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

using System.Numerics;

namespace SixLabors.Fonts.Tables.General.Kern {
  /// <summary>
  /// Represents a kerning subtable in the OpenType 'kern' table.
  /// Each subtable contains kerning data in a specific format.
  /// <see href="https://learn.microsoft.com/en-us/typography/opentype/spec/kern"/>
  /// </summary>
  internal abstract class KerningSubTable
  {
      /// <summary>
      /// The coverage flags describing the properties of this subtable.
      /// </summary>
      private readonly KerningCoverage coverage;

      /// <summary>
      /// Initializes a new instance of the <see cref="KerningSubTable"/> class.
      /// </summary>
      /// <param name="coverage">The coverage flags for this subtable.</param>
      public KerningSubTable(KerningCoverage coverage)
          => this.coverage = coverage;

      /// <summary>
      /// Loads a <see cref="KerningSubTable"/> from the specified binary reader.
      /// Returns <see langword="null"/> if the subtable format is not supported.
      /// </summary>
      /// <param name="reader">The binary reader positioned at the start of the subtable header.</param>
      /// <returns>The loaded <see cref="KerningSubTable"/>, or <see langword="null"/> for unsupported formats.</returns>
      public static KerningSubTable? Load(BigEndianBinaryReader reader)
      {
          // Kerning subtables will share the same header format.
          // This header is used to identify the format of the subtable and the kind of information it contains:
          // +--------+----------+----------------------------------------------------------+
          // | Type   | Field    | Description                                              |
          // +========+==========+==========================================================+
          // | uint16 | version  | Kern subtable version number                             |
          // +--------+----------+----------------------------------------------------------+
          // | uint16 | length   | Length of the subtable, in bytes(including this header). |
          // +--------+----------+----------------------------------------------------------+
          // | uint16 | coverage | What type of information is contained in this table.     |
          // +--------+----------+----------------------------------------------------------+
          ushort subVersion = reader.ReadUInt16();
          ushort length = reader.ReadUInt16();
          KerningCoverage coverage = KerningCoverage.Read(reader);
          if (coverage.Format == 0)
          {
              return Format0SubTable.Load(reader, coverage);
          }
          else
          {
              // we don't support versions other than 'Format 0' same as Windows
              return null;
          }
      }

      /// <summary>
      /// Attempts to get the kerning offset for the specified pair of glyph indices.
      /// </summary>
      /// <param name="index1">The glyph index of the first (left) glyph.</param>
      /// <param name="index2">The glyph index of the second (right) glyph.</param>
      /// <param name="offset">When this method returns, contains the kerning offset if found.</param>
      /// <returns><see langword="true"/> if a kerning value was found; otherwise, <see langword="false"/>.</returns>
      protected abstract bool TryGetOffset(ushort index1, ushort index2, out short offset);

      /// <summary>
      /// Attempts to apply the kerning offset for the specified glyph pair to the result vector.
      /// The offset is applied to the X component for horizontal kerning or the Y component for vertical kerning.
      /// </summary>
      /// <param name="index1">The glyph index of the first (left) glyph.</param>
      /// <param name="index2">The glyph index of the second (right) glyph.</param>
      /// <param name="result">The vector to which the kerning offset is applied.</param>
      /// <returns><see langword="true"/> if a kerning offset was applied; otherwise, <see langword="false"/>.</returns>
      public bool TryApplyOffset(ushort index1, ushort index2, ref Vector2 result)
      {
          if (this.TryGetOffset(index1, index2, out short offset))
          {
              if (this.coverage.Horizontal)
              {
                  // apply to X
                  if (this.coverage.OverrideAccumulator)
                  {
                      result.X = offset;
                  }
                  else
                  {
                      result.X += offset;
                  }
              }
              else
              {
                  // apply to Y
                  if (this.coverage.OverrideAccumulator)
                  {
                      result.Y = offset;
                  }
                  else
                  {
                      result.Y += offset;
                  }
              }

              return true;
          }

          return false;
      }
  }
}