using System.Collections.ObjectModel;
using System.Diagnostics;
using System.Globalization;
using System.Text;
using AvParser.Core.Parsing;
using AvParser.Core.Proxies;
using AvParser.Core.Settings;
using AvParser.UI.Localization;
using AvParser.UI.Navigation;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using ReactiveUI;
using ReactiveUI.Primitives;
using ReactiveUI.Primitives.Concurrency;
using ReactiveUI.Primitives.Signals;
using ReactiveUI.SourceGenerators;

namespace AvParser.UI.ViewModels;

/// <summary>Runs a parser over pasted text and streams the results into the UI.</summary>
/// <remarks>
/// This page exists to exercise the whole <see cref="IParser{TInput,TOutput}"/> contract —
/// streaming, progress and cancellation — rather than to be a finished feature.
/// </remarks>
public partial class ParseViewModel : PageViewModel, IDisposable
{
    /// <summary>Records buffered before being pushed to the UI collection in one go.</summary>
    private const int BatchSize = 512;

    /// <summary>
    /// Upper bound on rows shown. Beyond this the parse still completes and the count stays
    /// accurate, but the list stops growing — truncation is reported, never silent.
    /// </summary>
    private const int MaxDisplayedRecords = 20_000;

    private readonly IParserCatalog _catalog;
    private readonly ISettingsService _settings;
    private readonly IProxyPool _proxyPool;
    private readonly IServiceProvider _services;
    private readonly ILogger<ParseViewModel> _logger;
    private readonly ISequencer _mainThread;
    private readonly ObservableAsPropertyHelper<bool> _isBusy;

    private readonly Signal<RxVoid> _proxyChanged = new();

    private CancellationTokenSource? _cancellation;

    /// <summary>Text to parse.</summary>
    [Reactive]
    public partial string InputText { get; set; }

    /// <summary>Parser applied by <see cref="ParseCommand"/>.</summary>
    [Reactive]
    public partial ParserViewModel SelectedParser { get; set; }

    /// <summary>Completion of the running parse, 0.0 to 1.0.</summary>
    [Reactive]
    public partial double Progress { get; set; }

    /// <summary>Outcome summary shown under the toolbar; <see langword="null"/> when idle.</summary>
    [Reactive]
    public partial string? StatusMessage { get; set; }

    /// <summary>
    /// Whether the selected parser needs the network but has no working proxy to use.
    /// </summary>
    /// <remarks>
    /// Only network parsers are gated. A parser that works on text the user pasted has nothing to
    /// route, and blocking it would make the app unusable whenever the public lists are down.
    /// </remarks>
    [Reactive]
    public partial bool IsBlockedWithoutProxy { get; set; }

    /// <summary>Creates the page.</summary>
    /// <param name="catalog">Available parsers.</param>
    /// <param name="settings">Used to remember the selected parser.</param>
    /// <param name="proxyPool">Consulted for the live count that gates network parsers.</param>
    /// <param name="services">Resolves the navigation service lazily, to keep pages acyclic.</param>
    /// <param name="logger">Diagnostics.</param>
    /// <param name="mainThread">
    /// Scheduler used to marshal collection and progress updates back to the UI thread. Tests
    /// pass <see cref="ImmediateSequencer.Instance"/> to make everything synchronous.
    /// </param>
    public ParseViewModel(
        IParserCatalog catalog,
        ISettingsService settings,
        IProxyPool proxyPool,
        IServiceProvider services,
        ILogger<ParseViewModel> logger,
        ISequencer? mainThread = null
    )
    {
        _catalog = catalog ?? throw new ArgumentNullException(nameof(catalog));
        _settings = settings ?? throw new ArgumentNullException(nameof(settings));
        _proxyPool = proxyPool ?? throw new ArgumentNullException(nameof(proxyPool));
        _services = services ?? throw new ArgumentNullException(nameof(services));
        _logger = logger ?? throw new ArgumentNullException(nameof(logger));
        _mainThread = mainThread ?? RxSchedulers.MainThreadScheduler;

        InputText = string.Empty;
        Parsers = [.. catalog.Parsers.Select(parser => new ParserViewModel(parser))];
        SelectedParser = Parsers.First(parser => parser.Id == catalog.FindOrDefault(settings.Current.LastParserId).Id);

        // The pool changes on every lease outcome and on every probe, so coalesce before
        // re-evaluating whether the parser is allowed to run.
        _proxyPool.Changed += OnProxyPoolChanged;
        _proxyChanged
            .Throttle(TimeSpan.FromMilliseconds(250), _mainThread)
            .ObserveOn(_mainThread)
            .Subscribe(_ => RefreshProxyGate());

        RefreshProxyGate();

        var canParse = this.WhenAnyValue(
                x => x.InputText,
                x => x.IsBlockedWithoutProxy,
                static (text, blocked) => (text, blocked)
            )
            .Select(static state => !string.IsNullOrWhiteSpace(state.text) && !state.blocked)
            .DistinctUntilChanged();

        ParseCommand = ReactiveCommand.CreateFromTask(RunParseAsync, canParse, _mainThread);

        GoToProxiesCommand = ReactiveCommand.Create(
            () => _services.GetRequiredService<INavigationService>().NavigateTo<ProxiesViewModel>(),
            outputScheduler: _mainThread
        );
        _isBusy = ParseCommand.IsExecuting.ToProperty(this, nameof(IsBusy), false, _mainThread);

        CancelCommand = ReactiveCommand.Create(() => _cancellation?.Cancel(), ParseCommand.IsExecuting, _mainThread);

        ClearCommand = ReactiveCommand.Create(
            () =>
            {
                InputText = string.Empty;
                ClearResults();
                StatusMessage = null;
                Progress = 0d;
            },
            ParseCommand.IsExecuting.Select(static running => !running),
            _mainThread
        );

        LoadSampleCommand = ReactiveCommand.Create(
            () => InputText = SampleFor(SelectedParser.Id),
            ParseCommand.IsExecuting.Select(static running => !running),
            _mainThread
        );

        GenerateLargeSampleCommand = ReactiveCommand.Create(
            () => InputText = LargeSampleFor(SelectedParser.Id),
            ParseCommand.IsExecuting.Select(static running => !running),
            _mainThread
        );

        // Remember the parser choice; the debounced settings service coalesces the writes.
        // Switching parser can also change whether the gate applies, since only network parsers
        // are gated.
        this.WhenAnyValue(x => x.SelectedParser)
            .Where(static parser => parser is not null)
            .Subscribe(parser =>
            {
                _settings.Update(current => current with { LastParserId = parser.Id });
                RefreshProxyGate();
            });

        _settings.Changes.Subscribe(_ => RefreshProxyGate());

        // Errors surfacing from any command must not tear the process down.
        ParseCommand.ThrownExceptions.Subscribe(OnCommandFailed);
    }

    /// <inheritdoc />
    public override string TitleKey => "Page.Parse";

    /// <inheritdoc />
    public override string IconKey => "IconDocument";

    /// <summary>Every registered parser, for the picker.</summary>
    public IReadOnlyList<ParserViewModel> Parsers { get; }

    /// <summary>Successfully parsed records, capped at <see cref="MaxDisplayedRecords"/>.</summary>
    public ObservableCollection<ParsedRecord> Records { get; } = [];

    /// <summary>Per-line failures. A failure never aborts the parse.</summary>
    public ObservableCollection<ParseErrorViewModel> Errors { get; } = [];

    /// <summary>Whether a parse is currently running.</summary>
    public bool IsBusy => _isBusy.Value;

    /// <summary>Runs <see cref="SelectedParser"/> over <see cref="InputText"/>.</summary>
    public ReactiveCommand<RxVoid, RxVoid> ParseCommand { get; }

    /// <summary>Cancels the running parse.</summary>
    public ReactiveCommand<RxVoid, RxVoid> CancelCommand { get; }

    /// <summary>Clears the input and all results.</summary>
    public ReactiveCommand<RxVoid, RxVoid> ClearCommand { get; }

    /// <summary>Fills the input with a small example for the selected parser.</summary>
    public ReactiveCommand<RxVoid, string> LoadSampleCommand { get; }

    /// <summary>Fills the input with 50 000 rows, so progress and cancellation are observable.</summary>
    public ReactiveCommand<RxVoid, string> GenerateLargeSampleCommand { get; }

    /// <summary>Takes the user to the page where the proxy problem can be fixed.</summary>
    public ReactiveCommand<RxVoid, RxVoid> GoToProxiesCommand { get; }

    /// <summary>Explains why parsing is blocked.</summary>
    public string ProxyRequiredMessage => Localizer.Instance["Parse.ProxyRequired"];

    /// <summary>Re-evaluates the proxy gate. Exposed so tests can drive it without waiting.</summary>
    public void RefreshProxyGate()
    {
        var settings = _settings.Current;

        IsBlockedWithoutProxy =
            SelectedParser.Parser.RequiresNetwork && !settings.AllowDirectConnection && _proxyPool.LiveCount == 0;
    }

    private void OnProxyPoolChanged(object? sender, EventArgs e) => _proxyChanged.OnNext(RxVoid.Default);

    private async Task RunParseAsync(CancellationToken commandToken)
    {
        using var cancellation = CancellationTokenSource.CreateLinkedTokenSource(commandToken);
        _cancellation = cancellation;

        var parser = SelectedParser.Parser;
        var input = InputText;
        var token = cancellation.Token;

        ClearResults();
        Progress = 0d;
        StatusMessage = null;

        var recordBuffer = new List<ParsedRecord>(BatchSize);
        var errorBuffer = new List<ParseError>(16);
        var progress = new Progress<ParseProgress>(value => OnUi(() => Progress = value.Fraction));

        var stopwatch = Stopwatch.StartNew();
        var succeeded = 0;
        var failed = 0;
        var truncated = false;
        var cancelled = false;

        try
        {
            await foreach (var outcome in parser.ParseAsync(input, progress, token).ConfigureAwait(false))
            {
                if (outcome.IsSuccess)
                {
                    succeeded++;
                    if (succeeded <= MaxDisplayedRecords)
                    {
                        recordBuffer.Add(outcome.Value!);
                    }
                    else
                    {
                        truncated = true;
                    }
                }
                else
                {
                    failed++;
                    errorBuffer.Add(outcome.Error);
                }

                if (recordBuffer.Count >= BatchSize)
                {
                    FlushBuffers(recordBuffer, errorBuffer);
                }
            }
        }
        catch (OperationCanceledException)
        {
            cancelled = true;
        }
        finally
        {
            _cancellation = null;
            FlushBuffers(recordBuffer, errorBuffer);
            stopwatch.Stop();
        }

        var summary = BuildSummary(succeeded, failed, stopwatch.Elapsed, truncated, cancelled);
        OnUi(() =>
        {
            StatusMessage = summary;
            Progress = cancelled ? Progress : 1d;
        });

        _logger.LogInformation(
            "Parsed with {Parser}: {Succeeded} record(s), {Failed} error(s) in {Elapsed}",
            parser.Id,
            succeeded,
            failed,
            stopwatch.Elapsed
        );
    }

    /// <summary>
    /// Builds the outcome line out of translated fragments.
    /// </summary>
    /// <remarks>
    /// Assembled from plural-aware pieces rather than one format string per case: Russian needs
    /// three forms for a counted noun, so "{0} records" with an English plural glued on cannot be
    /// translated correctly.
    /// </remarks>
    internal static string BuildSummary(int succeeded, int failed, TimeSpan elapsed, bool truncated, bool cancelled)
    {
        var loc = Localizer.Instance;
        var records = loc.Plural("Parse.Count.Records", succeeded);
        var milliseconds = elapsed.TotalMilliseconds.ToString("N0", CultureInfo.CurrentCulture);

        var text = loc.Format(cancelled ? "Parse.Status.Cancelled" : "Parse.Status.Done", records, milliseconds);

        if (failed > 0)
        {
            text = loc.Format("Parse.Status.WithErrors", text.TrimEnd('.'), loc.Plural("Parse.Count.Errors", failed));
        }

        if (truncated)
        {
            text += " " + loc.Format("Parse.Status.Truncated", loc.Plural("Parse.Count.Records", MaxDisplayedRecords));
        }

        return text;
    }

    private void FlushBuffers(List<ParsedRecord> records, List<ParseError> errors)
    {
        if (records.Count == 0 && errors.Count == 0)
        {
            return;
        }

        // Copy before clearing: the scheduled callback may run after the loop has refilled these.
        var recordBatch = records.ToArray();
        var errorBatch = errors.ToArray();
        records.Clear();
        errors.Clear();

        OnUi(() =>
        {
            foreach (var record in recordBatch)
            {
                Records.Add(record);
            }

            foreach (var error in errorBatch)
            {
                Errors.Add(new ParseErrorViewModel(error));
            }
        });
    }

    /// <inheritdoc />
    /// <remarks>The pool is a singleton and would otherwise keep this page alive for the process.</remarks>
    public void Dispose()
    {
        _proxyPool.Changed -= OnProxyPoolChanged;
        _proxyChanged.Dispose();
        GC.SuppressFinalize(this);
    }

    /// <inheritdoc />
    protected override void OnLanguageChanged()
    {
        base.OnLanguageChanged();

        this.RaisePropertyChanged(nameof(ProxyRequiredMessage));

        // The summary and the listed errors were both rendered in the previous language.
        StatusMessage = null;

        foreach (var error in Errors)
        {
            error.Refresh();
        }
    }

    private void ClearResults()
    {
        Records.Clear();
        Errors.Clear();
    }

    private void OnCommandFailed(Exception exception)
    {
        _logger.LogError(exception, "Parse failed");
        OnUi(() => StatusMessage = Localizer.Instance.Format("Parse.Status.Failed", exception.Message));
    }

    /// <summary>Marshals a mutation onto the UI thread; the parse loop runs on the thread pool.</summary>
    private void OnUi(Action action) => _mainThread.Schedule(action);

    private static string SampleFor(string parserId) =>
        parserId switch
        {
            "key-value" => """
                # Sample configuration
                host = localhost
                port: 8080
                enabled = true
                name = av-parser
                """,
            _ => """
                id,name,role
                1,Ada Lovelace,Analyst
                2,Grace Hopper,Compiler
                3,Alan Turing,Cryptanalyst
                """,
        };

    private static string LargeSampleFor(string parserId)
    {
        const int rows = 50_000;
        var text = new StringBuilder(rows * 24);

        if (parserId == "key-value")
        {
            for (var i = 0; i < rows; i++)
            {
                text.Append("key").Append(i).Append(" = value").Append(i).Append('\n');
            }

            return text.ToString();
        }

        text.Append("id,name,score\n");
        for (var i = 0; i < rows; i++)
        {
            text.Append(i).Append(",item-").Append(i).Append(',').Append(i % 100).Append('\n');
        }

        return text.ToString();
    }
}