PNG/Decoder.cs

namespace Uwaa.PNG;

internal static class Decoder
{
    public static (byte bytesPerPixel, byte samplesPerPixel) GetBytesAndSamplesPerPixel(ImageHeader header)
    {
        var bitDepthCorrected = (header.BitDepth + 7) / 8;
        var samplesPerPixel = SamplesPerPixel(header);
        return ((byte)(samplesPerPixel * bitDepthCorrected), samplesPerPixel);
    }

    public static byte[] Decode(byte[] decompressedData, ImageHeader header, byte bytesPerPixel, byte samplesPerPixel)
    {
        switch (header.InterlaceMethod)
        {
            case InterlaceMethod.None:
                {
                    var bytesPerScanline = BytesPerScanline(header, samplesPerPixel);

                    var currentRowStartByteAbsolute = 1;
                    for (var rowIndex = 0; rowIndex < header.Height; rowIndex++)
                    {
                        var filterType = (FilterType)decompressedData[currentRowStartByteAbsolute - 1];

                        var previousRowStartByteAbsolute = rowIndex + (bytesPerScanline * (rowIndex - 1));

                        var end = currentRowStartByteAbsolute + bytesPerScanline;
                        for (var currentByteAbsolute = currentRowStartByteAbsolute; currentByteAbsolute < end; currentByteAbsolute++)
                        {
                            ReverseFilter(decompressedData, filterType, previousRowStartByteAbsolute, currentRowStartByteAbsolute, currentByteAbsolute, currentByteAbsolute - currentRowStartByteAbsolute, bytesPerPixel);
                        }

                        currentRowStartByteAbsolute += bytesPerScanline + 1;
                    }

                    return decompressedData;
                }
            case InterlaceMethod.Adam7:
                {
                    int byteHack = bytesPerPixel == 1 ? 1 : 0; // TODO: Further investigation required.
                    int pixelsPerRow = (header.Width * bytesPerPixel) + byteHack; // Add an extra byte per line.
                    byte[] newBytes = new byte[header.Height * pixelsPerRow];
                    int i = 0;
                    int previousStartRowByteAbsolute = -1;
                    // 7 passes
                    for (int pass = 0; pass < 7; pass++)
                    {
                        int numberOfScanlines = Adam7.GetNumberOfScanlinesInPass(header, pass);
                        int numberOfPixelsPerScanline = Adam7.GetPixelsPerScanlineInPass(header, pass);

                        if (numberOfScanlines <= 0 || numberOfPixelsPerScanline <= 0)
                        {
                            continue;
                        }

                        for (int scanlineIndex = 0; scanlineIndex < numberOfScanlines; scanlineIndex++)
                        {
                            FilterType filterType = (FilterType)decompressedData[i++];
                            int rowStartByte = i;

                            for (int j = 0; j < numberOfPixelsPerScanline; j++)
                            {
                                (int x, int y) pixelIndex = Adam7.GetPixelIndexForScanlineInPass(pass, scanlineIndex, j);
                                for (int k = 0; k < bytesPerPixel; k++)
                                {
                                    int byteLineNumber = (j * bytesPerPixel) + k;
                                    ReverseFilter(decompressedData, filterType, previousStartRowByteAbsolute, rowStartByte, i, byteLineNumber, bytesPerPixel);
                                    i++;
                                }

                                int start = byteHack + (pixelsPerRow * pixelIndex.y) + (pixelIndex.x * bytesPerPixel);
                                Array.ConstrainedCopy(decompressedData, rowStartByte + (j * bytesPerPixel), newBytes, start, bytesPerPixel);
                            }

                            previousStartRowByteAbsolute = rowStartByte;
                        }
                    }

                    return newBytes;
                }
            default:
                throw new ArgumentOutOfRangeException($"Invalid interlace method: {header.InterlaceMethod}.");
        }
    }

    static byte SamplesPerPixel(ImageHeader header)
    {
        return header.ColorType switch
        {
            ColorType.None => 1,
            ColorType.PaletteUsed or ColorType.PaletteUsed | ColorType.ColorUsed => 1,
            ColorType.ColorUsed => 3,
            ColorType.AlphaChannelUsed => 2,
            ColorType.ColorUsed | ColorType.AlphaChannelUsed => 4,
            _ => 0,
        };
    }

    static int BytesPerScanline(ImageHeader header, byte samplesPerPixel)
    {
        int width = header.Width;

        return header.BitDepth switch
        {
            1 => (width + 7) / 8,
            2 => (width + 3) / 4,
            4 => (width + 1) / 2,
            8 or 16 => width * samplesPerPixel * (header.BitDepth / 8),
            _ => 0,
        };
    }

    static void ReverseFilter(byte[] data, FilterType type, int previousRowStartByteAbsolute, int rowStartByteAbsolute, int byteAbsolute, int rowByteIndex, int bytesPerPixel)
    {
        byte GetLeftByteValue()
        {
            int leftIndex = rowByteIndex - bytesPerPixel;
            byte leftValue = leftIndex >= 0 ? data[rowStartByteAbsolute + leftIndex] : (byte)0;
            return leftValue;
        }

        byte GetAboveByteValue()
        {
            int upIndex = previousRowStartByteAbsolute + rowByteIndex;
            return upIndex >= 0 ? data[upIndex] : (byte)0;
        }

        byte GetAboveLeftByteValue()
        {
            int index = previousRowStartByteAbsolute + rowByteIndex - bytesPerPixel;
            return index < previousRowStartByteAbsolute || previousRowStartByteAbsolute < 0 ? (byte)0 : data[index];
        }

        // Moved out of the switch for performance.
        if (type == FilterType.Up)
        {
            int above = previousRowStartByteAbsolute + rowByteIndex;
            if (above < 0)
                return;

            data[byteAbsolute] += data[above];
            return;
        }

        if (type == FilterType.Sub)
        {
            int leftIndex = rowByteIndex - bytesPerPixel;
            if (leftIndex < 0)
                return;

            data[byteAbsolute] += data[rowStartByteAbsolute + leftIndex];
            return;
        }

        switch (type)
        {
            case FilterType.None:
                return;
            case FilterType.Average:
                data[byteAbsolute] += (byte)((GetLeftByteValue() + GetAboveByteValue()) / 2);
                break;
            case FilterType.Paeth:
                byte a = GetLeftByteValue();
                byte b = GetAboveByteValue();
                byte c = GetAboveLeftByteValue();
                data[byteAbsolute] += GetPaethValue(a, b, c);
                break;
            default:
                throw new ArgumentOutOfRangeException(nameof(type), type, null);
        }
    }

    /// <summary>
    /// Computes a simple linear function of the three neighboring pixels (left, above, upper left),
    /// then chooses as predictor the neighboring pixel closest to the computed value.
    /// </summary>
    static byte GetPaethValue(byte a, byte b, byte c)
    {
        int p = a + b - c;
        int pa = Math.Abs(p - a);
        int pb = Math.Abs(p - b);
        int pc = Math.Abs(p - c);

        if (pa <= pb && pa <= pc)
            return a;
        else
            return pb <= pc ? b : c;
    }
}