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

using System;
using System.Buffers;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;
using System.Text;
using static SixLabors.Fonts.Native.CoreFoundation;
using static SixLabors.Fonts.Native.CoreText;

namespace SixLabors.Fonts.Native {
  /// <summary>
  /// An enumerator that enumerates over available macOS system fonts.
  /// The enumerated strings are the absolute paths to the font files.
  /// </summary>
  /// <remarks>
  /// Internally, it calls the native CoreText's <see cref="CTFontManagerCopyAvailableFontURLs"/> method to retrieve
  /// the list of fonts so using this class must be guarded by <c>RuntimeInformation.IsOSPlatform(OSPlatform.OSX)</c>.
  /// </remarks>
  internal sealed class MacSystemFontsEnumerator : IEnumerable<string>, IEnumerator<string>
  {
      private static readonly ArrayPool<byte> BytePool = ArrayPool<byte>.Shared;

      private readonly IntPtr fontUrls;
      private readonly bool releaseFontUrls;
      private int fontIndex;

      public MacSystemFontsEnumerator()
          : this(CTFontManagerCopyAvailableFontURLs(), releaseFontUrls: true, fontIndex: 0)
      {
      }

      private MacSystemFontsEnumerator(IntPtr fontUrls, bool releaseFontUrls, int fontIndex)
      {
          if (fontUrls == IntPtr.Zero)
          {
              throw new ArgumentException($"The {nameof(fontUrls)} must not be NULL.", nameof(fontUrls));
          }

          this.fontUrls = fontUrls;
          this.releaseFontUrls = releaseFontUrls;
          this.fontIndex = fontIndex;

          this.Current = null!;
      }

      public string Current { get; private set; }

      object IEnumerator.Current => this.Current;

      public bool MoveNext()
      {
          Debug.Assert(CFGetTypeID(this.fontUrls) == CFArrayGetTypeID(), "The fontUrls array must be a CFArrayRef");
          if (this.fontIndex < CFArrayGetCount(this.fontUrls))
          {
              IntPtr fontUrl = CFArrayGetValueAtIndex(this.fontUrls, this.fontIndex);
              Debug.Assert(CFGetTypeID(fontUrl) == CFURLGetTypeID(), "The elements of the fontUrls array must be a CFURLRef");
              IntPtr fontPath = CFURLCopyFileSystemPath(fontUrl, CFURLPathStyle.kCFURLPOSIXPathStyle);

              int fontPathLength = (int)CFStringGetLength(fontPath);
              int fontPathBufferSize = (fontPathLength + 1) * 2; // +1 for the NULL byte and *2 for UTF-16
              byte[] fontPathBuffer = BytePool.Rent(fontPathBufferSize);
              CFStringGetCString(fontPath, fontPathBuffer, fontPathBufferSize, CFStringEncoding.kCFStringEncodingUTF16LE);
              this.Current = Encoding.Unicode.GetString(fontPathBuffer, 0, fontPathBufferSize - 2); // -2 for the UTF-16 NULL
              BytePool.Return(fontPathBuffer);

              CFRelease(fontPath);

              this.fontIndex++;

              return true;
          }

          return false;
      }

      public void Reset() => this.fontIndex = 0;

      public void Dispose()
      {
          if (this.releaseFontUrls)
          {
              CFRelease(this.fontUrls);
          }
      }

      public IEnumerator<string> GetEnumerator() => new MacSystemFontsEnumerator(this.fontUrls, releaseFontUrls: false, this.fontIndex);

      IEnumerator IEnumerable.GetEnumerator() => this.GetEnumerator();
  }
}