using InboxIntel.Application.Abstractions;
using InboxIntel.Infrastructure.Persistence;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Microsoft.Extensions.Logging;

namespace InboxIntel.Infrastructure.Ai;

/// <summary>
/// Fills <c>Email.Embedding</c> (pgvector) for semantic search, in small background batches
/// so interactive requests are never starved (per docs/discovery/06: embeddings are the
/// small always-on model; the batch pause keeps VRAM/CPU pressure low). Exits immediately
/// when the embedding provider is unavailable (AI disabled / Ollama down) — semantic search
/// simply stays dormant and lexical search is unaffected.
/// </summary>
public class EmbeddingBackfillWorker : BackgroundService
{
    private const int BatchSize = 32;
    private static readonly TimeSpan BatchPause = TimeSpan.FromSeconds(2);
    private static readonly TimeSpan IdleRescan = TimeSpan.FromMinutes(15);

    private readonly IServiceScopeFactory _scopeFactory;
    private readonly ILogger<EmbeddingBackfillWorker> _logger;

    public EmbeddingBackfillWorker(IServiceScopeFactory scopeFactory, ILogger<EmbeddingBackfillWorker> logger)
    {
        _scopeFactory = scopeFactory;
        _logger = logger;
    }

    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
    {
        // Provider availability is fixed by configuration for the process lifetime.
        using (var probe = _scopeFactory.CreateScope())
        {
            if (!probe.ServiceProvider.GetRequiredService<IEmbeddingProvider>().IsAvailable)
            {
                _logger.LogDebug("EmbeddingBackfillWorker idle: no embedding provider (AI disabled).");
                return;
            }
        }

        _logger.LogInformation("EmbeddingBackfillWorker started (batch {Batch}, pause {Pause}s)",
            BatchSize, BatchPause.TotalSeconds);

        while (!stoppingToken.IsCancellationRequested)
        {
            int processed;
            try
            {
                processed = await ProcessBatchAsync(stoppingToken);
            }
            catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested) { break; }
            catch (Exception ex)
            {
                // Ollama hiccups must never crash the host; back off and retry.
                _logger.LogWarning(ex, "Embedding batch failed; retrying after idle pause.");
                processed = 0;
            }

            await Task.Delay(processed > 0 ? BatchPause : IdleRescan, stoppingToken);
        }
    }

    /// <summary>Embeds one batch. Public-ish (internal) for direct testing.</summary>
    internal async Task<int> ProcessBatchAsync(CancellationToken ct)
    {
        using var scope = _scopeFactory.CreateScope();
        var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
        var embeddings = scope.ServiceProvider.GetRequiredService<IEmbeddingProvider>();

        var batch = await db.Emails
            .Where(e => e.Embedding == null)
            .OrderByDescending(e => e.SentAtUtc) // newest mail becomes searchable first
            .Take(BatchSize)
            .ToListAsync(ct);
        if (batch.Count == 0) return 0;

        // Subject + snippet is the semantic core; bodies are noisy (signatures, quoting)
        // and slow to embed. Truncate defensively to keep well inside the model context.
        var texts = batch
            .Select(e => Truncate($"{e.Subject}\n{e.Snippet ?? e.BodyText}", 2000))
            .ToList();
        var vectors = await embeddings.EmbedBatchAsync(texts, ct);
        if (vectors.Count != batch.Count)
        {
            _logger.LogWarning("Embedding batch returned {Got} vectors for {Want} emails; skipping batch.",
                vectors.Count, batch.Count);
            return 0;
        }

        for (var i = 0; i < batch.Count; i++)
        {
            if (vectors[i].Length == 0) continue; // provider soft-failure for one item
            batch[i].Embedding = new Pgvector.Vector(vectors[i]);
        }
        await db.SaveChangesAsync(ct);
        _logger.LogDebug("Embedded {Count} emails", batch.Count);
        return batch.Count;
    }

    private static string Truncate(string s, int max) => s.Length <= max ? s : s[..max];
}