using System.Globalization;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace Elternbeirat.PocketBase;

/// <summary>
/// Shared parsing of a PocketBase date string as wall-clock time. The trailing
/// "Z" is stripped rather than honoured, so the number is taken at face value and
/// the result carries <see cref="DateTimeKind.Unspecified"/> -- no timezone shift.
/// </summary>
internal static class WallClock
{
    public static DateTime Parse(string raw)
    {
        // Drop a trailing "Z" so DateTime.Parse does not treat the value as UTC
        // and convert it to local time (which would shift 19:30 to 20:30/21:30).
        var value = raw.EndsWith('Z') ? raw[..^1] : raw;
        var parsed = DateTime.Parse(value, CultureInfo.InvariantCulture,
            DateTimeStyles.None);
        return DateTime.SpecifyKind(parsed, DateTimeKind.Unspecified);
    }
}

/// <summary>
/// Reads PocketBase date values as local wall-clock time.
/// <para>
/// PocketBase stores every date in UTC and serializes it with a trailing "Z"
/// (e.g. "2026-10-08 19:30:00.000Z"). By project convention the stored number
/// IS the local time (Europe/Berlin) and the "Z" is ignored -- see the timezone
/// decision in the data model. This converter therefore parses the value and
/// returns it as an <see cref="DateTimeKind.Unspecified"/> instant, so no
/// timezone shift is ever applied when the value is later formatted.
/// </para>
/// </summary>
public sealed class LocalDateTimeConverter : JsonConverter<DateTime>
{
    public override DateTime Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        var raw = reader.GetString();
        return string.IsNullOrEmpty(raw) 
            ? default 
            : WallClock.Parse(raw);
    }

    public override void Write(Utf8JsonWriter writer, DateTime value, JsonSerializerOptions options)
        => writer.WriteStringValue(value.ToString("yyyy-MM-dd HH:mm:ss.fff'Z'",
            CultureInfo.InvariantCulture));
}

/// <summary>
/// Nullable counterpart of <see cref="LocalDateTimeConverter"/>. PocketBase sends
/// an empty string for an unset optional date (e.g. an event without an end);
/// that maps to null.
/// </summary>
public sealed class NullableLocalDateTimeConverter : JsonConverter<DateTime?>
{
    public override DateTime? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        var raw = reader.GetString();
        if (string.IsNullOrEmpty(raw))
            return null;

        return WallClock.Parse(raw);
    }

    public override void Write(Utf8JsonWriter writer, DateTime? value, JsonSerializerOptions options)
    {
        if (value is null)
            writer.WriteStringValue("");
        else
            writer.WriteStringValue(value.Value.ToString("yyyy-MM-dd HH:mm:ss.fff'Z'",
                CultureInfo.InvariantCulture));
    }
}