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

using System;
using System.Diagnostics.CodeAnalysis;

namespace SixLabors.ImageSharp.Formats.Webp {
  /// <summary>
  /// Detects Webp file headers.
  /// </summary>
  public sealed class WebpImageFormatDetector : IImageFormatDetector
  {
      /// <inheritdoc />
      public int HeaderSize => 12;

      /// <inheritdoc />
      public bool TryDetectFormat(ReadOnlySpan<byte> header, [NotNullWhen(true)] out IImageFormat? format)
      {
          format = this.IsSupportedFileFormat(header) ? WebpFormat.Instance : null;
          return format != null;
      }

      private bool IsSupportedFileFormat(ReadOnlySpan<byte> header)
          => header.Length >= this.HeaderSize && IsRiffContainer(header) && IsWebpFile(header);

      /// <summary>
      /// Checks, if the header starts with a valid RIFF FourCC.
      /// </summary>
      /// <param name="header">The header bytes.</param>
      /// <returns>True, if its a valid RIFF FourCC.</returns>
      private static bool IsRiffContainer(ReadOnlySpan<byte> header)
          => header[..4].SequenceEqual(WebpConstants.RiffFourCc);

      /// <summary>
      /// Checks if 'WEBP' is present in the header.
      /// </summary>
      /// <param name="header">The header bytes.</param>
      /// <returns>True, if its a webp file.</returns>
      private static bool IsWebpFile(ReadOnlySpan<byte> header)
          => header.Slice(8, 4).SequenceEqual(WebpConstants.WebpHeader);
  }
}