using System.Buffers.Binary;

namespace Lskj.AgentPet.Host.Core.ErpBridge;

public static class BridgeFrameCodec
{
    public const int MaxMessageBytes = 1024 * 1024;

    public static async Task WriteAsync(
        Stream stream,
        ReadOnlyMemory<byte> body,
        CancellationToken cancellationToken = default)
    {
        ArgumentNullException.ThrowIfNull(stream);
        if (body.Length is <= 0 or > MaxMessageBytes)
            throw new HostError("bridge_protocol_error", "ERP 桥请求长度无效或超过 1 MB。");
        byte[] header = new byte[4];
        BinaryPrimitives.WriteInt32LittleEndian(header, body.Length);
        await stream.WriteAsync(header, cancellationToken).ConfigureAwait(false);
        await stream.WriteAsync(body, cancellationToken).ConfigureAwait(false);
        await stream.FlushAsync(cancellationToken).ConfigureAwait(false);
    }

    public static async Task<byte[]> ReadAsync(
        Stream stream,
        CancellationToken cancellationToken = default)
    {
        ArgumentNullException.ThrowIfNull(stream);
        byte[] header = new byte[4];
        await ReadExactlyAsync(stream, header, cancellationToken).ConfigureAwait(false);
        int length = BinaryPrimitives.ReadInt32LittleEndian(header);
        if (length is <= 0 or > MaxMessageBytes)
            throw new HostError("bridge_protocol_error", "ERP 桥响应长度无效或超过 1 MB。");
        byte[] body = new byte[length];
        await ReadExactlyAsync(stream, body, cancellationToken).ConfigureAwait(false);
        return body;
    }

    private static async Task ReadExactlyAsync(
        Stream stream,
        Memory<byte> target,
        CancellationToken cancellationToken)
    {
        int offset = 0;
        while (offset < target.Length)
        {
            int count = await stream.ReadAsync(target[offset..], cancellationToken).ConfigureAwait(false);
            if (count == 0)
                throw new HostError("bridge_disconnected", "ERP 桥在完整消息到达前关闭。");
            offset += count;
        }
    }
}