using System.IO.Compression;

namespace AgentWikiUi.Tests;

/// <summary>Scrive PNG minimali (colore pieno) senza dipendere da Avalonia.</summary>
static class PngFactory
{
    public static byte[] Create(int width, int height)
    {
        var raw = new byte[height * (1 + width * 3)];
        for (var y = 0; y < height; y++)
            for (var x = 0; x < width; x++) raw[y * (1 + width * 3) + 1 + x * 3] = 200; // filtro 0 per riga, canale rosso
        using var ms = new MemoryStream();
        ms.Write([0x89, 0x50, 0x4E, 0x47, 0x0D, 0x0A, 0x1A, 0x0A]);
        var ihdr = new byte[13];
        WriteInt(ihdr, 0, width); WriteInt(ihdr, 4, height);
        ihdr[8] = 8; ihdr[9] = 2; // 8 bit, RGB
        Chunk(ms, "IHDR", ihdr);
        using (var z = new MemoryStream())
        {
            using (var zs = new ZLibStream(z, CompressionLevel.Fastest, leaveOpen: true)) zs.Write(raw);
            Chunk(ms, "IDAT", z.ToArray());
        }
        Chunk(ms, "IEND", []);
        return ms.ToArray();
    }

    static void WriteInt(byte[] b, int at, int v) { b[at] = (byte)(v >> 24); b[at + 1] = (byte)(v >> 16); b[at + 2] = (byte)(v >> 8); b[at + 3] = (byte)v; }

    static void Chunk(Stream s, string type, byte[] data)
    {
        var len = new byte[4]; WriteInt(len, 0, data.Length);
        s.Write(len);
        var body = new byte[4 + data.Length];
        for (var i = 0; i < 4; i++) body[i] = (byte)type[i];
        data.CopyTo(body, 4);
        s.Write(body);
        var crc = new byte[4]; WriteInt(crc, 0, (int)Crc32(body));
        s.Write(crc);
    }

    static uint Crc32(byte[] data)
    {
        var crc = 0xFFFFFFFFu;
        foreach (var b in data)
        {
            crc ^= b;
            for (var k = 0; k < 8; k++) crc = (crc & 1) != 0 ? (crc >> 1) ^ 0xEDB88320u : crc >> 1;
        }
        return ~crc;
    }
}