social-media/backend/src/Socialize.Api/Modules/CalendarIntegrations/Services/CalendarImportSyncService.cs

using Microsoft.EntityFrameworkCore;
using Socialize.Api.Data;
using Socialize.Api.Modules.CalendarIntegrations.Data;
using System.Text.Json;

namespace Socialize.Api.Modules.CalendarIntegrations.Services;

public sealed class CalendarImportSyncService(
    AppDbContext dbContext,
    IHttpClientFactory httpClientFactory,
    IcsCalendarParser parser)
{
    public async Task RefreshSourceAsync(Guid sourceId, CancellationToken ct)
    {
        CalendarSource? source = await dbContext.CalendarSources
            .SingleOrDefaultAsync(candidate => candidate.Id == sourceId, ct);
        if (source is null)
        {
            throw new InvalidOperationException("Calendar source was not found.");
        }

        source.LastAttemptedSyncAt = DateTimeOffset.UtcNow;

        if (string.IsNullOrWhiteSpace(source.SourceUrl))
        {
            source.LastSyncError = "Calendar source does not have a source URL.";
            await dbContext.SaveChangesAsync(ct);
            return;
        }

        try
        {
            using HttpClient httpClient = httpClientFactory.CreateClient();
            DateOnly rangeStart = DateOnly.FromDateTime(DateTime.UtcNow.AddYears(-1));
            DateOnly rangeEnd = DateOnly.FromDateTime(DateTime.UtcNow.AddYears(2));
            IReadOnlyCollection<ParsedCalendarEvent> parsedEvents = await GetParsedEventsAsync(
                httpClient,
                source.SourceUrl,
                rangeStart,
                rangeEnd,
                ct);

            await ReplaceEventsAsync(source.Id, parsedEvents, ct);

            source.LastSuccessfulSyncAt = DateTimeOffset.UtcNow;
            source.LastSyncError = null;
            source.LastAttemptedSyncAt = source.LastSuccessfulSyncAt;
            await dbContext.SaveChangesAsync(ct);
        }
        catch (HttpRequestException ex)
        {
            await RecordSyncFailureAsync(source, ex.Message, ct);
        }
        catch (FormatException ex)
        {
            await RecordSyncFailureAsync(source, ex.Message, ct);
        }
        catch (InvalidOperationException ex)
        {
            await RecordSyncFailureAsync(source, ex.Message, ct);
        }
    }

    public async Task RefreshDueSourcesAsync(CancellationToken ct)
    {
        DateTimeOffset staleBefore = DateTimeOffset.UtcNow.AddHours(-12);
        Guid[] sourceIds = await dbContext.CalendarSources
            .Where(source => source.IsEnabled && source.SourceUrl != null)
            .Where(source => source.LastAttemptedSyncAt == null || source.LastAttemptedSyncAt < staleBefore)
            .OrderBy(source => source.LastAttemptedSyncAt)
            .Select(source => source.Id)
            .Take(25)
            .ToArrayAsync(ct);

        foreach (Guid sourceId in sourceIds)
        {
            await RefreshSourceAsync(sourceId, ct);
        }
    }

    private async Task ReplaceEventsAsync(
        Guid sourceId,
        IReadOnlyCollection<ParsedCalendarEvent> parsedEvents,
        CancellationToken ct)
    {
        await dbContext.CalendarEvents
            .Where(calendarEvent => calendarEvent.CalendarSourceId == sourceId)
            .ExecuteDeleteAsync(ct);

        DateTimeOffset importedAt = DateTimeOffset.UtcNow;
        foreach (ParsedCalendarEvent parsedEvent in parsedEvents)
        {
            dbContext.CalendarEvents.Add(new CalendarEvent
            {
                Id = Guid.NewGuid(),
                CalendarSourceId = sourceId,
                SourceEventUid = parsedEvent.SourceEventUid,
                Title = parsedEvent.Title,
                Description = parsedEvent.Description,
                IsAllDay = parsedEvent.IsAllDay,
                IsFloatingTime = parsedEvent.IsFloatingTime,
                StartDate = parsedEvent.StartDate,
                EndDate = parsedEvent.EndDate,
                StartLocalDateTime = parsedEvent.StartLocalDateTime,
                EndLocalDateTime = parsedEvent.EndLocalDateTime,
                StartUtc = parsedEvent.StartUtc,
                EndUtc = parsedEvent.EndUtc,
                TimeZoneId = parsedEvent.TimeZoneId,
                RecurrenceId = parsedEvent.RecurrenceId,
                Location = parsedEvent.Location,
                SourceUrl = parsedEvent.SourceUrl,
                SourceLastModifiedAt = parsedEvent.SourceLastModifiedAt,
                ImportedAt = importedAt,
            });
        }
    }

    private async Task<IReadOnlyCollection<ParsedCalendarEvent>> GetParsedEventsAsync(
        HttpClient httpClient,
        string sourceUrl,
        DateOnly rangeStart,
        DateOnly rangeEnd,
        CancellationToken ct)
    {
        if (TryGetNagerCountryCode(sourceUrl, out string? countryCode))
        {
            return await GetNagerEventsAsync(httpClient, sourceUrl, countryCode!, rangeStart, rangeEnd, ct);
        }

        string content = await httpClient.GetStringAsync(sourceUrl, ct);
        return parser.Parse(content, rangeStart, rangeEnd);
    }

    private static async Task<IReadOnlyCollection<ParsedCalendarEvent>> GetNagerEventsAsync(
        HttpClient httpClient,
        string sourceUrl,
        string countryCode,
        DateOnly rangeStart,
        DateOnly rangeEnd,
        CancellationToken ct)
    {
        List<ParsedCalendarEvent> events = [];
        for (int year = rangeStart.Year; year <= rangeEnd.Year; year++)
        {
            string yearUrl = BuildNagerYearUrl(sourceUrl, countryCode, year);
            string json = await httpClient.GetStringAsync(yearUrl, ct);
            NagerHoliday[] holidays = JsonSerializer.Deserialize<NagerHoliday[]>(
                json,
                new JsonSerializerOptions(JsonSerializerDefaults.Web)) ?? [];

            foreach (NagerHoliday holiday in holidays)
            {
                if (!DateOnly.TryParse(holiday.Date, out DateOnly date) ||
                    date < rangeStart ||
                    date > rangeEnd)
                {
                    continue;
                }

                events.Add(ToParsedEvent(
                    $"nager-{countryCode}-{date:yyyyMMdd}-{NormalizeUidPart(holiday.Name)}",
                    string.IsNullOrWhiteSpace(holiday.Name) ? holiday.LocalName : holiday.Name,
                    holiday.LocalName,
                    date,
                    string.Join(", ", holiday.Types ?? []),
                    yearUrl));
            }

            events.AddRange(GetSupplementalCountryEvents(countryCode, year, rangeStart, rangeEnd));
        }

        return events
            .GroupBy(calendarEvent => calendarEvent.SourceEventUid)
            .Select(group => group.First())
            .OrderBy(calendarEvent => calendarEvent.StartDate)
            .ThenBy(calendarEvent => calendarEvent.Title)
            .ToArray();
    }

    private static ParsedCalendarEvent ToParsedEvent(
        string uid,
        string? title,
        string? localName,
        DateOnly date,
        string? types,
        string sourceUrl)
    {
        string? description = string.IsNullOrWhiteSpace(types)
            ? localName
            : $"{localName}\nTypes: {types}";

        return new ParsedCalendarEvent(
            uid,
            string.IsNullOrWhiteSpace(title) ? "Untitled event" : title,
            description,
            IsAllDay: true,
            IsFloatingTime: false,
            date,
            date.AddDays(1),
            StartLocalDateTime: null,
            EndLocalDateTime: null,
            StartUtc: null,
            EndUtc: null,
            TimeZoneId: null,
            RecurrenceId: null,
            Location: null,
            sourceUrl,
            SourceLastModifiedAt: null);
    }

    private static IReadOnlyCollection<ParsedCalendarEvent> GetSupplementalCountryEvents(
        string countryCode,
        int year,
        DateOnly rangeStart,
        DateOnly rangeEnd)
    {
        if (!countryCode.Equals("CA", StringComparison.OrdinalIgnoreCase))
        {
            return [];
        }

        DateOnly mothersDay = NthWeekdayOfMonth(year, month: 5, DayOfWeek.Sunday, occurrence: 2);
        if (mothersDay < rangeStart || mothersDay > rangeEnd)
        {
            return [];
        }

        return
        [
            ToParsedEvent(
                $"socialize-ca-mothers-day-{year}",
                "Mother's Day",
                "Mother's Day",
                mothersDay,
                "Observance",
                "socialize://calendar-observances/CA"),
        ];
    }

    private static DateOnly NthWeekdayOfMonth(int year, int month, DayOfWeek dayOfWeek, int occurrence)
    {
        DateOnly date = new(year, month, 1);
        while (date.DayOfWeek != dayOfWeek)
        {
            date = date.AddDays(1);
        }

        return date.AddDays((occurrence - 1) * 7);
    }

    private static bool TryGetNagerCountryCode(string sourceUrl, out string? countryCode)
    {
        countryCode = null;
        if (!Uri.TryCreate(sourceUrl, UriKind.Absolute, out Uri? uri) ||
            !uri.Host.Contains("date.nager.at", StringComparison.OrdinalIgnoreCase))
        {
            return false;
        }

        string[] segments = uri.AbsolutePath
            .Split('/', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries);

        string? candidate = segments.LastOrDefault(segment => segment.Length == 2);
        if (candidate is null)
        {
            return false;
        }

        countryCode = candidate.ToUpperInvariant();
        return true;
    }

    private static string BuildNagerYearUrl(string sourceUrl, string countryCode, int year)
    {
        if (Uri.TryCreate(sourceUrl, UriKind.Absolute, out Uri? uri))
        {
            return $"{uri.Scheme}://{uri.Host}/api/v3/PublicHolidays/{year}/{countryCode}";
        }

        return $"https://date.nager.at/api/v3/PublicHolidays/{year}/{countryCode}";
    }

    private static string NormalizeUidPart(string? value)
    {
        return new string((value ?? "holiday")
            .ToLowerInvariant()
            .Select(character => char.IsLetterOrDigit(character) ? character : '-')
            .ToArray())
            .Trim('-');
    }

    private async Task RecordSyncFailureAsync(
        CalendarSource source,
        string message,
        CancellationToken ct)
    {
        source.LastSyncError = NormalizeSyncError(message);
        await dbContext.SaveChangesAsync(ct);
    }

    public static string NormalizeSyncError(string message)
    {
        ArgumentNullException.ThrowIfNull(message);

        return message.Length > 2048 ? message[..2048] : message;
    }

    private sealed record NagerHoliday(
        string Date,
        string LocalName,
        string Name,
        string[]? Types);
}