Inboxintel/src/InboxIntel.Infrastructure/Notifications/DigestService.cs

using System.Text;
using InboxIntel.Application.Abstractions;
using InboxIntel.Infrastructure.Persistence;
using Microsoft.EntityFrameworkCore;

namespace InboxIntel.Infrastructure.Notifications;

/// <summary>Builds an HTML inbox-summary email and sends it via <see cref="IEmailSender"/>.</summary>
public class DigestService : IDigestService
{
    private readonly AppDbContext _db;
    private readonly IAnalyticsService _analytics;
    private readonly IEmailSender _emailSender;

    public DigestService(AppDbContext db, IAnalyticsService analytics, IEmailSender emailSender)
    {
        _db = db;
        _analytics = analytics;
        _emailSender = emailSender;
    }

    public async Task SendDigestAsync(Guid userId, CancellationToken ct = default)
    {
        var user = await _db.Users.FirstOrDefaultAsync(u => u.Id == userId, ct);
        if (user is null || string.IsNullOrWhiteSpace(user.Email)) return;

        var dashboard = await _analytics.GetDashboardAsync(userId, ct);
        var html = BuildHtml(dashboard);

        await _emailSender.SendAsync(user.Email, "Your InboxIntel digest", html, ct);

        user.LastDigestSentUtc = DateTimeOffset.UtcNow;
        await _db.SaveChangesAsync(ct);
    }

    private static string BuildHtml(Application.DTOs.DashboardSummaryDto d)
    {
        var sb = new StringBuilder();
        sb.Append("<div style=\"font-family:sans-serif;max-width:480px\">");
        sb.Append("<h2>Your inbox this week</h2>");
        sb.Append($"<p><strong>{d.Health.Score}/100</strong> inbox health (grade {d.Health.Grade})</p>");
        sb.Append($"<p>{d.UnreadEmails:N0} unread of {d.TotalEmails:N0} total emails</p>");
        if (d.Health.SafeToUnsubscribeCount > 0)
            sb.Append($"<p>{d.Health.SafeToUnsubscribeCount} senders are safe to unsubscribe from.</p>");

        if (d.TopSenders.Count > 0)
        {
            sb.Append("<h3>Top senders</h3><ul>");
            foreach (var s in d.TopSenders.Take(5))
                sb.Append($"<li>{System.Net.WebUtility.HtmlEncode(s.DisplayName ?? s.Address)} — {s.EmailCount:N0} emails</li>");
            sb.Append("</ul>");
        }

        if (d.Health.Recommendations.Count > 0)
        {
            sb.Append("<h3>Recommendations</h3><ul>");
            foreach (var r in d.Health.Recommendations)
                sb.Append($"<li>{System.Net.WebUtility.HtmlEncode(r)}</li>");
            sb.Append("</ul>");
        }

        sb.Append("</div>");
        return sb.ToString();
    }
}