namespace XFESystem.Core;

/// <summary>A copy of resources already assigned by the PCI bus, never a request to map or resize a BAR.</summary>
public readonly record struct PciGpuMemoryBar(
    int Index, bool IsAssigned, bool IsIo, bool Is64Bit, bool IsPrefetchable, ulong Address, ulong Length);

/// <summary>PCI discovery evidence. It is deliberately separate from an initialized rendering backend.</summary>
public sealed record PciGpuDiscovery(
    string Address, ushort VendorId, ushort DeviceId, byte ClassCode, byte Subclass,
    bool PciExpress, int MessageInterruptCount, PciGpuMemoryBar[] Bars)
{
    public bool IsNvidiaDisplayController => VendorId == 0x10DE && ClassCode == 0x03;

    /// <summary>
    /// Checks copied resource geometry before a future driver may map it. This does not
    /// initialize GSP firmware, an address space, a GPU channel, or the CUDA driver ABI.
    /// </summary>
    public string ValidateRegisterWindow()
    {
        if (!IsNvidiaDisplayController) return "Not an NVIDIA display controller.";
        if (!PciExpress) return "PCI Express capability is absent.";
        if (Bars is null || Bars.Length is 0 or > 6) return "Invalid PCI BAR snapshot.";
        bool registers = false;
        for (int i = 0; i < Bars.Length; i++)
        {
            var bar = Bars[i];
            if (bar.Index is < 0 or > 5) return "Invalid PCI BAR index.";
            for (int previous = 0; previous < i; previous++)
                if (Bars[previous].Index == bar.Index) return "Duplicate PCI BAR index.";
            if (!bar.IsAssigned) continue;
            if (bar.IsIo) continue;
            if (bar.Address == 0 || bar.Length == 0 || bar.Length - 1 > ulong.MaxValue - bar.Address)
                return "Assigned memory BAR has an empty or overflowing address range.";
            if ((bar.Length & (bar.Length - 1)) != 0 || (bar.Address & (bar.Length - 1)) != 0)
                return "Assigned memory BAR is not aligned to its decoded size.";
            if (!bar.Is64Bit && (bar.Address > uint.MaxValue || bar.Length - 1 > uint.MaxValue - bar.Address))
                return "A 32-bit memory BAR extends beyond 4 GiB.";
            if (bar.Index == 0 && !bar.IsPrefetchable) registers = true;
        }
        return registers ? string.Empty : "NVIDIA BAR 0 has no assigned non-prefetchable register window.";
    }
}