add a bunch more performance improvements

filter provider becomes single instance and no longer has constructor parameters.

tokenizers use list and stringbuilder pools to reduce allocations.

system font finder becomes static to preserve file cache across all documents.

src/UglyToad.PdfPig.Fonts/SystemFonts/SystemFontFinder.cs

@@ -1,4 +1,6 @@
-namespace UglyToad.PdfPig.Fonts.SystemFonts
+using System.Collections.Concurrent;
+
+namespace UglyToad.PdfPig.Fonts.SystemFonts
 {
     using System;
     using System.Collections.Generic;
@@ -14,10 +16,16 @@
     public class SystemFontFinder : ISystemFontFinder
     {
         private static readonly IReadOnlyDictionary<string, string[]> NameSubstitutes;
+        private static readonly Lazy<IReadOnlyList<SystemFontRecord>> AvailableFonts;
 
         private static readonly object CacheLock = new object();
         private static readonly Dictionary<string, TrueTypeFont> Cache = new Dictionary<string, TrueTypeFont>(StringComparer.OrdinalIgnoreCase);
 
+        /// <summary>
+        /// The instance of <see cref="SystemFontFinder"/>.
+        /// </summary>
+        public static readonly ISystemFontFinder Instance = new SystemFontFinder();
+
         static SystemFontFinder()
         {
             var dict = new Dictionary<string, string[]>
@@ -66,18 +74,7 @@
             }
 
             NameSubstitutes = dict;
-        }
 
-        private readonly Lazy<IReadOnlyList<SystemFontRecord>> availableFonts;
-
-        private readonly Dictionary<string, string> nameToFileNameMap = new Dictionary<string, string>(StringComparer.OrdinalIgnoreCase);
-        private readonly HashSet<string> readFiles = new HashSet<string>();
-
-        /// <summary>
-        /// Create a new <see cref="SystemFontFinder"/>.
-        /// </summary>
-        public SystemFontFinder()
-        {
             ISystemFontLister lister;
 #if NETSTANDARD2_0
             if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
@@ -100,7 +97,18 @@
             lister = new WindowsSystemFontLister();
 #endif
 
-            availableFonts = new Lazy<IReadOnlyList<SystemFontRecord>>(() => lister.GetAllFonts().ToList());
+            AvailableFonts = new Lazy<IReadOnlyList<SystemFontRecord>>(() => lister.GetAllFonts().ToList());
+        }
+
+        private readonly ConcurrentDictionary<string, string> nameToFileNameMap = new ConcurrentDictionary<string, string>(StringComparer.OrdinalIgnoreCase);
+        private readonly object readFilesLock = new object();
+        private readonly HashSet<string> readFiles = new HashSet<string>();
+
+        /// <summary>
+        /// Create a new <see cref="SystemFontFinder"/>.
+        /// </summary>
+        private SystemFontFinder()
+        {
         }
 
         /// <inheritdoc />
@@ -179,7 +187,7 @@
                 return null;
             }
 
-            var nameCandidates = availableFonts.Value.Where(x => Path.GetFileName(x.Path)?.StartsWith(name[0].ToString(), StringComparison.OrdinalIgnoreCase) == true);
+            var nameCandidates = AvailableFonts.Value.Where(x => Path.GetFileName(x.Path)?.StartsWith(name[0].ToString(), StringComparison.OrdinalIgnoreCase) == true);
 
             foreach (var systemFontRecord in nameCandidates)
             {
@@ -189,7 +197,7 @@
                 }
             }
 
-            foreach (var record in availableFonts.Value)
+            foreach (var record in AvailableFonts.Value)
             {
                 if (TryGetTrueTypeFont(name, record, out var font))
                 {
@@ -207,9 +215,12 @@
             font = null;
             if (record.Type == SystemFontType.TrueType)
             {
-                if (readFiles.Contains(record.Path))
+                lock (readFilesLock)
                 {
-                    return false;
+                    if (readFiles.Contains(record.Path))
+                    {
+                        return false;
+                    }
                 }
 
                 return TryReadFile(record.Path, true, name, out font);
@@ -221,7 +232,6 @@
         private bool TryReadFile(string fileName, bool readNameFirst, string fontName, out TrueTypeFont font)
         {
             font = null;
-            readFiles.Add(fileName);
 
             var bytes = File.ReadAllBytes(fileName);
 
@@ -233,15 +243,25 @@
 
                 if (name == null)
                 {
+                    lock (readFilesLock)
+                    {
+                        readFiles.Add(fileName);
+                    }
+
                     return false;
                 }
 
                 var fontNameFromFile = name.GetPostscriptName() ?? name.FontName;
 
-                nameToFileNameMap[fontNameFromFile] = fileName;
+                nameToFileNameMap.TryAdd(fontNameFromFile, fileName);
 
                 if (!string.Equals(fontNameFromFile, fontName, StringComparison.OrdinalIgnoreCase))
                 {
+                    lock (readFilesLock)
+                    {
+                        readFiles.Add(fileName);
+                    }
+
                     return false;
                 }
             }
@@ -258,6 +278,11 @@
                 }
             }
 
+            lock (readFilesLock)
+            {
+                readFiles.Add(fileName);
+            }
+
             return true;
         }
     }