using System.Security.Authentication;
using CommunityToolkit.Mvvm.ComponentModel;
using CommunityToolkit.Mvvm.Input;
using DodoSSH.Client.Auth;
using DodoSSH.Client.Session;
using DodoSSH.Client.Ssh;
using DodoSSH.Client.Storage;
using DodoSSH.Client.Terminal;
using DodoSSH.Crypto;

namespace DodoSSH.Client.App.ViewModels;

/// <summary>Which of the shell's mutually exclusive screens is showing.</summary>
internal enum ShellState
{
    /// <summary>Reading the cache to find out whether this machine is enrolled.</summary>
    Starting = 0,

    /// <summary>Nothing is cached. The user has to name a server and sign in, which needs a network.</summary>
    NeedsServer = 1,

    /// <summary>Signed in, but the account has no vault key yet.</summary>
    NeedsEnrollment = 2,

    /// <summary>
    /// Showing the recovery code, and refusing to move on until the user confirms they have it.
    /// </summary>
    /// <remarks>
    /// A separate state rather than a dismissible banner, because this is the only moment the code exists.
    /// Losing it along with the passphrase means the vault is unrecoverable and there is no server-side
    /// reset by design — so this is the one screen a user must not be able to click past.
    /// </remarks>
    ShowingRecoveryCode = 3,

    /// <summary>Enrolled. The passphrase opens the vault, with or without a network.</summary>
    Locked = 4,

    /// <summary>Open.</summary>
    Unlocked = 5,
}

/// <summary>
/// The shell: get to an unlocked vault, then hand over to <see cref="VaultViewModel"/>.
/// </summary>
/// <remarks>
/// <para>
/// The order of these states is the product's onboarding story. A fresh machine needs a server URL and one
/// browser sign-in; everything about the identity provider comes from
/// <c>/.well-known/dodossh-configuration</c>, so the user never configures an authority or a client id.
/// After that the network is optional — the cached salt and wrapped bundle mean the passphrase alone
/// unlocks, which is the state the application spends nearly all of its life in.
/// </para>
/// <para>
/// Key derivation runs on a worker thread. At the shipped profile it is a third of a second of solid CPU,
/// and doing that on the UI thread would freeze the window at exactly the moment the user is watching it.
/// </para>
/// </remarks>
internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisposable
{
    private readonly ClientPaths paths;
    private readonly ClientCacheFactory caches;
    private readonly TerminalWorkspace workspace;
    private readonly IKnownHostStore knownHosts;
    private readonly SignInHandler signIn;
    private readonly TimeProvider clock;
    private readonly Argon2Profile? passphraseProfile;

    private IVaultServer? connection;
    private bool disposed;

    /// <summary>
    /// Establishes a connection to a server.
    /// </summary>
    /// <remarks>
    /// A delegate rather than a direct call to <see cref="ServerConnection.SignInAsync"/>, so this whole
    /// state machine can be driven by a test against an in-memory server. Sign-in is the one step that
    /// genuinely needs a browser and a network, and letting it be the reason nothing else is testable
    /// would be the wrong trade.
    /// </remarks>
    internal delegate Task<IVaultServer> SignInHandler(Uri serverUrl, CancellationToken cancellationToken);

    internal MainWindowViewModel(
        ClientPaths paths,
        ClientCacheFactory caches,
        TerminalWorkspace workspace,
        IKnownHostStore knownHosts,
        SignInHandler signIn,
        TimeProvider clock,
        Argon2Profile? passphraseProfile = null)
    {
        this.paths = paths;
        this.caches = caches;
        this.workspace = workspace;
        this.knownHosts = knownHosts;
        this.signIn = signIn;
        this.clock = clock;
        this.passphraseProfile = passphraseProfile;
    }

    [ObservableProperty]
    private ShellState state = ShellState.Starting;

    [ObservableProperty]
    private string statusMessage = "Opening the local cache…";

    [ObservableProperty]
    private bool isBusy;

    /// <remarks>
    /// The address <c>dotnet run --project src/DodoSSH.Api</c> actually serves, so the first launch after
    /// a clone works without the user having to know a port. This was <c>https://localhost:7217</c>, which
    /// is the API's <em>second</em> launch profile: the first is HTTP on 5233 and is the one both the
    /// README and a plain <c>dotnet run</c> select, so nothing was listening on 7217. Pointing an HTTPS
    /// client at a plaintext port fails as "The SSL connection could not be established", which sends
    /// people looking for a certificate problem — see <see cref="ExplainSignInFailure" />. A real
    /// deployment is HTTPS behind a proxy and its address is typed over this one; the placeholder in the
    /// setup card shows that shape.
    /// </remarks>
    [ObservableProperty]
    private string serverUrl = "http://localhost:5233";

    [ObservableProperty]
    private string passphrase = string.Empty;

    [ObservableProperty]
    private string confirmPassphrase = string.Empty;

    /// <summary>Shown once, immediately after enrolling, and never stored anywhere.</summary>
    [ObservableProperty]
    private string? recoveryCode;

    [ObservableProperty]
    private bool recoveryCodeWrittenDown;

    /// <summary>Who this machine is enrolled as, readable without the passphrase.</summary>
    [ObservableProperty]
    private string? accountName;

    [ObservableProperty]
    private VaultViewModel? vault;

    /// <summary>
    /// Shells that were left running when the vault was locked.
    /// </summary>
    /// <remarks>
    /// Refreshed by <see cref="LockAsync"/>, which is where the policy this reports is explained.
    /// </remarks>
    [ObservableProperty]
    private int liveSessionCount;

    internal bool HasLiveSessions => LiveSessionCount > 0;

    /// <summary>The count as a sentence, because a bare number on a lock screen explains nothing.</summary>
    internal string LiveSessionSummary => LiveSessionCount == 1
        ? "1 shell is still connected and still running."
        : $"{LiveSessionCount} shells are still connected and still running.";

    /// <summary>Where the embedded browser should navigate.</summary>
    internal Uri TerminalPageUrl => workspace.PageUrl;

    /// <summary>
    /// Raised when a terminal session opens, so the view can hand the terminal the keyboard.
    /// </summary>
    /// <remarks>
    /// Forwarded from <see cref="VaultViewModel.SessionOpened"/> rather than exposed there directly,
    /// because <see cref="Vault"/> is replaced on every unlock and the view would have to re-subscribe
    /// each time. This shell is the window's data context for the life of the process, so one
    /// subscription is enough.
    /// </remarks>
    internal event EventHandler? TerminalSessionOpened;

    internal bool IsStarting => State == ShellState.Starting;

    internal bool IsNeedingServer => State == ShellState.NeedsServer;

    internal bool IsNeedingEnrollment => State == ShellState.NeedsEnrollment;

    internal bool IsShowingRecoveryCode => State == ShellState.ShowingRecoveryCode;

    internal bool IsLocked => State == ShellState.Locked;

    internal bool IsUnlocked => State == ShellState.Unlocked;

    /// <summary>Whether a connection to the server is currently held.</summary>
    internal bool IsOnline => connection is not null;

    /// <summary>
    /// Brings the schema up to date and works out which screen to show.
    /// </summary>
    /// <remarks>
    /// Migrating happens before unlock and touches no encrypted content — only the shape of the tables.
    /// That is the point of migrating rather than recreating: a user who upgrades while offline must still
    /// be able to open their vault.
    /// </remarks>
    internal async Task StartAsync(CancellationToken cancellationToken)
    {
        try
        {
            paths.EnsureCreated();
            await caches.MigrateAsync(cancellationToken).ConfigureAwait(true);

            var profile = await Opener().ReadProfileAsync(cancellationToken).ConfigureAwait(true);

            if (profile is null)
            {
                State = ShellState.NeedsServer;
                StatusMessage = "Sign in to a DodoSSH server to set this machine up.";
                return;
            }

            AccountName = profile.DisplayName ?? profile.Email ?? profile.Subject;
            ServerUrl = profile.ServerUrl;
            State = ShellState.Locked;
            StatusMessage = $"Enrolled against {profile.ServerUrl}.";
        }
        catch (Exception exception) when (exception is not OperationCanceledException)
        {
            State = ShellState.NeedsServer;
            StatusMessage = $"The local cache could not be opened: {exception.Message}";
        }
    }

    /// <summary>Discovers the server and runs the browser sign-in.</summary>
    [RelayCommand]
    private async Task SignInAsync(CancellationToken cancellationToken)
    {
        if (!Uri.TryCreate(ServerUrl, UriKind.Absolute, out var url))
        {
            StatusMessage = "That is not a valid server URL.";
            return;
        }

        // Checked separately from parsing, because "localhost:5233" parses perfectly well as an absolute
        // URI whose scheme is "localhost" — and then fails much later with something unrelated to the
        // actual mistake.
        if (url.Scheme is not ("http" or "https"))
        {
            StatusMessage = $"A server URL has to start with http:// or https://, not {url.Scheme}:.";
            return;
        }

        await RunAsync(
            "Opening your browser to sign in…",
            explain: exception => ExplainSignInFailure(exception, url),
            work: async () =>
            {
                connection?.Dispose();
                connection = null;

                connection = await signIn(url, cancellationToken).ConfigureAwait(true);

                OnPropertyChanged(nameof(IsOnline));

                var outcome = await Provisioner()!
                    .RefreshAsync(ServerUrl, cancellationToken)
                    .ConfigureAwait(true);

                AccountName = outcome.Me.DisplayName ?? outcome.Me.Email ?? outcome.Me.Subject;
                StatusMessage = outcome.Message;

                State = outcome.Status == ProvisionStatus.EnrollmentRequired
                    ? ShellState.NeedsEnrollment
                    : ShellState.Locked;
            }).ConfigureAwait(true);
    }

    /// <summary>Creates the identity key and the personal vault.</summary>
    [RelayCommand]
    private async Task EnrollAsync(CancellationToken cancellationToken)
    {
        if (Provisioner() is not { } provisioner)
        {
            StatusMessage = "Sign in first.";
            return;
        }

        if (!ValidateNewPassphrase())
        {
            return;
        }

        await RunAsync(
            "Creating your vault. This deliberately takes a moment…",
            async () =>
            {
                var chosen = Passphrase;

                var outcome = await Task
                    .Run(
                        () => provisioner.EnrollAsync(
                            ServerUrl,
                            chosen,
                            Environment.MachineName,
                            "Personal",
                            cancellationToken),
                        cancellationToken)
                    .ConfigureAwait(true);

                ConfirmPassphrase = string.Empty;
                RecoveryCode = outcome.RecoveryCode;
                RecoveryCodeWrittenDown = false;
                StatusMessage = outcome.Message;

                // A brand-new account always yields a code. An account someone else already enrolled does
                // not, and there is nothing to show.
                State = RecoveryCode is null ? ShellState.Locked : ShellState.ShowingRecoveryCode;
            }).ConfigureAwait(true);
    }

    /// <summary>Leaves the recovery-code screen, once the user says they have it.</summary>
    [RelayCommand]
    private void ConfirmRecoveryCode()
    {
        if (!RecoveryCodeWrittenDown)
        {
            StatusMessage = "Confirm you have written the recovery code down first.";
            return;
        }

        // Cleared from memory as well as from the screen. It was never persisted, and keeping it in a view
        // model for the rest of the session would undo that.
        RecoveryCode = null;
        State = ShellState.Locked;
        StatusMessage = "Unlock with the passphrase you just chose.";
    }

    /// <summary>Opens the vault.</summary>
    [RelayCommand]
    private async Task UnlockAsync(CancellationToken cancellationToken)
    {
        if (Passphrase.Length == 0)
        {
            StatusMessage = "Enter your vault passphrase.";
            return;
        }

        await RunAsync(
            "Unlocking…",
            async () =>
            {
                var entered = Passphrase;

                // Off the UI thread: Argon2id at the shipped profile is a third of a second of solid CPU
                // and would otherwise freeze the window mid-unlock.
                var outcome = await Task
                    .Run(() => Opener().UnlockAsync(entered, cancellationToken), cancellationToken)
                    .ConfigureAwait(true);

                StatusMessage = outcome.Message;

                if (!outcome.IsUnlocked)
                {
                    return;
                }

                Passphrase = string.Empty;

                Vault = new VaultViewModel(outcome.Session!, workspace, knownHosts, () => connection);
                State = ShellState.Unlocked;

                await Vault.LoadAsync(cancellationToken).ConfigureAwait(true);

                // After the first load, so the list is on screen before anything talks to a server. The
                // loop is started from the UI thread deliberately: every pass resumes here, which is what
                // keeps the observable collections single-threaded.
                Vault.StartAutoSync();
            }).ConfigureAwait(true);
    }

    /// <summary>
    /// Closes the vault and forgets every key it held. Open shells keep running.
    /// </summary>
    /// <remarks>
    /// <para>
    /// <b>Lock is a vault operation, and deliberately not a disconnect.</b> The reason a person locks is
    /// that they are walking away from the machine, which is exactly the moment a long upgrade, build or
    /// transfer is most likely to be in flight — so killing every shell would make Lock a button that
    /// destroys work, and the predictable response is to stop using it and leave the vault open instead.
    /// The same argument decides it for the idle auto-lock this will grow: an unattended timeout that
    /// terminated a running job would be worse than the exposure it removes.
    /// </para>
    /// <para>
    /// <b>What "locked" therefore describes.</b> Disposing the vault zeroes the identity keys, the vault
    /// keys and the cache key, so nothing on disk can be read without the passphrase again. It says
    /// nothing about this machine's access to remote hosts: an SSH channel authenticated at connect time
    /// needs no vault key to keep running, and the credential it used was already spent. Locking cannot
    /// retroactively un-authorise a session any more than revocation can — the same honest limit the
    /// README records for a removed team member. So a locked DodoSSH still holds open, authenticated
    /// channels, and <see cref="LiveSessionCount"/> is shown on the unlock screen rather than left to be
    /// inferred from a terminal that the lock screen hides.
    /// </para>
    /// <para>
    /// The count is a snapshot taken here. While locked it can only fall — opening a session needs the
    /// vault — so a stale value over-reports and never under-reports, which is the safe direction for a
    /// warning of this kind.
    /// </para>
    /// </remarks>
    [RelayCommand]
    private async Task LockAsync()
    {
        if (Vault is { } open)
        {
            Vault = null;
            await open.DisposeAsync().ConfigureAwait(true);
        }

        LiveSessionCount = workspace.LiveSessionCount;

        State = ShellState.Locked;
        StatusMessage = "Locked.";
    }

    /// <inheritdoc />
    public async ValueTask DisposeAsync()
    {
        if (disposed)
        {
            return;
        }

        disposed = true;

        if (Vault is { } open)
        {
            await open.DisposeAsync().ConfigureAwait(false);
        }

        connection?.Dispose();
    }

    /// <remarks>
    /// The passphrase is the entire defence for the vault — docs/crypto.md §2 says so plainly, and no
    /// server-side reset exists. A length floor is a crude check and still the one that matters most.
    /// </remarks>
    private bool ValidateNewPassphrase()
    {
        if (Passphrase.Length < 12)
        {
            StatusMessage = "Use a passphrase of at least 12 characters.";
            return false;
        }

        if (!string.Equals(Passphrase, ConfirmPassphrase, StringComparison.Ordinal))
        {
            StatusMessage = "The two passphrases do not match.";
            return false;
        }

        return true;
    }

    /// <remarks>
    /// Sync limits come from the server when there is one, so a batch is never larger than this
    /// particular deployment accepts. Offline, the defaults apply and nothing is pushed anyway.
    /// </remarks>
    private SessionOpener Opener() => new(caches, clock, connection?.SyncOptions);

    private AccountProvisioner? Provisioner() =>
        connection is null
            ? null
            : new AccountProvisioner(
                connection.Account, connection.KeyBinding, caches, clock, passphraseProfile);

    /// <remarks>
    /// Every command funnels through here so the busy flag and the failure message are handled once. A
    /// command that forgot either would leave the window permanently disabled or silently doing nothing.
    /// </remarks>
    /// <param name="busyMessage">Shown while the work runs.</param>
    /// <param name="work">The work.</param>
    /// <param name="explain">
    /// Turns a failure into something a user can act on. Optional, because most failures here already
    /// carry their own explanation; the ones that do not are the ones crossing into another process's
    /// vocabulary, where the exception describes a symptom and not the mistake.
    /// </param>
    private async Task RunAsync(
        string busyMessage,
        Func<Task> work,
        Func<Exception, string>? explain = null)
    {
        if (IsBusy)
        {
            return;
        }

        IsBusy = true;
        StatusMessage = busyMessage;

        try
        {
            await work().ConfigureAwait(true);
        }
        catch (OperationCanceledException)
        {
            StatusMessage = "Cancelled.";
        }
        catch (Exception exception)
        {
            StatusMessage = explain?.Invoke(exception) ?? exception.Message;
        }
        finally
        {
            IsBusy = false;
        }
    }

    /// <remarks>
    /// One case earns a translation rather than the exception's own words: pointing an HTTPS client at a
    /// plaintext port reports "The SSL connection could not be established", which sends people looking
    /// for a certificate problem. The scheme is the mistake, and the development stack serves HTTP, so
    /// this is the first thing a new user will hit.
    /// </remarks>
    private static string ExplainSignInFailure(Exception exception, Uri server)
    {
        var secureChannelFailed = exception is HttpRequestException
            && exception.GetBaseException() is AuthenticationException;

        if (secureChannelFailed && server.Scheme is "https")
        {
            var plain = new UriBuilder(server) { Scheme = "http" }.Uri;

            return $"{exception.Message} {server.Host} answered, but not with TLS. If this is a "
                + $"development server it probably serves plain HTTP — try {plain.GetLeftPart(UriPartial.Authority)}.";
        }

        return exception.Message;
    }

    /// <remarks>
    /// One place for the subscription, so unlocking, locking and disposing all route through it rather
    /// than each remembering to detach.
    /// </remarks>
    partial void OnVaultChanged(VaultViewModel? oldValue, VaultViewModel? newValue)
    {
        if (oldValue is not null)
        {
            oldValue.SessionOpened -= OnVaultSessionOpened;
        }

        if (newValue is not null)
        {
            newValue.SessionOpened += OnVaultSessionOpened;
        }
    }

    private void OnVaultSessionOpened(object? sender, EventArgs e) =>
        TerminalSessionOpened?.Invoke(this, e);

    partial void OnLiveSessionCountChanged(int value)
    {
        OnPropertyChanged(nameof(HasLiveSessions));
        OnPropertyChanged(nameof(LiveSessionSummary));
    }

    partial void OnStateChanged(ShellState value)
    {
        OnPropertyChanged(nameof(IsStarting));
        OnPropertyChanged(nameof(IsNeedingServer));
        OnPropertyChanged(nameof(IsNeedingEnrollment));
        OnPropertyChanged(nameof(IsShowingRecoveryCode));
        OnPropertyChanged(nameof(IsLocked));
        OnPropertyChanged(nameof(IsUnlocked));
    }
}