Improve the performance of the RtxtParser. (#9839)

* Improve the performance of the RtxtParser.

There are some small improvements in the RtxtParser that can be done to improve the performance of the parser. This commit includes the following changes:

1. Update the `enum` to be `byte`. This saves us a few bytes per instance.
2. Reorder the fields to reduce padding.
3. Encode is a type is a stylable into the `RType` enum. 
4. Pre-allocate the array so we reduce the number of allocations.

The following table shows the improvements for 10000 items. We have to use a large number to 
see the improvements. Note that the default Maui template has about 7000 entries in the R.txt file. 
So this is not an unreasonable test size.

| Method             | Mean     | Error    | StdDev   | Gen0      | Gen1      | Gen2     | Allocated |
|------------------- |---------:|---------:|---------:|----------:|----------:|---------:|----------:|
| RtxtParserBaseLine | 53.40 ms | 0.315 ms | 0.246 ms | 7500.0000 | 3000.0000 | 300.0000 |  67.32 MB |
| RtxtParser         | 48.58 ms | 0.192 ms | 0.179 ms | 7454.5455 | 2454.5455 | 272.7273 |  60.44 MB |

Author: dellis1972

src/Xamarin.Android.Build.Tasks/Tasks/GenerateResourceDesignerAssembly.cs

@@ -177,6 +177,7 @@ bool Run (DirectoryAssemblyResolver res)
 				foreach (var r in resources) {
 					switch (r.Type) {
 						case RType.Integer:
+						case RType.Integer_Styleable:
 							if (IsApplication)
 								CreateIntField (cache, r.ResourceTypeName, r.Identifier, r.Id, constDesigner, module);
 							CreateIntProperty (cache, r.ResourceTypeName, r.Identifier, r.Id, resourceDesigner, module);

src/Xamarin.Android.Build.Tasks/Utilities/FileResourceParser.cs

@@ -156,7 +156,7 @@ public IList<R> Parse (string resourceDirectory, IEnumerable<string> additionalR
 				foreach (R r in resources[t].OrderBy(x => x.ToSortedString(), StringComparer.Ordinal)) {
 
 					int id = Convert.ToInt32 (itemPackageId + typeid.ToString ("X2") + itemid.ToString ("X4"), fromBase: 16);
-					if (r.Type == RType.Integer && r.Id == -1) {
+					if ((r.Type != RType.Array) && r.Id == -1) {
 						itemid++;
 						r.UpdateId (id);
 					} else {
@@ -352,6 +352,7 @@ resources [field.ResourceTypeName].Add (new R () {
 						ResourceTypeName = field.ResourceTypeName,
 						Identifier = $"{field.Identifier}_{r.Replace (":", "_")}",
 						Id = id++,
+						Type = RType.Integer_Styleable,
 					});
 				}
 			}

src/Xamarin.Android.Build.Tasks/Utilities/ManagedResourceParser.cs

@@ -295,6 +295,7 @@ void ProcessRtxtFile (string file)
 				var cl = CreateClass (r.ResourceTypeName);
 				switch (r.Type) {
 					case RType.Integer:
+					case RType.Integer_Styleable:
 						CreateIntField (cl, r.Identifier, r.Id);
 						break;
 					case RType.Array:

src/Xamarin.Android.Build.Tasks/Utilities/RtxtParser.cs

@@ -5,35 +5,35 @@
 using System.Text.RegularExpressions;
 using Microsoft.Android.Build.Tasks;
 using Microsoft.Build.Utilities;
+using System.Runtime.InteropServices;
 
 namespace Xamarin.Android.Tasks
 {
-	public enum RType {
+	public enum RType : byte {
 		Integer,
+		Integer_Styleable,
 		Array,
 	}
 
-	public enum ResourceType {
+	public enum ResourceType : byte {
 		System,
 		Custom,
 	}
 
+	[StructLayout(LayoutKind.Sequential)]
 	public struct R : IComparable<R> {
-		public RType Type;
 		public int Id;
-		public int [] Ids;
+		public RType Type;
+		public ResourceType ResourceType;
 		public string Identifier;
 		public string ResourceTypeName;
-		public ResourceType ResourceType;
-
+		public int [] Ids;
 		public string Key => $"{ResourceTypeName}:{Identifier}";
 
 		public override string ToString ()
 		{
-			if (Type == RType.Integer) {
-				if (ResourceTypeName == "styleable")
-					return $"int {ResourceTypeName} {Identifier} {Id}";
-				return $"int {ResourceTypeName} {Identifier} 0x{Id.ToString ("x8")}";
+			if (Type != RType.Array) {
+				return $"int {ResourceTypeName} {Identifier} { (Type == RType.Integer ? $"0x{Id.ToString ("x8")}" : $"{Id}")}";
 			}
 			return $"int[] {ResourceTypeName} {Identifier} {{ {String.Join (", ", Ids.Select (x => $"0x{x.ToString ("x8")}"))} }}";
 		}
@@ -60,7 +60,7 @@ public void UpdateIds (int [] newIds)
 	}
 
 	public class RtxtParser {
-
+		static readonly int DefaultCapacity = 10000; // a Maui template project has 7000+ resource entires.
 		static readonly char[] EmptyChar = new char [] { ' ' };
 		static readonly char[] CurlyBracketsChar = new char [] { '{', '}' };
 		static readonly char[] CommaChar = new char [] { ',' };
@@ -98,16 +98,16 @@ public IEnumerable<R> Parse (string file, TaskLoggingHelper logger, Dictionary<s
 		{
 			log = logger;
 			map = mapping;
-			var result = new List<R> ();
+			var result = new List<R> (DefaultCapacity);
 			if (File.Exists (file))
 				ProcessRtxtFile (file, result);
 			return result;
 		}
 
-		void ProcessRtxtFile (string file, IList<R> result)
+		void ProcessRtxtFile (string file, List<R> result)
 		{
 			int lineNumber = 0;
-			foreach (var line in File.ReadLines (file)) {
+			foreach (var line in File.ReadLines (file) ) {
 				lineNumber++;
 				var items = line.Split (EmptyChar, 4);
 				if (items.Length < 4) {
@@ -124,6 +124,7 @@ void ProcessRtxtFile (string file, IList<R> result)
 					if (items [1] != "styleable") {
 						result.Add (new R () {
 							ResourceTypeName = items [1],
+							Type = RType.Integer,
 							Identifier = itemName,
 							Id = value,
 						});
@@ -133,6 +134,7 @@ void ProcessRtxtFile (string file, IList<R> result)
 						case "int":
 							result.Add (new R () {
 								ResourceTypeName = items [1],
+								Type = RType.Integer_Styleable,
 								Identifier = itemName,
 								Id = Convert.ToInt32 (items [3].Trim (), 10),
 							});