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

using System;

namespace SixLabors.Fonts.Tables.AdvancedTypographic.Variations {
  /// <summary>
  /// Implements reading the Font Variations Table `fvar`.
  /// <see href="https://docs.microsoft.com/de-de/typography/opentype/spec/fvar"/>
  /// </summary>
  internal class FVarTable : Table
  {
      /// <summary>
      /// The table name identifier for the fvar table.
      /// </summary>
      internal const string TableName = "fvar";

      /// <summary>
      /// Initializes a new instance of the <see cref="FVarTable"/> class.
      /// </summary>
      /// <param name="axisCount">The number of variation axes.</param>
      /// <param name="axes">The array of variation axis records.</param>
      /// <param name="instances">The array of named instance records.</param>
      public FVarTable(ushort axisCount, VariationAxisRecord[] axes, InstanceRecord[] instances)
      {
          this.AxisCount = axisCount;
          this.Axes = axes;
          this.Instances = instances;
      }

      /// <summary>
      /// Gets the number of variation axes defined in this font.
      /// </summary>
      public ushort AxisCount { get; }

      /// <summary>
      /// Gets the array of variation axis records defining each axis (e.g. weight, width).
      /// </summary>
      public VariationAxisRecord[] Axes { get; }

      /// <summary>
      /// Gets the array of named instance records defined in this font.
      /// </summary>
      public InstanceRecord[] Instances { get; }

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

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

      /// <summary>
      /// Loads the fvar table from the specified binary reader.
      /// </summary>
      /// <param name="reader">The big-endian binary reader positioned at the start of the fvar table.</param>
      /// <returns>The <see cref="FVarTable"/>.</returns>
      public static FVarTable Load(BigEndianBinaryReader reader)
      {
          // VariationsTable `fvar`
          // +-----------------+----------------------------------------+----------------------------------------------------------------+
          // | Type            | Name                                   | Description                                                    |
          // +=================+========================================+================================================================+
          // | uint16          | majorVersion                           | Major version number of the font variations table — set to 1.  |
          // +-----------------+----------------------------------------+----------------------------------------------------------------+
          // | uint16          | minorVersion                           | Minor version number of the font variations table — set to 0.  |
          // +-----------------+----------------------------------------+----------------------------------------------------------------+
          // | Offset16        | axesArrayOffset                        | Offset in bytes from the beginning of the table to the start   |
          // |                 |                                        | of the VariationAxisRecord array.                              |
          // +-----------------+----------------------------------------+----------------------------------------------------------------+
          // | uint16          | (reserved)                             | This field is permanently reserved. Set to 2.                  |
          // +-----------------+----------------------------------------+----------------------------------------------------------------+
          // | uint16          | axisCount                              | The number of variation axes in the font                       |
          // |                 |                                        | (the number of records in the axes array).                     |
          // +-----------------+----------------------------------------+----------------------------------------------------------------+
          // | uint16          | axisSize                               | The size in bytes of each VariationAxisRecord                  |
          // |                 |                                        | — set to 20 (0x0014) for this version.                         |
          // +-----------------+----------------------------------------+----------------------------------------------------------------+
          // | uint16          | instanceCount                          | The number of named instances defined in the font              |
          // |                 |                                        | (the number of records in the instances array).                |
          // +-----------------+----------------------------------------+----------------------------------------------------------------+
          // | uint16          | instanceSize                           | The size in bytes of each InstanceRecord                       |
          // |                 |                                        | — set to either axisCount * sizeof(Fixed) + 4,                 |
          // |                 |                                        | or to axisCount * sizeof(Fixed) + 6.                           |
          // +-----------------+----------------------------------------+----------------------------------------------------------------+
          long startOffset = reader.BaseStream.Position;
          ushort major = reader.ReadUInt16();
          ushort minor = reader.ReadUInt16();
          ushort axesArrayOffset = reader.ReadOffset16();
          ushort reserved = reader.ReadUInt16();
          ushort axisCount = reader.ReadUInt16();
          ushort axisSize = reader.ReadUInt16();
          ushort instanceCount = reader.ReadUInt16();
          ushort instanceSize = reader.ReadUInt16();

          if (major != 1)
          {
              throw new NotSupportedException("Only version 1 of fvar table is supported");
          }

          VariationAxisRecord[] axesArray = new VariationAxisRecord[axisCount];
          for (int i = 0; i < axisCount; i++)
          {
              axesArray[i] = VariationAxisRecord.Load(reader, axesArrayOffset + (axisSize * i));
          }

          InstanceRecord[] instances = new InstanceRecord[instanceCount];
          long instancesOffset = reader.BaseStream.Position - startOffset;
          for (int i = 0; i < instanceCount; i++)
          {
              instances[i] = InstanceRecord.Load(reader, instancesOffset + (i * instanceSize), axisCount);
          }

          return new FVarTable(axisCount, axesArray, instances);
      }
  }
}