using System.Runtime.CompilerServices;
using System.Text;

namespace Lskj.AgentPet.Host.Core.AstrBot;

public sealed record SseFrame(string Event, string Data, string? Id);

public static class SseParser
{
    public const int MaxEventCharacters = 1024 * 1024;

    public static async IAsyncEnumerable<SseFrame> ReadAsync(
        Stream source,
        [EnumeratorCancellation] CancellationToken cancellationToken = default)
    {
        ArgumentNullException.ThrowIfNull(source);
        using StreamReader reader = new(
            source,
            new UTF8Encoding(false, true),
            detectEncodingFromByteOrderMarks: true,
            bufferSize: 4096,
            leaveOpen: true);
        string eventName = "message";
        string? id = null;
        StringBuilder data = new();

        while (true)
        {
            string? line = await reader.ReadLineAsync(cancellationToken).ConfigureAwait(false);
            if (line is null)
            {
                if (data.Length > 0)
                    yield return new SseFrame(eventName, TrimFinalNewline(data), id);
                yield break;
            }
            if (line.Length == 0)
            {
                if (data.Length > 0)
                    yield return new SseFrame(eventName, TrimFinalNewline(data), id);
                eventName = "message";
                data.Clear();
                continue;
            }
            if (line[0] == ':') continue;

            int separator = line.IndexOf(':');
            string field = separator < 0 ? line : line[..separator];
            string value = separator < 0 ? string.Empty : line[(separator + 1)..];
            if (value.StartsWith(' ')) value = value[1..];
            switch (field)
            {
                case "event":
                    eventName = string.IsNullOrWhiteSpace(value) ? "message" : value;
                    break;
                case "data":
                    if (data.Length + value.Length + 1 > MaxEventCharacters)
                        throw new HostError("astrbot_sse_event_too_large", "AstrBot SSE 单个事件超过 1 MB。 ");
                    data.Append(value).Append('\n');
                    break;
                case "id":
                    if (!value.Contains('\0')) id = value;
                    break;
            }
        }
    }

    private static string TrimFinalNewline(StringBuilder value)
    {
        return value.Length > 0 && value[value.Length - 1] == '\n'
            ? value.ToString(0, value.Length - 1)
            : value.ToString();
    }
}