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

namespace GB5Shared.DateConverter
{
    /// <summary>
    /// GB5 standard DateTime converter.
    /// READ : ISO 8601 — "yyyy-MM-ddTHH:mm:ssZ" / "yyyy-MM-dd"
    ///         Legacy  — "/Date(milliseconds)/" or "/Date(milliseconds+offset)/"
    /// WRITE: "yyyy-MM-ddTHH:mm:ss.fffZ"  (UTC ISO 8601)
    /// </summary>
    public class MicrosoftDateConverter : JsonConverter<DateTime>
    {
        public override DateTime Read(
            ref Utf8JsonReader reader,
            Type typeToConvert,
            JsonSerializerOptions options)
        {
            if (reader.TokenType == JsonTokenType.Null)
                return DateTime.MinValue;

            if (reader.TokenType == JsonTokenType.String)
            {
                var value = reader.GetString();

                if (string.IsNullOrWhiteSpace(value))
                    return DateTime.MinValue;

                // Legacy Microsoft /Date(milliseconds)/ or /Date(milliseconds+offset)/ format
                if (value.StartsWith("/Date(", StringComparison.Ordinal) && value.EndsWith(")/", StringComparison.Ordinal))
                {
                    var inner = value.Substring(6, value.Length - 8);
                    var signIdx = inner.IndexOfAny(new[] { '+', '-' });
                    var msStr = signIdx > 0 ? inner.Substring(0, signIdx) : inner;
                    if (long.TryParse(msStr, out long ms))
                        return DateTimeOffset.FromUnixTimeMilliseconds(ms).UtcDateTime;
                    return DateTime.MinValue;
                }

                if (DateTime.TryParse(value, null,
                        System.Globalization.DateTimeStyles.RoundtripKind, out DateTime parsed))
                    return parsed.Kind == DateTimeKind.Unspecified
                        ? DateTime.SpecifyKind(parsed, DateTimeKind.Utc)
                        : parsed.ToUniversalTime();

                return DateTime.MinValue;
            }

            return DateTime.MinValue;
        }

        public override void Write(
            Utf8JsonWriter writer,
            DateTime value,
            JsonSerializerOptions options)
        {
            if (value == DateTime.MinValue || value == default)
            {
                writer.WriteNullValue();
                return;
            }

            writer.WriteStringValue(value.ToUniversalTime().ToString("yyyy-MM-ddTHH:mm:ss.fffZ"));
        }
    }
}