Migrate existing code

System/IO/Paths.cs

@@ -0,0 +1,113 @@
+#if !(LINUX || MACOS || WINDOWS)
+#define TARGET_ANY
+#endif
+
+#if TARGET_ANY || LINUX
+#define TARGET_LINUX
+#endif
+#if TARGET_ANY || MACOS
+#define TARGET_MACOS
+#endif
+#if TARGET_ANY || WINDOWS
+#define TARGET_WINDOWS
+#endif
+
+using System.Buffers;
+
+using CommunityToolkit.HighPerformance.Buffers;
+
+namespace System.IO;
+
+public static class PathEx
+{
+	public static string ExpandPath(string path)
+	{
+#if TARGET_ANY
+		if (OperatingSystem.IsLinux() || OperatingSystem.IsMacOS())
+		{
+#endif
+#if TARGET_LINUX || TARGET_MACOS
+			return ExpandPathUnixImpl(path);
+#endif
+#if TARGET_ANY
+		}
+		else if (OperatingSystem.IsWindows())
+		{
+#endif
+#if TARGET_WINDOWS
+			return ExpandPathWindowsImpl(path);
+#endif
+#if TARGET_ANY
+		}
+		else
+		{
+			throw new PlatformNotSupportedException();
+		}
+#endif
+	}
+
+#if TARGET_LINUX || TARGET_MACOS
+	private static string ExpandPathUnixImpl(string path)
+	{
+		if (string.IsNullOrWhiteSpace(path))
+		{
+			return path;
+		}
+
+		var reader = new SequenceReader<char>(new(path.AsMemory()));
+		if (!reader.TryReadTo(out ReadOnlySpan<char> read, '$'))
+		{
+			return path;
+		}
+
+		using ArrayPoolBufferWriter<char> result = new();
+		while (true)
+		{
+			result.Write(read);
+
+			int skip = 0;
+			if (reader.UnreadSpan[0] == '{' && reader.UnreadSpan.IndexOf('}') is not -1 and int s)
+			{
+				read = reader.UnreadSpan[1..s];
+				skip = s + 1;
+			}
+			else
+			{
+				var propertyReader = new SequenceReader<char>(reader.UnreadSequence);
+				while (!propertyReader.End && propertyReader.UnreadSpan[0] is char c && (c == '_' || char.IsAsciiLetterOrDigit(c)))
+				{
+					propertyReader.Advance(1);
+				}
+
+				read = reader.UnreadSpan[..(int)propertyReader.Consumed];
+				skip = read.Length;
+			}
+
+			if (skip != 0 && Environment.GetEnvironmentVariable(read.ToString()) is { } env)
+			{
+				result.Write(env);
+				reader.Advance(skip);
+			}
+			else
+			{
+				result.Write(['$']);
+			}
+
+			if (!reader.TryReadTo(out read, '$'))
+			{
+				break;
+			}
+		}
+
+		result.Write(reader.UnreadSpan);
+		return result.WrittenSpan.ToString();
+	}
+#endif
+
+#if TARGET_WINDOWS
+	private static string ExpandPathWindowsImpl(string path)
+	{
+		return Environment.ExpandEnvironmentVariables(path);
+	}
+#endif
+}