using System.Reflection;
using Newtonsoft.Json;

namespace Idoklad.Infrastructure;

/// <summary>
/// Read support for the SDK's <c>NullableProperty&lt;T&gt;</c>, used by Patch models to tell
/// "not set" apart from "set to null". The SDK converter only writes: its <c>ReadJson</c> throws
/// <see cref="NotImplementedException"/>, because the SDK never deserializes a Patch model. This
/// service does, so a PATCH body carrying such a member failed while binding. Writing stays with
/// the SDK converter so the payload sent upstream is unchanged.
/// </summary>
public sealed class SdkNullablePropertyConverter : JsonConverter
{
    private readonly JsonConverter _sdkConverter;

    public SdkNullablePropertyConverter(JsonConverter sdkConverter)
    {
        _sdkConverter = sdkConverter;
    }

    public override bool CanRead => true;

    public override bool CanWrite => _sdkConverter.CanWrite;

    public override bool CanConvert(Type objectType) => _sdkConverter.CanConvert(objectType);

    /// <summary>
    /// Reads the JSON scalar the SDK converter writes: a value, or null for an explicit reset.
    /// A member missing from the body is never read at all and keeps its unset default.
    /// </summary>
    public override object? ReadJson(JsonReader reader, Type objectType, object? existingValue, JsonSerializer serializer)
    {
        var propertyType = Nullable.GetUnderlyingType(objectType) ?? objectType;
        var valueType = propertyType.GetGenericArguments()[0];

        var value = reader.TokenType == JsonToken.Null
            ? null
            : serializer.Deserialize(reader, typeof(Nullable<>).MakeGenericType(valueType));

        // The member converter puts the reader in DateParseHandling.None, so the date arrives as a
        // string and the serializer's DateTimeZoneHandling never gets applied to it. The SDK
        // rejects a DateTime whose Kind is not Utc, so apply the same normalization here.
        if (value is DateTime date)
        {
            value = ToUtc(date);
        }

        // NullableProperty<T>(T? value) marks the member as set, including when the value is null.
        return Activator.CreateInstance(propertyType, BindingFlags.Instance | BindingFlags.Public | BindingFlags.CreateInstance, null, new[] { value }, null);
    }

    public override void WriteJson(JsonWriter writer, object? value, JsonSerializer serializer)
        => _sdkConverter.WriteJson(writer, value, serializer);

    /// <summary>
    /// Matches <see cref="DateTimeZoneHandling.Utc"/>: a value that carries an offset is converted,
    /// a value without one is taken as already being UTC and is only marked as such.
    /// </summary>
    private static DateTime ToUtc(DateTime value) => value.Kind switch
    {
        DateTimeKind.Utc => value,
        DateTimeKind.Local => value.ToUniversalTime(),
        _ => DateTime.SpecifyKind(value, DateTimeKind.Utc),
    };
}