using System.ComponentModel;
using System.Runtime.InteropServices;
using System.Runtime.Versioning;
using Microsoft.Win32.SafeHandles;

/// <summary>Byte-for-byte the Windows descriptor observer of Sdk2LifecycleWindowsDescriptors.cs (lmxopcua#799), hosted for the SDK1 comparator.</summary>
internal static class WindowsDescriptors
{
    [SupportedOSPlatform("windows")]
    internal static Task<(int Total, int Pki)> CountAsync(CancellationToken ct, IReadOnlyList<string> roots)
    {
        var result = new TaskCompletionSource<(int Total, int Pki)>(TaskCreationOptions.RunContinuationsAsynchronously);
        var thread = new Thread(() =>
        {
            try { result.TrySetResult(WindowsHandleCounts(roots)); }
            catch (Exception error) { result.TrySetException(error); }
        }) { IsBackground = true, Name = "soak-descriptor-probe" };
        thread.Start();
        return BoundAsync();

        async Task<(int Total, int Pki)> BoundAsync()
        {
            try { return await result.Task.WaitAsync(TimeSpan.FromSeconds(5), ct).ConfigureAwait(false); }
            catch (TimeoutException) { throw new TimeoutException("Descriptor probe exceeded its handle-enumeration deadline."); }
        }
    }

    [SupportedOSPlatform("windows")]
    private static (int Total, int Pki) WindowsHandleCounts(IReadOnlyList<string> roots)
    {
        // Canonicalise the roots first and close those directory handles before the snapshot, so they are not counted.
        var canonicalRoots = roots.Select(WindowsCanonicalRoot).Distinct(StringComparer.OrdinalIgnoreCase).ToArray();
        var self = (nuint)Environment.ProcessId;
        var length = 1 << 20;
        while (true)
        {
            var buffer = Marshal.AllocHGlobal(length);
            try
            {
                var status = NtQuerySystemInformation(SystemExtendedHandleInformation, buffer, length, out var needed);
                if (status == StatusInfoLengthMismatch)
                {
                    length = Math.Max(length * 2, needed + (1 << 16));
                    continue;
                }
                if (status != 0) throw new Win32Exception(RtlNtStatusToDosError(status), $"NtQuerySystemInformation failed (0x{status:X8}).");
                var count = (long)Marshal.ReadIntPtr(buffer);
                var entries = buffer + 2 * IntPtr.Size;
                var entrySize = Marshal.SizeOf<HandleEntry>();
                int total = 0, pki = 0;
                var path = new char[32768];
                for (long i = 0; i < count; i++)
                {
                    var entry = Marshal.PtrToStructure<HandleEntry>(entries + (nint)(i * entrySize));
                    if (entry.UniqueProcessId != self) continue;
                    total++;
                    var handle = (nint)entry.HandleValue;
                    // GetFileType is answered by the I/O manager without the file-object lock; only disk files and
                    // directories go on to the name query (pipes, sockets and consoles cannot hold PKI material).
                    if (GetFileType(handle) != FileTypeDisk) continue;
                    var name = FinalPath(handle, path);
                    if (name is not null && WithinWindowsRoots(name, canonicalRoots)) pki++;
                }
                return (total, pki);
            }
            finally { Marshal.FreeHGlobal(buffer); }
        }
    }

    [SupportedOSPlatform("windows")]
    private static string WindowsCanonicalRoot(string root)
    {
        var full = Path.TrimEndingDirectorySeparator(Path.GetFullPath(root));
        if (!Directory.Exists(full)) return BaselineMetrics.CanonicalPath(full);
        using var directory = CreateFileW(full, 0, FileShareAll, 0, OpenExisting, FileFlagBackupSemantics, 0);
        if (directory.IsInvalid) throw new Win32Exception(Marshal.GetLastWin32Error(), "Cannot open a PKI root for canonicalisation.");
        return FinalPath(directory.DangerousGetHandle(), new char[32768])
            ?? throw new IOException("A PKI root has no final path.");
    }

    private static string? FinalPath(nint handle, char[] buffer)
    {
        var written = GetFinalPathNameByHandleW(handle, buffer, buffer.Length, 0);
        if (written == 0 || written >= buffer.Length) return null; // Closed/reused between snapshot and query, or not a named file.
        var name = new string(buffer, 0, (int)written);
        if (name.StartsWith(@"\\?\UNC\", StringComparison.Ordinal)) return @"\\" + name[8..];
        return name.StartsWith(@"\\?\", StringComparison.Ordinal) ? name[4..] : name;
    }

    private static bool WithinWindowsRoots(string path, IReadOnlyList<string> roots)
    {
        path = Path.TrimEndingDirectorySeparator(path);
        return roots.Any(root => path.Equals(root, StringComparison.OrdinalIgnoreCase)
            || path.StartsWith(root + Path.DirectorySeparatorChar, StringComparison.OrdinalIgnoreCase));
    }

    private const int SystemExtendedHandleInformation = 64;
    private const int StatusInfoLengthMismatch = unchecked((int)0xC0000004);
    private const uint FileTypeDisk = 1;
    private const uint FileShareAll = 7;
    private const uint OpenExisting = 3;
    private const uint FileFlagBackupSemantics = 0x02000000;

    [StructLayout(LayoutKind.Sequential)]
    private struct HandleEntry
    {
        public nint Object;
        public nuint UniqueProcessId;
        public nuint HandleValue;
        public uint GrantedAccess;
        public ushort CreatorBackTraceIndex;
        public ushort ObjectTypeIndex;
        public uint HandleAttributes;
        public uint Reserved;
    }

    [DllImport("ntdll.dll")]
    private static extern int NtQuerySystemInformation(int informationClass, nint information, int length, out int returnLength);

    [DllImport("ntdll.dll")]
    private static extern int RtlNtStatusToDosError(int status);

    [DllImport("kernel32.dll", SetLastError = true)]
    private static extern uint GetFileType(nint handle);

    [DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
    private static extern uint GetFinalPathNameByHandleW(nint handle, [Out] char[] path, int length, uint flags);

    [DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Unicode)]
    private static extern SafeFileHandle CreateFileW(string name, uint access, uint share, nint security, uint disposition,
        uint flags, nint template);
}