using System.Collections.Generic;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace GB5Shared.JsonConverter
{
    // Legacy grid-style clients occasionally wrap a single-record save payload in a
    // one-element JSON array (a holdover from batch-save grid components) instead of
    // posting the bare object [FromBody] expects. Apply via
    // [JsonConverter(typeof(SingleOrArrayJsonConverter<TheDTO>))] on the DTO class:
    // a plain object deserializes normally; a JSON array deserializes its first
    // element (an empty array yields default(T)).
    public class SingleOrArrayJsonConverter<T> : JsonConverter<T>
    {
        public override T? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        {
            var innerOptions = WithoutSelf(options);

            if (reader.TokenType == JsonTokenType.StartArray)
            {
                var list = JsonSerializer.Deserialize<List<T>>(ref reader, innerOptions);
                return (list != null && list.Count > 0) ? list[0] : default;
            }

            return JsonSerializer.Deserialize<T>(ref reader, innerOptions);
        }

        public override void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
        {
            JsonSerializer.Serialize(writer, value, WithoutSelf(options));
        }

        // Re-entering JsonSerializer for the same type T would otherwise pick this
        // same converter back up (it's registered on T via the class-level attribute)
        // and recurse forever, so strip it from a cloned options instance first.
        private static JsonSerializerOptions WithoutSelf(JsonSerializerOptions options)
        {
            var clone = new JsonSerializerOptions(options);
            for (int i = clone.Converters.Count - 1; i >= 0; i--)
            {
                if (clone.Converters[i] is SingleOrArrayJsonConverter<T>)
                    clone.Converters.RemoveAt(i);
            }
            return clone;
        }
    }
}