// Copyright (c) Six Labors.
// Licensed under the Six Labors Split License.

namespace SixLabors.Fonts.Tables {
  /// <summary>
  /// Represents a font collection header (for .ttc font collections).
  /// A font collection contains one or more fonts where typically the glyf table is shared by multiple fonts to save space,
  /// but other tables are not.
  /// Each font in the collection has its own set of tables.
  /// </summary>
  internal class TtcHeader
  {
      internal const string TableName = "ttcf";

      /// <summary>
      /// Initializes a new instance of the <see cref="TtcHeader"/> class.
      /// </summary>
      /// <param name="ttcTag">The TTC tag, expected to be "ttcf".</param>
      /// <param name="majorVersion">The major version of the TTC header (1 or 2).</param>
      /// <param name="minorVersion">The minor version of the TTC header.</param>
      /// <param name="numFonts">The number of fonts in the collection.</param>
      /// <param name="offsetTable">Array of byte offsets to each font's offset table.</param>
      /// <param name="dsigTag">The DSIG table tag (version 2+ only, otherwise 0).</param>
      /// <param name="dsigLength">The length of the DSIG table in bytes (version 2+ only, otherwise 0).</param>
      /// <param name="dsigOffset">The byte offset of the DSIG table (version 2+ only, otherwise 0).</param>
      public TtcHeader(string ttcTag, ushort majorVersion, ushort minorVersion, uint numFonts, uint[] offsetTable, uint dsigTag, uint dsigLength, uint dsigOffset)
      {
          this.TtcTag = ttcTag;
          this.MajorVersion = majorVersion;
          this.MinorVersion = minorVersion;
          this.NumFonts = numFonts;
          this.OffsetTable = offsetTable;
          this.DsigTag = dsigTag;
          this.DsigLength = dsigLength;
          this.DsigOffset = dsigOffset;
      }

      /// <summary>
      /// Gets the tag, should be "ttcf".
      /// </summary>
      public string TtcTag { get; }

      /// <summary>
      /// Gets the major version of the TTC header. Version 1 has no DSIG; version 2 includes DSIG fields.
      /// </summary>
      public ushort MajorVersion { get; }

      /// <summary>
      /// Gets the minor version of the TTC header.
      /// </summary>
      public ushort MinorVersion { get; }

      /// <summary>
      /// Gets the number of fonts contained in the collection.
      /// </summary>
      public uint NumFonts { get; }

      /// <summary>
      /// Gets the array of offsets to the OffsetTable of each font. Use <see cref="FontReader"/> for each font.
      /// </summary>
      public uint[] OffsetTable { get; }

      /// <summary>
      /// Gets the tag of the DSIG (digital signature) table. Only present in version 2+ headers.
      /// </summary>
      public uint DsigTag { get; }

      /// <summary>
      /// Gets the length of the DSIG table in bytes. Only present in version 2+ headers.
      /// </summary>
      public uint DsigLength { get; }

      /// <summary>
      /// Gets the byte offset of the DSIG table from the beginning of the file. Only present in version 2+ headers.
      /// </summary>
      public uint DsigOffset { get; }

      /// <summary>
      /// Reads a <see cref="TtcHeader"/> from the given reader.
      /// </summary>
      /// <param name="reader">The binary reader positioned at the start of the TTC header.</param>
      /// <returns>The parsed <see cref="TtcHeader"/>.</returns>
      /// <exception cref="InvalidFontTableException">Thrown when the tag is not "ttcf".</exception>
      public static TtcHeader Read(BigEndianBinaryReader reader)
      {
          string tag = reader.ReadTag();

          if (tag != TableName)
          {
              throw new InvalidFontTableException($"Expected tag = {TableName} found {tag}", TableName);
          }

          ushort majorVersion = reader.ReadUInt16();
          ushort minorVersion = reader.ReadUInt16();
          uint numFonts = reader.ReadUInt32();
          uint[] offsetTable = new uint[numFonts];
          for (int i = 0; i < numFonts; ++i)
          {
              offsetTable[i] = reader.ReadOffset32();
          }

          // Version 2 fields
          uint dsigTag = 0;
          uint dsigLength = 0;
          uint dsigOffset = 0;
          if (majorVersion >= 2)
          {
              dsigTag = reader.ReadUInt32();
              dsigLength = reader.ReadUInt32();
              dsigOffset = reader.ReadUInt32();
          }

          return new TtcHeader(tag, majorVersion, minorVersion, numFonts, offsetTable, dsigTag, dsigLength, dsigOffset);
      }
  }
}