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

using System.IO;

namespace SixLabors.Fonts.Tables.General {
  /// <summary>
  /// Represents the color palette table, which contains one or more palettes of colors
  /// used by color fonts (e.g., COLR table).
  /// <see href="https://learn.microsoft.com/en-us/typography/opentype/spec/cpal"/>
  /// </summary>
  internal class CpalTable : Table
  {
      /// <summary>
      /// The table name identifier.
      /// </summary>
      internal const string TableName = "CPAL";

      /// <summary>
      /// The offsets into the palette entries array for each palette.
      /// </summary>
      private readonly ushort[] paletteOffsets;

      /// <summary>
      /// The combined array of color records for all palettes.
      /// </summary>
      private readonly GlyphColor[] paletteEntries;

      /// <summary>
      /// Initializes a new instance of the <see cref="CpalTable"/> class.
      /// </summary>
      /// <param name="paletteOffsets">The index of each palette's first color record.</param>
      /// <param name="paletteEntries">The combined color records for all palettes.</param>
      public CpalTable(ushort[] paletteOffsets, GlyphColor[] paletteEntries)
      {
          this.paletteEntries = paletteEntries;
          this.paletteOffsets = paletteOffsets;
      }

      /// <summary>
      /// Gets the glyph color at the specified palette and entry indices.
      /// </summary>
      /// <param name="paletteIndex">The zero-based palette index.</param>
      /// <param name="paletteEntryIndex">The zero-based entry index within the palette.</param>
      /// <returns>The <see cref="GlyphColor"/>.</returns>
      public GlyphColor GetGlyphColor(int paletteIndex, int paletteEntryIndex)
          => this.paletteEntries[this.paletteOffsets[paletteIndex] + paletteEntryIndex];

      /// <summary>
      /// Loads the <see cref="CpalTable"/> from the specified font reader.
      /// </summary>
      /// <param name="fontReader">The font reader.</param>
      /// <returns>The <see cref="CpalTable"/>, or <see langword="null"/> if the table is not present.</returns>
      public static CpalTable? Load(FontReader fontReader)
      {
          if (!fontReader.TryGetReaderAtTablePosition(TableName, out BigEndianBinaryReader? binaryReader))
          {
              return null;
          }

          using (binaryReader)
          {
              return Load(binaryReader);
          }
      }

      /// <summary>
      /// Loads the <see cref="CpalTable"/> from the specified binary reader.
      /// </summary>
      /// <param name="reader">The big-endian binary reader.</param>
      /// <returns>The <see cref="CpalTable"/>.</returns>
      public static CpalTable Load(BigEndianBinaryReader reader)
      {
          // FORMAT 0

          // Type      | Name                            | Description
          // ----------|---------------------------------|----------------------------------------------------------------------------------------------------
          // uint16    | version                         | Table version number (=0).
          // uint16    | numPaletteEntries               | Number of palette entries in each palette.
          // uint16    | numPalettes                     | Number of palettes in the table.
          // uint16    | numColorRecords                 | Total number of color records, combined for all palettes.
          // Offset32  | offsetFirstColorRecord          | Offset from the beginning of CPAL table to the first ColorRecord.
          // uint16    | colorRecordIndices[numPalettes] | Index of each palette’s first color record in the combined color record array.

          // additional format 1 fields
          // Offset32  | offsetPaletteTypeArray          | Offset from the beginning of CPAL table to the Palette Type Array. Set to 0 if no array is provided.
          // Offset32  | offsetPaletteLabelArray         | Offset from the beginning of CPAL table to the Palette Labels Array. Set to 0 if no array is provided.
          // Offset32  | offsetPaletteEntryLabelArray    | Offset from the beginning of CPAL table to the Palette Entry Label Array.Set to 0 if no array is provided.
          ushort version = reader.ReadUInt16();
          ushort numPaletteEntries = reader.ReadUInt16();
          ushort numPalettes = reader.ReadUInt16();
          ushort numColorRecords = reader.ReadUInt16();
          uint offsetFirstColorRecord = reader.ReadOffset32();

          ushort[]? colorRecordIndices = reader.ReadUInt16Array(numPalettes);

          uint offsetPaletteTypeArray = 0;
          uint offsetPaletteLabelArray = 0;
          uint offsetPaletteEntryLabelArray = 0;
          if (version == 1)
          {
              offsetPaletteTypeArray = reader.ReadOffset32();
              offsetPaletteLabelArray = reader.ReadOffset32();
              offsetPaletteEntryLabelArray = reader.ReadOffset32();
          }

          reader.Seek(offsetFirstColorRecord, SeekOrigin.Begin);
          GlyphColor[] palettes = new GlyphColor[numColorRecords];
          for (int n = 0; n < numColorRecords; n++)
          {
              byte blue = reader.ReadByte();
              byte green = reader.ReadByte();
              byte red = reader.ReadByte();
              byte alpha = reader.ReadByte();
              palettes[n] = new GlyphColor(red, green, blue, alpha);
          }

          return new CpalTable(colorRecordIndices, palettes);
      }
  }
}