// Adapted from https://github.com/dotnet/runtime/blob/v5.0.12/src/libraries/Common/src/System/IO/PathInternal.Windows.cs

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using System.Runtime.CompilerServices;
using System.IO;

namespace Duplicati.Library.Common.IO
{
    /// <summary>Contains internal path helpers that are shared between many projects.</summary>
    internal static class PathInternalWindows
    {
        // All paths in Win32 ultimately end up becoming a path to a File object in the Windows object manager. Passed in paths get mapped through
        // DosDevice symbolic links in the object tree to actual File objects under \Devices. To illustrate, this is what happens with a typical
        // path "Foo" passed as a filename to any Win32 API:
        //
        //  1. "Foo" is recognized as a relative path and is appended to the current directory (say, "C:\" in our example)
        //  2. "C:\Foo" is prepended with the DosDevice namespace "\??\"
        //  3. CreateFile tries to create an object handle to the requested file "\??\C:\Foo"
        //  4. The Object Manager recognizes the DosDevices prefix and looks
        //      a. First in the current session DosDevices ("\Sessions\1\DosDevices\" for example, mapped network drives go here)
        //      b. If not found in the session, it looks in the Global DosDevices ("\GLOBAL??\")
        //  5. "C:" is found in DosDevices (in our case "\GLOBAL??\C:", which is a symbolic link to "\Device\HarddiskVolume6")
        //  6. The full path is now "\Device\HarddiskVolume6\Foo", "\Device\HarddiskVolume6" is a File object and parsing is handed off
        //      to the registered parsing method for Files
        //  7. The registered open method for File objects is invoked to create the file handle which is then returned
        //
        // There are multiple ways to directly specify a DosDevices path. The final format of "\??\" is one way. It can also be specified
        // as "\\.\" (the most commonly documented way) and "\\?\". If the question mark syntax is used the path will skip normalization
        // (essentially GetFullPathName()) and path length checks.

        // Windows Kernel-Mode Object Manager
        // https://msdn.microsoft.com/en-us/library/windows/hardware/ff565763.aspx
        // https://channel9.msdn.com/Shows/Going+Deep/Windows-NT-Object-Manager
        //
        // Introduction to MS-DOS Device Names
        // https://msdn.microsoft.com/en-us/library/windows/hardware/ff548088.aspx
        //
        // Local and Global MS-DOS Device Names
        // https://msdn.microsoft.com/en-us/library/windows/hardware/ff554302.aspx

        internal const string ExtendedDevicePathPrefix = @"\\?\";
        internal const string UncPathPrefix = @"\\";
        internal const string UncDevicePrefixToInsert = @"?\UNC\";
        internal const string UncExtendedPathPrefix = @"\\?\UNC\";
        internal const string DevicePathPrefix = @"\\.\";

        internal const int MaxShortPath = 260;

        // \\?\, \\.\, \??\
        internal const int DevicePrefixLength = 4;

        /// <summary>
        /// Returns true if the given character is a valid drive letter
        /// </summary>
        internal static bool IsValidDriveChar(char value)
        {
            return ((value >= 'A' && value <= 'Z') || (value >= 'a' && value <= 'z'));
        }

        /// <summary>
        /// Returns true if the path specified is relative to the current drive or working directory.
        /// Returns false if the path is fixed to a specific drive or UNC path.  This method does no
        /// validation of the path (URIs will be returned as relative as a result).
        /// </summary>
        /// <remarks>
        /// Handles paths that use the alternate directory separator.  It is a frequent mistake to
        /// assume that rooted paths (Path.IsPathRooted) are not relative.  This isn't the case.
        /// "C:a" is drive relative- meaning that it will be resolved against the current directory
        /// for C: (rooted, but relative). "C:\a" is rooted and not relative (the current directory
        /// will not be used to modify the path).
        /// </remarks>
        internal static bool IsPartiallyQualified(string path)
        {
            if (path.Length < 2)
            {
                // It isn't fixed, it must be relative.  There is no way to specify a fixed
                // path with one character (or less).
                return true;
            }

            if (IsDirectorySeparator(path[0]))
            {
                // There is no valid way to specify a relative path with two initial slashes or
                // \? as ? isn't valid for drive relative paths and \??\ is equivalent to \\?\
                return !(path[1] == '?' || IsDirectorySeparator(path[1]));
            }

            // The only way to specify a fixed path that doesn't begin with two slashes
            // is the drive, colon, slash format- i.e. C:\
            return !((path.Length >= 3)
                && (path[1] == Path.VolumeSeparatorChar)
                && IsDirectorySeparator(path[2])
                // To match old behavior we'll check the drive character for validity as the path is technically
                // not qualified if you don't have a valid drive. "=:\" is the "=" file's default data stream.
                && IsValidDriveChar(path[0]));
        }

        /// <summary>
        /// True if the given character is a directory separator.
        /// </summary>
        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal static bool IsDirectorySeparator(char c)
        {
            return c == Path.DirectorySeparatorChar || c == Path.AltDirectorySeparatorChar;
        }
    }
}