength)
-                : null;
+            return ConvertNameCore(_separator, _lowercase, name.AsSpan());
+        }
 
-            int resultUsedLength = 0;
-            Span<char> result = rentedBuffer is null
+        private static string ConvertNameCore(char separator, bool lowercase, ReadOnlySpan<char> chars)
+        {
+            char[]? rentedBuffer = null;
+
+            // While we can't predict the expansion factor of the resultant string,
+            // start with a buffer that is at least 20% larger than the input.
+            int initialBufferLength = (int)(1.2 * chars.Length);
+            Span<char> destination = initialBufferLength <= JsonConstants.StackallocCharThreshold
                 ? stackalloc char[JsonConstants.StackallocCharThreshold]
-                : rentedBuffer;
+                : (rentedBuffer = ArrayPool<char>.Shared.Rent(initialBufferLength));
 
-            void ExpandBuffer(ref Span<char> result)
-            {
-                char[] newBuffer = ArrayPool<char>.Shared.Rent(result.Length * 2);
+            SeparatorState state = SeparatorState.NotStarted;
+            int charsWritten = 0;
 
-                result.CopyTo(newBuffer);
+            for (int i = 0; i < chars.Length; i++)
+            {
+                char current = chars[i];
 
-                if (rentedBuffer is not null)
+                switch (char.GetUnicodeCategory(current))
                 {
-                    result.Slice(0, resultUsedLength).Clear();
-                    ArrayPool<char>.Shared.Return(rentedBuffer);
-                }
+                    case UnicodeCategory.UppercaseLetter:
 
-                rentedBuffer = newBuffer;
-                result = rentedBuffer;
-            }
+                        switch (state)
+                        {
+                            case SeparatorState.NotStarted:
+                                break;
+
+                            case SeparatorState.LowercaseLetterOrDigit:
+                            case SeparatorState.SpaceSeparator:
+                                // An uppercase letter following a sequence of lowercase letters or spaces
+                                // denotes the start of a new grouping: emit a separator character.
+                                WriteChar(separator, ref destination);
+                                break;
+
+                            case SeparatorState.UppercaseLetter:
+                                // We are reading through a sequence of two or more uppercase letters.
+                                // Uppercase letters are grouped together with the exception of the
+                                // final letter, assuming it is followed by lowercase letters.
+                                // For example, the value 'XMLReader' should render as 'xml_reader',
+                                // however 'SHA512Hash' should render as 'sha512-hash'.
+                                if (i + 1 < chars.Length && char.IsLower(chars[i + 1]))
+                                {
+                                    WriteChar(separator, ref destination);
+                                }
+                                break;
+
+                            default:
+                                Debug.Fail($"Unexpected state {state}");
+                                break;
+                        }
 
-            void WriteWord(ReadOnlySpan<char> word, ref Span<char> result)
-            {
-                if (word.IsEmpty)
-                {
-                    return;
-                }
+                        if (lowercase)
+                        {
+                            current = char.ToLowerInvariant(current);
+                        }
 
-                int written;
-                while (true)
-                {
-                    var destinationOffset = resultUsedLength != 0
-                        ? resultUsedLength + 1
-                        : resultUsedLength;
+                        WriteChar(current, ref destination);
+                        state = SeparatorState.UppercaseLetter;
+                        break;
 
-                    if (destinationOffset < result.Length)
-                    {
-                        Span<char> destination = result.Slice(destinationOffset);
+                    case UnicodeCategory.LowercaseLetter:
+                    case UnicodeCategory.DecimalDigitNumber:
 
-                        written = _lowercase
-                            ? word.ToLowerInvariant(destination)
-                            : word.ToUpperInvariant(destination);
+                        if (state is SeparatorState.SpaceSeparator)
+                        {
+                            // Normalize preceding spaces to one separator.
+                            WriteChar(separator, ref destination);
+                        }
 
-                        if (written > 0)
+                        if (!lowercase)
                         {
-                            break;
+                            current = char.ToUpperInvariant(current);
                         }
-                    }
 
-                    ExpandBuffer(ref result);
-                }
+                        WriteChar(current, ref destination);
+                        state = SeparatorState.LowercaseLetterOrDigit;
+                        break;
 
-                if (resultUsedLength != 0)
-                {
-                    result[resultUsedLength] = _separator;
-                    resultUsedLength += 1;
-                }
+                    case UnicodeCategory.SpaceSeparator:
+                        // Space characters are trimmed from the start and end of the input string
+                        // but are normalized to separator characters if between letters.
+                        if (state != SeparatorState.NotStarted)
+                        {
+                            state = SeparatorState.SpaceSeparator;
+                        }
+                        break;
 
-                resultUsedLength += written;
+                    default:
+                        // Non-alphanumeric characters (including the separator character itself)
+                        // are written as-is to the output and reset the separator state.
+                        // E.g. 'ABC???def' maps to 'abc???def' in snake_case.
+
+                        WriteChar(current, ref destination);
+                        state = SeparatorState.NotStarted;
+                        break;
+                }
             }
 
-            int first = 0;
-            ReadOnlySpan<char> chars = name.AsSpan();
-            CharCategory previousCategory = CharCategory.Boundary;
+            string result = destination.Slice(0, charsWritten).ToString();
 
-            for (int index = 0; index < chars.Length; index++)
+            if (rentedBuffer is not null)
             {
-                char current = chars[index];
-                UnicodeCategory currentCategoryUnicode = char.GetUnicodeCategory(current);
-
-                if (currentCategoryUnicode == UnicodeCategory.SpaceSeparator ||
-                    currentCategoryUnicode >= UnicodeCategory.ConnectorPunctuation &&
-                    currentCategoryUnicode <= UnicodeCategory.OtherPunctuation)
-                {
-                    WriteWord(chars.Slice(first, index - first), ref result);
-
-                    previousCategory = CharCategory.Boundary;
-                    first = index + 1;
+                destination.Slice(0, charsWritten).Clear();
+                ArrayPool<char>.Shared.Return(rentedBuffer);
+            }
 
-                    continue;
-                }
+            return result;
 
-                if (index + 1 < chars.Length)
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            void WriteChar(char value, ref Span<char> destination)
+            {
+                if (charsWritten == destination.Length)
                 {
-                    char next = chars[index + 1];
-                    CharCategory currentCategory = currentCategoryUnicode switch
-                    {
-                        UnicodeCategory.LowercaseLetter => CharCategory.Lowercase,
-                        UnicodeCategory.UppercaseLetter => CharCategory.Uppercase,
-                        _ => previousCategory
-                    };
-
-                    if (currentCategory == CharCategory.Lowercase && char.IsUpper(next) ||
-                        next == '_')
-                    {
-                        WriteWord(chars.Slice(first, index - first + 1), ref result);
-
-                        previousCategory = CharCategory.Boundary;
-                        first = index + 1;
-
-                        continue;
-                    }
-
-                    if (previousCategory == CharCategory.Uppercase &&
-                        currentCategoryUnicode == UnicodeCategory.UppercaseLetter &&
-                        char.IsLower(next))
-                    {
-                        WriteWord(chars.Slice(first, index - first), ref result);
-
-                        previousCategory = CharCategory.Boundary;
-                        first = index;
-
-                        continue;
-                    }
-
-                    previousCategory = currentCategory;
+                    ExpandBuffer(ref destination);
                 }
+
+                destination[charsWritten++] = value;
             }
 
-            WriteWord(chars.Slice(first), ref result);
+            void ExpandBuffer(ref Span<char> destination)
+            {
+                int newSize = checked(destination.Length * 2);
+                char[] newBuffer = ArrayPool<char>.Shared.Rent(newSize);
+                destination.CopyTo(newBuffer);
 
-            name = result.Slice(0, resultUsedLength).ToString();
+                if (rentedBuffer is not null)
+                {
+                    destination.Slice(0, charsWritten).Clear();
+                    ArrayPool<char>.Shared.Return(rentedBuffer);
+                }
 
-            if (rentedBuffer is not null)
-            {
-                result.Slice(0, resultUsedLength).Clear();
-                ArrayPool<char>.Shared.Return(rentedBuffer);
+                rentedBuffer = newBuffer;
+                destination = rentedBuffer;
             }
-
-            return name;
         }
 
-        private enum CharCategory
+        private enum SeparatorState
         {
-            Boundary,
-            Lowercase,
-            Uppercase,
+            NotStarted,
+            UppercaseLetter,
+            LowercaseLetterOrDigit,
+            SpaceSeparator,
         }
     }
 }