using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;
using ZB.MOM.WW.OtOpcUa.Core.Stability;

namespace ZB.MOM.WW.OtOpcUa.Server.Hosting;

/// <summary>
///     Drives one or more <see cref="ScheduledRecycleScheduler"/> instances on a fixed tick
///     cadence. Closes Phase 6.1 Stream B.4 by turning the shipped-as-pure-logic scheduler
///     into a running background feature.
/// </summary>
/// <remarks>
///     <para>Registered as a singleton in Program.cs. Each Tier C driver instance that wants a
///     scheduled recycle registers its scheduler via
///     <see cref="AddScheduler(ScheduledRecycleScheduler)"/> at startup. The hosted service
///     wakes every <see cref="TickInterval"/> (default 1 min) and calls
///     <see cref="ScheduledRecycleScheduler.TickAsync"/> on each registered scheduler.</para>
///
///     <para>Scheduler registration is closed after <see cref="ExecuteAsync"/> starts — callers
///     must register before the host starts, typically during DI setup. Adding a scheduler
///     mid-flight throws to avoid confusing "some ticks saw my scheduler, some didn't" races.</para>
/// </remarks>
public sealed class ScheduledRecycleHostedService : BackgroundService
{
    private readonly List<ScheduledRecycleScheduler> _schedulers = [];
    private readonly ILogger<ScheduledRecycleHostedService> _logger;
    private readonly TimeProvider _timeProvider;
    private bool _started;

    /// <summary>How often <see cref="ScheduledRecycleScheduler.TickAsync"/> fires on each registered scheduler.</summary>
    public TimeSpan TickInterval { get; }

    public ScheduledRecycleHostedService(
        ILogger<ScheduledRecycleHostedService> logger,
        TimeProvider? timeProvider = null,
        TimeSpan? tickInterval = null)
    {
        _logger = logger;
        _timeProvider = timeProvider ?? TimeProvider.System;
        TickInterval = tickInterval ?? TimeSpan.FromMinutes(1);
    }

    /// <summary>Register a scheduler to drive. Must be called before the host starts.</summary>
    public void AddScheduler(ScheduledRecycleScheduler scheduler)
    {
        ArgumentNullException.ThrowIfNull(scheduler);
        if (_started)
            throw new InvalidOperationException(
                "Cannot register a ScheduledRecycleScheduler after the hosted service has started. " +
                "Register all schedulers during DI configuration / startup.");
        _schedulers.Add(scheduler);
    }

    /// <summary>Snapshot of the current tick count — diagnostics only.</summary>
    public int TickCount { get; private set; }

    /// <summary>Snapshot of the number of registered schedulers — diagnostics only.</summary>
    public int SchedulerCount => _schedulers.Count;

    public override Task StartAsync(CancellationToken cancellationToken)
    {
        _started = true;
        return base.StartAsync(cancellationToken);
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        _logger.LogInformation(
            "ScheduledRecycleHostedService starting — {Count} scheduler(s), tick interval = {Interval}",
            _schedulers.Count, TickInterval);

        while (!stoppingToken.IsCancellationRequested)
        {
            try
            {
                await Task.Delay(TickInterval, _timeProvider, stoppingToken).ConfigureAwait(false);
            }
            catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested)
            {
                break;
            }

            await TickOnceAsync(stoppingToken).ConfigureAwait(false);
        }

        _logger.LogInformation("ScheduledRecycleHostedService stopping after {TickCount} tick(s).", TickCount);
    }

    /// <summary>
    ///     Execute one scheduler tick against every registered scheduler. Factored out of the
    ///     <see cref="ExecuteAsync"/> loop so tests can drive it directly without needing to
    ///     synchronize with <see cref="Task.Delay(TimeSpan, TimeProvider, CancellationToken)"/>.
    /// </summary>
    public async Task TickOnceAsync(CancellationToken cancellationToken)
    {
        var now = _timeProvider.GetUtcNow().UtcDateTime;
        TickCount++;

        foreach (var scheduler in _schedulers)
        {
            try
            {
                var fired = await scheduler.TickAsync(now, cancellationToken).ConfigureAwait(false);
                if (fired)
                    _logger.LogInformation("Scheduled recycle fired at {Now:o}; next = {Next:o}",
                        now, scheduler.NextRecycleUtc);
            }
            catch (OperationCanceledException) { throw; }
            catch (Exception ex)
            {
                // A single scheduler fault must not take down the rest — log + continue.
                _logger.LogError(ex,
                    "ScheduledRecycleScheduler tick failed at {Now:o}; continuing to other schedulers.", now);
            }
        }
    }
}
