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

using System;
using System.Collections.Generic;

namespace SixLabors.Fonts.Tables.Cff {
  /// <summary>
  /// Resolves the Font DICT index for a given glyph using the FDSelect data from a CIDFont.
  /// Supports FDSelect format 0 (per-glyph map) and formats 3/4 (range-based).
  /// </summary>
  internal struct FDRangeProvider
  {
      private readonly int format;
      private readonly FDRange[] ranges;
      private readonly Dictionary<int, byte> fdSelectMap;
      private uint currentGlyphIndex;
      private uint endGlyphIndexMax;
      private FDRange currentRange;
      private int currentSelectedRangeIndex;

      /// <summary>
      /// Initializes a new instance of the <see cref="FDRangeProvider"/> struct.
      /// </summary>
      /// <param name="cidFontInfo">The CIDFont information containing FDSelect data.</param>
      public FDRangeProvider(CidFontInfo cidFontInfo)
      {
          this.format = cidFontInfo.FdSelectFormat;
          this.ranges = cidFontInfo.FdRanges;
          this.fdSelectMap = cidFontInfo.FdSelectMap;
          this.currentGlyphIndex = 0;
          this.currentSelectedRangeIndex = 0;

          if (this.ranges.Length is not 0)
          {
              this.currentRange = this.ranges[0];
              this.endGlyphIndexMax = this.ranges[1].First;
          }
          else
          {
              // empty
              this.currentRange = default;
              this.endGlyphIndexMax = 0;
          }

          this.SelectedFDArray = 0;
      }

      /// <summary>
      /// Gets the currently selected Font DICT array index.
      /// </summary>
      public ushort SelectedFDArray { get; private set; }

      /// <summary>
      /// Sets the current glyph index and resolves the corresponding Font DICT index.
      /// </summary>
      /// <param name="index">The glyph index to look up.</param>
      public void SetCurrentGlyphIndex(ushort index)
      {
          switch (this.format)
          {
              case 0:
                  this.currentGlyphIndex = this.fdSelectMap[index];
                  break;

              case 3:
              case 4:
                  // Find proper range for selected index.
                  if (index >= this.currentRange.First && index < this.endGlyphIndexMax)
                  {
                      // Ok, in current range.
                      this.SelectedFDArray = this.currentRange.FontDictionary;
                  }
                  else
                  {
                      // Move to next range.
                      this.currentSelectedRangeIndex++;
                      this.currentRange = this.ranges[this.currentSelectedRangeIndex];

                      this.endGlyphIndexMax = this.ranges[this.currentSelectedRangeIndex + 1].First;
                      if (index >= this.currentRange.First && index < this.endGlyphIndexMax)
                      {
                          this.SelectedFDArray = this.currentRange.FontDictionary;
                      }
                      else
                      {
                          throw new NotSupportedException();
                      }
                  }

                  this.currentGlyphIndex = index;

                  break;

              default:
                  throw new NotSupportedException();
          }
      }
  }
}