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

using System.IO;

namespace SixLabors.Fonts.Tables {
  /// <summary>
  /// Represents a table record entry in the font directory.
  /// Each record contains the tag, checksum, offset, and length of a font table.
  /// <see href="https://learn.microsoft.com/en-us/typography/opentype/spec/otff#table-directory"/>
  /// </summary>
  internal class TableHeader
  {
      /// <summary>
      /// Initializes a new instance of the <see cref="TableHeader"/> class.
      /// </summary>
      /// <param name="tag">The four-byte table tag identifier.</param>
      /// <param name="checkSum">The checksum for the table.</param>
      /// <param name="offset">The byte offset of the table from the beginning of the font file.</param>
      /// <param name="len">The length of the table in bytes.</param>
      public TableHeader(string tag, uint checkSum, uint offset, uint len)
      {
          this.Tag = tag;
          this.CheckSum = checkSum;
          this.Offset = offset;
          this.Length = len;
      }

      /// <summary>
      /// Gets the four-byte table tag identifier (e.g. "head", "glyf", "cmap").
      /// </summary>
      public string Tag { get; }

      /// <summary>
      /// Gets the byte offset of the table from the beginning of the font file.
      /// </summary>
      public uint Offset { get; }

      /// <summary>
      /// Gets the checksum for the table, used to verify table integrity.
      /// </summary>
      public uint CheckSum { get; }

      /// <summary>
      /// Gets the length of the table data in bytes.
      /// </summary>
      public uint Length { get; }

      /// <summary>
      /// Reads a <see cref="TableHeader"/> from the given reader.
      /// </summary>
      /// <param name="reader">The binary reader positioned at the table record.</param>
      /// <returns>The parsed <see cref="TableHeader"/>.</returns>
      public static TableHeader Read(BigEndianBinaryReader reader) => new TableHeader(
              reader.ReadTag(),
              reader.ReadUInt32(),
              reader.ReadOffset32(),
              reader.ReadUInt32());

      /// <summary>
      /// Creates a <see cref="BigEndianBinaryReader"/> positioned at the start of this table's data.
      /// </summary>
      /// <param name="stream">The font file stream.</param>
      /// <returns>A reader positioned at the table data.</returns>
      public virtual BigEndianBinaryReader CreateReader(Stream stream)
      {
          stream.Seek(this.Offset, SeekOrigin.Begin);

          return new BigEndianBinaryReader(stream, true);
      }
  }
}