aa/Metadata/Models/Stateless/Dimensions.cs

using System.Drawing;

namespace View_by_Distance.Metadata.Models.Stateless.Methods;

internal static class Dimensions
{

    const string _ErrorMessage = "Could not recognize image format.";

#pragma warning disable IDE0230
    private static readonly Dictionary<byte[], Func<BinaryReader, Size>> _ImageFormatDecoders = new()
    {
        { new byte[] { 0x42, 0x4D }, DecodeBitmap },
        { new byte[] { 0x47, 0x49, 0x46, 0x38, 0x37, 0x61 }, DecodeGif },
        { new byte[] { 0x47, 0x49, 0x46, 0x38, 0x39, 0x61 }, DecodeGif },
        { new byte[] { 0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A }, DecodePng },
        { new byte[] { 0xff, 0xd8 }, DecodeJfif },
        { new byte[] { 0x52, 0x49, 0x46, 0x46 }, DecodeWebP },
    };
#pragma warning restore IDE0230

    private static bool StartsWith(byte[] thisBytes, byte[] thatBytes)
    {
        for (int i = 0; i < thatBytes.Length; i += 1)
        {
            if (thisBytes[i] != thatBytes[i])
            {
                return false;
            }
        }
        return true;
    }

    private static short ReadLittleEndianInt16(BinaryReader binaryReader)
    {
        byte[] bytes = new byte[sizeof(short)];
        for (int i = 0; i < sizeof(short); i += 1)
        {
            bytes[sizeof(short) - 1 - i] = binaryReader.ReadByte();
        }
        return BitConverter.ToInt16(bytes, 0);
    }

    private static int ReadLittleEndianInt32(BinaryReader binaryReader)
    {
        byte[] bytes = new byte[sizeof(int)];
        for (int i = 0; i < sizeof(int); i += 1)
        {
            bytes[sizeof(int) - 1 - i] = binaryReader.ReadByte();
        }
        return BitConverter.ToInt32(bytes, 0);
    }

    private static Size DecodeBitmap(BinaryReader binaryReader)
    {
        _ = binaryReader.ReadBytes(16);
        int width = binaryReader.ReadInt32();
        int height = binaryReader.ReadInt32();
        return new Size(width, height);
    }

    private static Size DecodeGif(BinaryReader binaryReader)
    {
        int width = binaryReader.ReadInt16();
        int height = binaryReader.ReadInt16();
        return new Size(width, height);
    }

    private static Size DecodePng(BinaryReader binaryReader)
    {
        _ = binaryReader.ReadBytes(8);
        int width = ReadLittleEndianInt32(binaryReader);
        int height = ReadLittleEndianInt32(binaryReader);
        return new Size(width, height);
    }

    private static Size DecodeJfif(BinaryReader binaryReader)
    {
        while (binaryReader.ReadByte() == 0xff)
        {
            byte marker = binaryReader.ReadByte();
            short chunkLength = ReadLittleEndianInt16(binaryReader);

            if (marker == 0xc0)
            {
                _ = binaryReader.ReadByte();

                int height = ReadLittleEndianInt16(binaryReader);
                int width = ReadLittleEndianInt16(binaryReader);
                return new Size(width, height);
            }

            if (chunkLength < 0)
            {
                ushort uChunkLength = (ushort)chunkLength;
                _ = binaryReader.ReadBytes(uChunkLength - 2);
            }
            else
            {
                _ = binaryReader.ReadBytes(chunkLength - 2);
            }
        }

        throw new ArgumentException(_ErrorMessage);
    }

    private static Size DecodeWebP(BinaryReader binaryReader)
    {
        _ = binaryReader.ReadUInt32(); // Size
        _ = binaryReader.ReadBytes(15); // WEBP, VP8 + more
        _ = binaryReader.ReadBytes(3); // SYNC

        int width = binaryReader.ReadUInt16() & 0b00_11111111111111; // 14 bits width
        int height = binaryReader.ReadUInt16() & 0b00_11111111111111; // 14 bits height

        return new Size(width, height);
    }

    /// <summary>
    /// Gets the dimensions of an image.
    /// </summary>
    /// <param name="path">The path of the image to get the dimensions of.</param>
    /// <returns>The dimensions of the specified image.</returns>
    /// <exception cref="ArgumentException">The image was of an unrecognized format.</exception>    
    public static Size GetDimensions(BinaryReader binaryReader)
    {
        int maxMagicBytesLength = _ImageFormatDecoders.Keys.OrderByDescending(x => x.Length).First().Length;

        byte[] magicBytes = new byte[maxMagicBytesLength];

        for (int i = 0; i < maxMagicBytesLength; i += 1)
        {
            magicBytes[i] = binaryReader.ReadByte();

            foreach (KeyValuePair<byte[], Func<BinaryReader, Size>> kvPair in _ImageFormatDecoders)
            {
                if (StartsWith(magicBytes, kvPair.Key))
                {
                    return kvPair.Value(binaryReader);
                }
            }
        }

        throw new ArgumentException(_ErrorMessage, nameof(binaryReader));
    }

    /// <summary>
    /// Gets the dimensions of an image.
    /// </summary>
    /// <param name="path">The path of the image to get the dimensions of.</param>
    /// <returns>The dimensions of the specified image.</returns>
    /// <exception cref="ArgumentException">The image was of an unrecognized format.</exception>
    public static Size GetDimensions(string path)
    {
        using BinaryReader binaryReader = new(File.OpenRead(path));
        try
        {
            return GetDimensions(binaryReader);
        }
        catch (ArgumentException e)
        {
            if (e.Message.StartsWith(_ErrorMessage))
            {
                throw new ArgumentException(_ErrorMessage, nameof(path), e);
            }
            else
            {
                throw;
            }
        }
    }

}