tidy up truetype tables

* improves the naming of truetype related classes.
* uses correct numeric type for the loca table.
* makes a few related classes public.

src/UglyToad.PdfPig/Fonts/TrueType/Parser/ITrueTypeTableParser.cs

@@ -2,7 +2,7 @@
 {
     using Tables;
 
-    internal interface ITrueTypeTableParser<out T> where T : ITable
+    internal interface ITrueTypeTableParser<out T> where T : ITrueTypeTable
     {
         T Parse(TrueTypeHeaderTable header, TrueTypeDataBytes data, TableRegister.Builder register);
     }

src/UglyToad.PdfPig/Fonts/TrueType/Parser/TableParser.cs

@@ -11,7 +11,7 @@
         private static readonly NameTableParser NameTableParser = new NameTableParser();
         private static readonly Os2TableParser Os2TableParser = new Os2TableParser();
 
-        public static T Parse<T>(TrueTypeHeaderTable table, TrueTypeDataBytes data, TableRegister.Builder register) where T : ITable
+        public static T Parse<T>(TrueTypeHeaderTable table, TrueTypeDataBytes data, TableRegister.Builder register) where T : ITrueTypeTable
         {
             if (typeof(T) == typeof(CMapTable))
             {

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

@@ -7,7 +7,7 @@
     using IO;
     using Util;
 
-    internal class CMapTable : ITable, IWriteable
+    internal class CMapTable : ITrueTypeTable, IWriteable
     {
         public IReadOnlyList<ICMapSubTable> SubTables { get; }
 

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

@@ -11,7 +11,7 @@
     /// The 'glyf' table contains the data that defines the appearance of the glyphs in the font. 
     /// This includes specification of the points that describe the contours that make up a glyph outline and the instructions that grid-fit that glyph.
     /// </summary>
-    internal class GlyphDataTable : ITable
+    internal class GlyphDataTable : ITrueTypeTable
     {
         public string Tag => TrueTypeHeaderTable.Glyf;
 
@@ -34,7 +34,7 @@
 
             var offsets = indexToLocationTable.GlyphOffsets;
 
-            var entryCount = offsets.Length;
+            var entryCount = offsets.Count;
 
             var glyphCount = entryCount - 1;
 

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

@@ -8,7 +8,7 @@
     /// The 'head' table contains global information about the font. 
     /// It contains things like as the font version number, the creation and modification dates, revision number and basic typographic data that applies to the font as a whole.
     /// </summary>
-    internal class HeaderTable : ITable
+    internal class HeaderTable : ITrueTypeTable
     {
         public string Tag => TrueTypeHeaderTable.Head;
         

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

@@ -4,7 +4,7 @@
     /// The 'hhea' table contains information needed to layout fonts whose characters are written horizontally, that is, either left to right or right to left. 
     /// This table contains information that is general to the font as a whole.
     /// </summary>
-    internal class HorizontalHeaderTable : ITable
+    internal class HorizontalHeaderTable : ITrueTypeTable
     {
         public string Tag => TrueTypeHeaderTable.Hhea;
 

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

@@ -8,7 +8,7 @@
     /// <summary>
     /// The 'hmtx' table contains metric information for the horizontal layout each of the glyphs in the font.
     /// </summary>
-    internal class HorizontalMetricsTable : ITable, IWriteable
+    internal class HorizontalMetricsTable : ITrueTypeTable, IWriteable
     {
         public string Tag => TrueTypeHeaderTable.Hmtx;
 

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

@@ -1,9 +0,0 @@
-namespace UglyToad.PdfPig.Fonts.TrueType.Tables
-{
-    internal interface ITable
-    {
-        string Tag { get; }
-
-        TrueTypeHeaderTable DirectoryTable { get; }
-    }
-}

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

@@ -0,0 +1,18 @@
+namespace UglyToad.PdfPig.Fonts.TrueType.Tables
+{
+    /// <summary>
+    /// A table in a TrueType font.
+    /// </summary>
+    public interface ITrueTypeTable
+    {
+        /// <summary>
+        /// The tag, a 4 letter/byte code, used to identify this table in a TrueType font.
+        /// </summary>
+        string Tag { get; }
+
+        /// <summary>
+        /// The directory entry from the font's offset subtable which indicates the length, offset, type and checksum of a table.
+        /// </summary>
+        TrueTypeHeaderTable DirectoryTable { get; }
+    }
+}

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

@@ -1,56 +1,95 @@
 namespace UglyToad.PdfPig.Fonts.TrueType.Tables
 {
     using System;
+    using System.Collections.Generic;
     using System.IO;
+    using Exceptions;
     using IO;
     using Parser;
     using Util;
 
+    /// <inheritdoc cref="ITrueTypeTable"/>
     /// <summary>
-    /// Stores the offset to the glyph locations relative to the start of the glyph data table.
+    /// Stores the offset to the glyph locations relative to the start of the <see cref="GlyphDataTable"/>.
     /// Index zero points to the "missing character" which is used for characters not provided by the font.
-    /// The number of glpyhs in this table should match the maximum profile table.
+    /// The number of glpyhs in this table should match the maximum profile table. The glyph offsets contains
+    /// an extra entry at the last index which points to the end of the glyph data, this makes it possible to compute
+    /// the length of the last glyph entry and supports empty glyphs. 
     /// </summary>
-    internal class IndexToLocationTable : ITable, IWriteable
+    public class IndexToLocationTable : ITrueTypeTable, IWriteable
     {
+        /// <inheritdoc />
         public string Tag => TrueTypeHeaderTable.Loca;
 
+        /// <inheritdoc />
         public TrueTypeHeaderTable DirectoryTable { get; }
 
+        /// <summary>
+        /// Indicates the format the offsets were stored in in the underlying file, for <see cref="EntryFormat.Short"/>
+        /// the values are divided by 2. The values in <see cref="GlyphOffsets"/> are the real offsets, with any format
+        /// changes removed.
+        /// </summary>
         public EntryFormat Format { get; }
 
         /// <summary>
         /// The glyph offsets relative to the start of the glyph data table.
         /// </summary>
-        public long[] GlyphOffsets { get; }
+        public IReadOnlyList<uint> GlyphOffsets { get; }
 
-        public IndexToLocationTable(TrueTypeHeaderTable directoryTable, EntryFormat format, long[] glyphOffsets)
+        /// <summary>
+        /// Create a new <see cref="IndexToLocationTable"/>.
+        /// </summary>
+        public IndexToLocationTable(TrueTypeHeaderTable directoryTable, EntryFormat format, IReadOnlyList<uint> glyphOffsets)
         {
             DirectoryTable = directoryTable;
             Format = format;
-            GlyphOffsets = glyphOffsets;
+            GlyphOffsets = glyphOffsets ?? throw new ArgumentNullException(nameof(glyphOffsets));
         }
 
-        public static IndexToLocationTable Load(TrueTypeDataBytes data, TrueTypeHeaderTable table, TableRegister.Builder tableRegister)
+        /// <summary>
+        /// Load the index to location (loca) table from the TrueType font. Requires the maximum profile (maxp) and header (head) table
+        /// to have been parsed.
+        /// </summary>
+        internal static IndexToLocationTable Load(TrueTypeDataBytes data, TrueTypeHeaderTable table, TableRegister.Builder tableRegister)
         {
+            if (data == null)
+            {
+                throw new ArgumentNullException(nameof(data));
+            }
+
+            if (tableRegister == null)
+            {
+                throw new ArgumentNullException(nameof(tableRegister));
+            }
+
             data.Seek(table.Offset);
 
             var headerTable = tableRegister.HeaderTable;
             var maximumProfileTable = tableRegister.MaximumProfileTable;
 
+            if (headerTable == null)
+            {
+                throw new InvalidFontFormatException("No header (head) table was defined in this font.");
+            }
+
+            if (maximumProfileTable == null)
+            {
+                throw new InvalidFontFormatException("No maximum profile (maxp) table was defined in this font.");
+            }
+
             var format = (EntryFormat)headerTable.IndexToLocFormat;
 
             var glyphCount = maximumProfileTable.NumberOfGlyphs + 1;
 
-            var offsets = new long[glyphCount];
+            var offsets = new uint[glyphCount];
 
             switch (format)
             {
                 case EntryFormat.Short:
                     { // The local offset divided by 2 is stored.
-                        for (int i = 0; i < glyphCount; i++)
+                        for (var i = 0; i < glyphCount; i++)
                         {
-                            offsets[i] = data.ReadUnsignedShort() * 2;
+                            offsets[i] = (uint)(data.ReadUnsignedShort() * 2);
                         }
                         break;
                     }
@@ -68,9 +107,10 @@
             return new IndexToLocationTable(table, format, offsets);
         }
 
+        /// <inheritdoc />
         public void Write(Stream stream)
         {
-            for (var i = 0; i < GlyphOffsets.Length; i++)
+            for (var i = 0; i < GlyphOffsets.Count; i++)
             {
                 var offset = GlyphOffsets[i];
                 switch (Format)
@@ -87,6 +127,9 @@
             }
         }
 
+        /// <summary>
+        /// The format of glyph offset entries stored in the raw TrueType data.
+        /// </summary>
         public enum EntryFormat : short
         {
             /// <summary>

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

@@ -5,7 +5,7 @@
     /// <summary>
     /// This table establishes the memory requirements for the font.
     /// </summary>
-    internal class BasicMaximumProfileTable : ITable
+    internal class BasicMaximumProfileTable : ITrueTypeTable
     {
         public string Tag => TrueTypeHeaderTable.Maxp;
 

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

@@ -4,7 +4,7 @@
     using Names;
     using Util.JetBrains.Annotations;
 
-    internal class NameTable : ITable
+    internal class NameTable : ITrueTypeTable
     {
         public string Tag => TrueTypeHeaderTable.Name;
 

src/UglyToad.PdfPig/Fonts/TrueType/Tables/Os2Table.cs

@@ -9,7 +9,7 @@
     /// <summary>
     /// The most basic format of the OS/2 table, excluding the fields not included in the Apple version of the specification.
     /// </summary>
-    internal class Os2Table : ITable, IWriteable
+    internal class Os2Table : ITrueTypeTable, IWriteable
     {
         public string Tag => TrueTypeHeaderTable.Os2;
 

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

@@ -7,7 +7,7 @@
     /// This table contains information for TrueType fonts on PostScript printers.
     /// This includes data for the FontInfo dictionary and the PostScript glyph names.
     /// </summary>
-    internal class PostScriptTable : ITable
+    internal class PostScriptTable : ITrueTypeTable
     {
         public string Tag => TrueTypeHeaderTable.Post;
 

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

@@ -147,11 +147,11 @@
             return result;
         }
 
-        public void ReadUnsignedIntArray(long[] offsets, int length)
+        public void ReadUnsignedIntArray(uint[] offsets, int length)
         {
-            for (int i = 0; i < length; i++)
+            for (var i = 0; i < length; i++)
             {
-                offsets[i] = ReadUnsignedInt();
+                offsets[i] = (uint)ReadUnsignedInt();
             }
         }
 

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

@@ -6,10 +6,12 @@
     using Util;
     using Util.JetBrains.Annotations;
 
+    /// <inheritdoc cref="IWriteable" />
     /// <summary>
-    /// A table directory entry from the TrueType font file.
+    /// A table directory entry from the TrueType font file. Indicates the position of the corresponding table
+    /// data in the TrueType font.
     /// </summary>
-    internal struct TrueTypeHeaderTable : IWriteable
+    public struct TrueTypeHeaderTable : IWriteable
     {
         #region RequiredTableTags
         /// <summary>
@@ -160,7 +162,11 @@
         #endregion
 
         #region PostScriptTableTags
-
+        /// <summary>
+        /// Compact font format table. The corresponding table contains a Compact Font Format font representation 
+        /// (also known as a PostScript Type 1, or CIDFont).
+        /// </summary>
+        /// <remarks>Optional</remarks>
         public const string Cff = "cff ";
         #endregion
 
@@ -176,18 +182,31 @@
         public uint CheckSum { get; }
 
         /// <summary>
-        /// Offset of the table from the beginning of the file.
+        /// Offset of the table data from the beginning of the file in bytes.
         /// </summary>
         public uint Offset { get; }
 
         /// <summary>
-        /// The length of the table.
+        /// The length of the table data in bytes.
         /// </summary>
         public uint Length { get; }
 
+        /// <summary>
+        /// Create a new <see cref="TrueTypeHeaderTable"/>.
+        /// </summary>
         public TrueTypeHeaderTable(string tag, uint checkSum, uint offset, uint length)
         {
-            Tag = tag ?? throw new ArgumentNullException(nameof(tag));
+            if (tag == null)
+            {
+                throw new ArgumentNullException(nameof(tag));
+            }
+
+            if (tag.Length != 4)
+            {
+                throw new ArgumentException($"A TrueType table tag must be a uint32, 4 bytes long, instead got: {tag}.", nameof(tag));
+            }
+
+            Tag = tag;
             CheckSum = checkSum;
             Offset = offset;
             Length = length;
@@ -201,11 +220,7 @@
             return new TrueTypeHeaderTable(tag, 0, 0, 0);
         }
 
-        public override string ToString()
-        {
-            return $"{Tag} {Offset} {Length} {CheckSum}";
-        }
-
+        /// <inheritdoc />
         public void Write(Stream stream)
         {
             for (var i = 0; i < Tag.Length; i++)
@@ -217,5 +232,11 @@
             stream.WriteUInt(Offset);
             stream.WriteUInt(Length);
         }
+
+        /// <inheritdoc />
+        public override string ToString()
+        {
+            return $"{Tag} - Offset: {Offset} Length: {Length} Checksum: {CheckSum}";
+        }
     }
 }

src/UglyToad.PdfPig/Writer/Fonts/Subsetting/TrueTypeGlyphTableSubsetter.cs

@@ -150,7 +150,7 @@
         {
             var indexToLocationTable = font.TableRegister.IndexToLocationTable;
 
-            var numGlyphs = indexToLocationTable.GlyphOffsets.Length - 1;
+            var numGlyphs = indexToLocationTable.GlyphOffsets.Count - 1;
 
             var glyphDirectory = font.TableRegister.GlyphTable.DirectoryTable;
 

src/UglyToad.PdfPig/Writer/Fonts/Subsetting/TrueTypeSubsetter.cs

@@ -3,7 +3,6 @@
     using System;
     using System.Collections.Generic;
     using System.IO;
-    using System.Linq;
     using IO;
     using PdfPig.Fonts.Exceptions;
     using PdfPig.Fonts.TrueType;
@@ -117,7 +116,7 @@
                     else if (entry.Tag == TrueTypeHeaderTable.Loca)
                     {
                         var table = new IndexToLocationTable(entry.DummyHeader, IndexToLocationTable.EntryFormat.Long,
-                            trueTypeSubsetGlyphTable.GlyphOffsets.Select(x => (long)x).ToArray());
+                            trueTypeSubsetGlyphTable.GlyphOffsets);
                         table.Write(stream);
                     }
                     else if (entry.Tag == TrueTypeHeaderTable.Head)