using System.Collections.Concurrent;
using GmRelay.Shared.Features.Confirmation.SendConfirmation;
using GmRelay.Shared.Features.Reminders.SendJoinLink;
using GmRelay.Shared.Features.Reminders.SendOneHourReminder;
using GmRelay.Shared.Platform;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;

namespace GmRelay.Shared.Infrastructure.Scheduling;

/// <summary>
/// Stateless scheduler: wakes every 60 seconds, queries PostgreSQL for actionable sessions.
/// All state is kept in the database so worker restarts do not lose scheduled work.
/// </summary>
public sealed class SessionSchedulerService(
    ISessionTriggerStore triggerStore,
    ISendConfirmationHandler confirmationHandler,
    ISendOneHourReminderHandler oneHourReminderHandler,
    ISendJoinLinkHandler joinLinkHandler,
    ISystemClock clock,
    ILogger<SessionSchedulerService> logger) : BackgroundService
{
    private static readonly TimeSpan TickInterval = TimeSpan.FromMinutes(1);
    private static readonly TimeSpan BackoffDuration = TimeSpan.FromMinutes(15);

    private readonly ConcurrentDictionary<Guid, DateTimeOffset> _confirmationBackoff = new();
    private readonly ConcurrentDictionary<Guid, DateTimeOffset> _oneHourBackoff = new();
    private readonly ConcurrentDictionary<Guid, DateTimeOffset> _joinLinkBackoff = new();

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        logger.LogInformation("Session scheduler started (interval: {Interval})", TickInterval);

        using var timer = new PeriodicTimer(TickInterval);

        do
        {
            try
            {
                await TickAsync(stoppingToken);
            }
            catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested)
            {
                break;
            }
            catch (Exception ex)
            {
                logger.LogError(ex, "Scheduler tick failed, will retry next tick");
            }
        }
        while (await timer.WaitForNextTickAsync(stoppingToken));

        logger.LogInformation("Session scheduler stopped");
    }

    public async Task TickAsync(CancellationToken ct)
    {
        var now = clock.UtcNow;

        await ProcessConfirmationTriggers(now, ct);
        await ProcessOneHourReminderTriggers(now, ct);
        await ProcessJoinLinkTriggers(now, ct);
    }

    private async Task ProcessConfirmationTriggers(DateTimeOffset now, CancellationToken ct)
    {
        IReadOnlyList<Guid> sessionIds;
        try
        {
            sessionIds = await triggerStore.GetSessionsNeedingConfirmationAsync(now, ct);
        }
        catch (Exception ex)
        {
            logger.LogError(ex, "Failed to query confirmation triggers");
            return;
        }

        foreach (var sessionId in sessionIds)
        {
            if (_confirmationBackoff.TryGetValue(sessionId, out var backoffUntil) && backoffUntil > now)
            {
                logger.LogDebug(
                    "Skipping confirmation for session {SessionId} until {Backoff}",
                    sessionId,
                    backoffUntil);
                continue;
            }

            try
            {
                await confirmationHandler.HandleAsync(sessionId, ct);
                _confirmationBackoff.TryRemove(sessionId, out _);
                logger.LogInformation("Confirmation sent for session {SessionId}", sessionId);
            }
            catch (Exception ex)
            {
                var nextAttempt = now.Add(BackoffDuration);
                _confirmationBackoff[sessionId] = nextAttempt;
                logger.LogError(
                    ex,
                    "Failed to send confirmation for session {SessionId}, backing off until {Backoff}",
                    sessionId,
                    nextAttempt);
            }
        }
    }

    private async Task ProcessOneHourReminderTriggers(DateTimeOffset now, CancellationToken ct)
    {
        IReadOnlyList<Guid> sessionIds;
        try
        {
            sessionIds = await triggerStore.GetSessionsNeedingOneHourReminderAsync(now, ct);
        }
        catch (Exception ex)
        {
            logger.LogError(ex, "Failed to query one-hour reminder triggers");
            return;
        }

        foreach (var sessionId in sessionIds)
        {
            if (_oneHourBackoff.TryGetValue(sessionId, out var backoffUntil) && backoffUntil > now)
            {
                logger.LogDebug(
                    "Skipping one-hour reminder for session {SessionId} until {Backoff}",
                    sessionId,
                    backoffUntil);
                continue;
            }

            try
            {
                await oneHourReminderHandler.HandleAsync(sessionId, ct);
                _oneHourBackoff.TryRemove(sessionId, out _);
                logger.LogInformation("One-hour reminder processed for session {SessionId}", sessionId);
            }
            catch (Exception ex)
            {
                var nextAttempt = now.Add(BackoffDuration);
                _oneHourBackoff[sessionId] = nextAttempt;
                logger.LogError(
                    ex,
                    "Failed to process one-hour reminder for session {SessionId}, backing off until {Backoff}",
                    sessionId,
                    nextAttempt);
            }
        }
    }

    private async Task ProcessJoinLinkTriggers(DateTimeOffset now, CancellationToken ct)
    {
        IReadOnlyList<Guid> sessionIds;
        try
        {
            sessionIds = await triggerStore.GetSessionsNeedingJoinLinkAsync(now, ct);
        }
        catch (Exception ex)
        {
            logger.LogError(ex, "Failed to query join-link triggers");
            return;
        }

        foreach (var sessionId in sessionIds)
        {
            if (_joinLinkBackoff.TryGetValue(sessionId, out var backoffUntil) && backoffUntil > now)
            {
                logger.LogDebug(
                    "Skipping join link for session {SessionId} until {Backoff}",
                    sessionId,
                    backoffUntil);
                continue;
            }

            try
            {
                await joinLinkHandler.HandleAsync(sessionId, ct);
                _joinLinkBackoff.TryRemove(sessionId, out _);
                logger.LogInformation("Join link sent for session {SessionId}", sessionId);
            }
            catch (Exception ex)
            {
                var nextAttempt = now.Add(BackoffDuration);
                _joinLinkBackoff[sessionId] = nextAttempt;
                logger.LogError(
                    ex,
                    "Failed to send join link for session {SessionId}, backing off until {Backoff}",
                    sessionId,
                    nextAttempt);
            }
        }
    }
}