namespace Uwaa.PNG;

internal static class Adam7
{
    /// <summary>
    /// For a given pass number (1 indexed) the scanline indexes of the lines included in that pass in the 8x8 grid.
    /// </summary>
    static readonly IReadOnlyDictionary<int, int[]> PassToScanlineGridIndex = new Dictionary<int, int[]>
    {
        { 1, [ 0 ] },
        { 2, [ 0 ] },
        { 3, [ 4 ] },
        { 4, [ 0, 4 ] },
        { 5, [ 2, 6 ] },
        { 6, [ 0, 2, 4, 6 ] },
        { 7, [ 1, 3, 5, 7 ] }
    };

    static readonly IReadOnlyDictionary<int, int[]> PassToScanlineColumnIndex = new Dictionary<int, int[]>
    {
        { 1, [ 0 ] },
        { 2, [ 4 ] },
        { 3, [ 0, 4 ] },
        { 4, [ 2, 6 ] },
        { 5, [ 0, 2, 4, 6 ] },
        { 6, [ 1, 3, 5, 7 ] },
        { 7, [ 0, 1, 2, 3, 4, 5, 6, 7 ] }
    };

    /*
     * To go from raw image data to interlaced:
     *
     * An 8x8 grid is repeated over the image. There are 7 passes and the indexes in this grid correspond to the
     * pass number including that pixel. Each row in the grid corresponds to a scanline.
     *
     * 1 6 4 6 2 6 4 6 - Scanline 0: pass 1 has pixel 0, 8, 16, etc. pass 2 has pixel 4, 12, 20, etc.
     * 7 7 7 7 7 7 7 7
     * 5 6 5 6 5 6 5 6
     * 7 7 7 7 7 7 7 7
     * 3 6 4 6 3 6 4 6
     * 7 7 7 7 7 7 7 7
     * 5 6 5 6 5 6 5 6
     * 7 7 7 7 7 7 7 7
     *
     *
     *
     */

    public static int GetNumberOfScanlinesInPass(ImageHeader header, int pass)
    {
        int[] indices = PassToScanlineGridIndex[pass + 1];

        int mod = header.Height % 8;

        if (mod == 0) //fits exactly
            return indices.Length * (header.Height / 8);

        int additionalLines = 0;
        for (int i = 0; i < indices.Length; i++)
            if (indices[i] < mod)
                additionalLines++;

        return (indices.Length * (header.Height / 8)) + additionalLines;
    }

    public static int GetPixelsPerScanlineInPass(ImageHeader header, int pass)
    {
        int[] indices = PassToScanlineColumnIndex[pass + 1];

        int mod = header.Width % 8;

        if (mod == 0) //fits exactly
            return indices.Length * (header.Width / 8);

        int additionalColumns = 0;
        for (int i = 0; i < indices.Length; i++)
            if (indices[i] < mod)
                additionalColumns++;

        return (indices.Length * (header.Width / 8)) + additionalColumns;
    }

    public static (int x, int y) GetPixelIndexForScanlineInPass(int pass, int scanlineIndex, int indexInScanline)
    {
        int[] columnIndices = PassToScanlineColumnIndex[pass + 1];
        int[] rows = PassToScanlineGridIndex[pass + 1];

        int actualRow = scanlineIndex % rows.Length;
        int actualCol = indexInScanline % columnIndices.Length;
        int precedingRows = 8 * (scanlineIndex / rows.Length);
        int precedingCols = 8 * (indexInScanline / columnIndices.Length);

        return (precedingCols + columnIndices[actualCol], precedingRows + rows[actualRow]);
    }
}