use lazy loading for glyph data

glyph data in TrueType fonts can be very large and slow to parse. to avoid this we store the raw table data at parsing time and enable lazy loading of glyph descriptions.

src/UglyToad.PdfPig.Fonts/TrueType/Tables/GlyphDataTable.cs

@@ -13,35 +13,69 @@
     /// </summary>
     internal class GlyphDataTable : ITrueTypeTable
     {
+        private readonly IReadOnlyList<uint> glyphOffsets;
+        private readonly PdfRectangle maxGlyphBounds;
+        private readonly TrueTypeDataBytes tableBytes;
+
         /// <inheritdoc />
         public string Tag => TrueTypeHeaderTable.Glyf;
 
         /// <inheritdoc />
         public TrueTypeHeaderTable DirectoryTable { get; }
 
-        public IReadOnlyList<IGlyphDescription> Glyphs { get; }
+        private readonly Lazy<IReadOnlyList<IGlyphDescription>> glyphs;
+        public IReadOnlyList<IGlyphDescription> Glyphs => glyphs.Value;
 
-        public GlyphDataTable(TrueTypeHeaderTable directoryTable, IReadOnlyList<IGlyphDescription> glyphs)
+        public GlyphDataTable(TrueTypeHeaderTable directoryTable, IReadOnlyList<uint> glyphOffsets, 
+            PdfRectangle maxGlyphBounds, 
+            TrueTypeDataBytes tableBytes)
         {
+            this.glyphOffsets = glyphOffsets;
+            this.maxGlyphBounds = maxGlyphBounds;
+            this.tableBytes = tableBytes;
             DirectoryTable = directoryTable;
-            Glyphs = glyphs ?? throw new ArgumentNullException(nameof(glyphs));
+            if (tableBytes == null)
+            {
+                throw new ArgumentNullException(nameof(tableBytes));
+            }
+
+            if (glyphOffsets == null)
+            {
+                throw new ArgumentNullException(nameof(glyphOffsets));
+            }
+
+            if (tableBytes.Length != directoryTable.Length)
+            {
+                throw new ArgumentException($"glyf table data should match length of directory entry. Expected: {directoryTable.Length}. Actual: {tableBytes.Length}.");
+            }
+
+            glyphs = new Lazy<IReadOnlyList<IGlyphDescription>>(ReadGlyphs);
         }
 
         public static GlyphDataTable Load(TrueTypeDataBytes data, TrueTypeHeaderTable table, TableRegister.Builder tableRegister)
         {
             data.Seek(table.Offset);
-            
-            var indexToLocationTable = tableRegister.IndexToLocationTable;
 
-            var offsets = indexToLocationTable.GlyphOffsets;
+            var bytes = data.ReadByteArray((int)table.Length);
+
+            return new GlyphDataTable(table, tableRegister.IndexToLocationTable.GlyphOffsets, 
+                tableRegister.HeaderTable.Bounds,
+                new TrueTypeDataBytes(bytes));
+        }
+
+        private IReadOnlyList<IGlyphDescription> ReadGlyphs()
+        {
+            var data = tableBytes;
+
+            var offsets = glyphOffsets;
 
             var entryCount = offsets.Count;
 
             var glyphCount = entryCount - 1;
 
-            var glyphs = new IGlyphDescription[glyphCount];
+            var result = new IGlyphDescription[glyphCount];
 
-            var emptyGlyph = Glyph.Empty(tableRegister.HeaderTable.Bounds);
+            var emptyGlyph = Glyph.Empty(maxGlyphBounds);
 
             var compositeLocations = new Dictionary<int, TemporaryCompositeLocation>();
 
@@ -50,11 +84,11 @@
                 if (offsets[i] == offsets[i + 1])
                 {
                     // empty glyph
-                    glyphs[i] = emptyGlyph;
+                    result[i] = emptyGlyph;
                     continue;
                 }
 
-                data.Seek(offsets[i] + table.Offset);
+                data.Seek(offsets[i]);
 
                 var contourCount = data.ReadSignedShort();
 
@@ -62,27 +96,27 @@
                 var minY = data.ReadSignedShort();
                 var maxX = data.ReadSignedShort();
                 var maxY = data.ReadSignedShort();
-                
+
                 var bounds = new PdfRectangle(minX, minY, maxX, maxY);
-                
+
                 // If the number of contours is greater than or equal zero it's a simple glyph.
                 if (contourCount >= 0)
                 {
-                    glyphs[i] = ReadSimpleGlyph(data, contourCount, bounds);
+                    result[i] = ReadSimpleGlyph(data, contourCount, bounds);
                 }
                 else
                 {
-                    compositeLocations.Add(i , new TemporaryCompositeLocation(data.Position, bounds, contourCount));
+                    compositeLocations.Add(i, new TemporaryCompositeLocation(data.Position, bounds, contourCount));
                 }
             }
 
             // Build composite glyphs by combining simple and other composite glyphs.
             foreach (var compositeLocation in compositeLocations)
             {
-                glyphs[compositeLocation.Key] = ReadCompositeGlyph(data, compositeLocation.Value, compositeLocations, glyphs, emptyGlyph);
+                result[compositeLocation.Key] = ReadCompositeGlyph(data, compositeLocation.Value, compositeLocations, result, emptyGlyph);
             }
-            
+
-            return new GlyphDataTable(table, glyphs);
+            return result;
         }
 
         private static Glyph ReadSimpleGlyph(TrueTypeDataBytes data, short contourCount, PdfRectangle bounds)

src/UglyToad.PdfPig.Fonts/TrueType/TrueTypeDataBytes.cs

@@ -31,6 +31,11 @@
         /// </summary>
         public long Position => inputBytes.CurrentOffset;
 
+        /// <summary>
+        /// The length of the data in bytes.
+        /// </summary>
+        public long Length => inputBytes.Length;
+
         /// <summary>
         /// Read a 32-fixed floating point value.
         /// </summary>