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

using SixLabors.Fonts.Unicode;
using SixLabors.Fonts.WellKnownIds;
using System.Collections.Generic;
using System.Linq;

namespace SixLabors.Fonts.Tables.General.CMap {
  /// <summary>
  /// Format 0 is a simple byte encoding subtable that maps character codes 0–255 to glyph indices.
  /// <see href="https://learn.microsoft.com/en-us/typography/opentype/spec/cmap#format-0-byte-encoding-table"/>
  /// </summary>
  internal sealed class Format0SubTable : CMapSubTable
  {
      /// <summary>
      /// Initializes a new instance of the <see cref="Format0SubTable"/> class.
      /// </summary>
      /// <param name="language">The language code for Macintosh platform subtables.</param>
      /// <param name="platform">The platform identifier.</param>
      /// <param name="encoding">The platform-specific encoding identifier.</param>
      /// <param name="glyphIds">The array of glyph indices indexed by character code.</param>
      public Format0SubTable(ushort language, PlatformIDs platform, ushort encoding, byte[] glyphIds)
          : base(platform, encoding, 0)
      {
          this.Language = language;
          this.GlyphIds = glyphIds;
      }

      /// <summary>
      /// Gets the language code for Macintosh platform subtables.
      /// </summary>
      public ushort Language { get; }

      /// <summary>
      /// Gets the array of glyph indices indexed by character code.
      /// </summary>
      public byte[] GlyphIds { get; }

      /// <inheritdoc/>
      public override bool TryGetGlyphId(CodePoint codePoint, out ushort glyphId)
      {
          int b = codePoint.Value;
          if (b >= this.GlyphIds.Length)
          {
              glyphId = 0;
              return false;
          }

          glyphId = this.GlyphIds[b];
          return true;
      }

      /// <inheritdoc/>
      public override bool TryGetCodePoint(ushort glyphId, out CodePoint codePoint)
      {
          for (int i = 0; i < this.GlyphIds.Length; i++)
          {
              if (this.GlyphIds[i] == glyphId)
              {
                  codePoint = new CodePoint(i);
                  return true;
              }
          }

          codePoint = default;
          return false;
      }

      /// <inheritdoc/>
      public override IEnumerable<int> GetAvailableCodePoints()
          => Enumerable.Range(0, this.GlyphIds.Length);

      /// <summary>
      /// Loads one or more <see cref="Format0SubTable"/> instances from the specified encoding records and reader.
      /// </summary>
      /// <param name="encodings">The encoding records that share this subtable.</param>
      /// <param name="reader">The binary reader positioned after the format field.</param>
      /// <returns>An enumerable of <see cref="Format0SubTable"/> instances, one per encoding record.</returns>
      public static IEnumerable<Format0SubTable> Load(IEnumerable<EncodingRecord> encodings, BigEndianBinaryReader reader)
      {
          // format has already been read by this point skip it
          ushort length = reader.ReadUInt16();
          ushort language = reader.ReadUInt16();
          int glyphsCount = length - 6;

          // char 'A' == 65 thus glyph = glyphIds[65];
          byte[] glyphIds = reader.ReadBytes(glyphsCount);

          foreach (EncodingRecord encoding in encodings)
          {
              yield return new Format0SubTable(language, encoding.PlatformID, encoding.EncodingID, glyphIds);
          }
      }
  }
}