aa/Metadata/Models/Stateless/Dimensions.cs

using System.Drawing;

namespace View_by_Distance.Metadata.Models.Stateless;

internal static class Dimensions
{

#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])
                continue;
            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)
                _ = binaryReader.ReadBytes(chunkLength - 2);
            else
            {
                ushort uChunkLength = (ushort)chunkLength;
                _ = binaryReader.ReadBytes(uChunkLength - 2);
            }
        }
        return null;
    }

    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);
    }

    internal 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);
            }
        }
        return null;
    }

    internal static Size? GetDimensions(string path)
    {
        using BinaryReader binaryReader = new(File.OpenRead(path));
        return GetDimensions(binaryReader);
    }

    internal static Size? GetDimensions(Stream stream)
    {
        using BinaryReader binaryReader = new(stream);
        return GetDimensions(binaryReader);
    }

}