const encodings = [
  { rid: 'h', scaleResolutionDownBy: 1, maxBitrate: 2000000, maxFramerate: 30, scalabilityMode: 'L1T3_KEY' },
  { rid: 'm', scaleResolutionDownBy: 2, maxBitrate: 800000,  maxFramerate: 15, scalabilityMode: 'L1T2' },
  { rid: 'l', scaleResolutionDownBy: 4, maxBitrate: 300000,  maxFramerate: 7,  scalabilityMode: 'L1T1' }
];

def select_layers(client_cap, available_layers, bw_estimate):
    """
    client_cap: {max_fps, max_width, max_height, codec_support}
    available_layers: list of {rid, spatial_layer, temporal_layer, bitrate}
    bw_estimate: 当前可用带宽 (bps)
    """
    # 1. 剔除超出设备能力的层
    feasible = [l for l in available_layers
                if l['bitrate'] <= bw_estimate * 0.85
                and l['max_fps'] <= client_cap['max_fps']
                and l['width'] <= client_cap['max_width']]
    # 2. 优先保留高时间层（流畅度优先）
    feasible.sort(key=lambda x: (x['temporal_layer'], x['spatial_layer']), reverse=True)
    return feasible[:1]  # 单流模式仅保留最优层

const encodings = [
  // L2T3 (720p) - 高配首选
  { rid: 'h', scaleResolutionDownBy: 1, scalabilityMode: 'L3T3_KEY', maxBitrate: 2000000, maxFramerate: 30 },
  // L1T2 (360p) - 中配兜底
  { rid: 'm', scaleResolutionDownBy: 2, scalabilityMode: 'L2T2', maxBitrate: 800000, maxFramerate: 15 },
  // L0T1 (180p) - 极弱网保底
  { rid: 'l', scaleResolutionDownBy: 4, scalabilityMode: 'L1T1', maxBitrate: 300000, maxFramerate: 7 }
];

class AdaptiveController:
    def __init__(self):
        self.ewma_bw = 0          # 指数加权移动平均带宽
        self.ewma_rtt = 0
        self.loss_rate = 0
        self.current_layer = (2, 2)  # (spatial, temporal)

    def on_rtcp_feedback(self, report):
        # 1. 更新带宽估计 (结合 Transport-CC 与 REMB)
        self.ewma_bw = 0.85 * self.ewma_bw + 0.15 * report.available_bitrate
        self.loss_rate = 0.9 * self.loss_rate + 0.1 * report.fraction_lost
        self.ewma_rtt = 0.8 * self.ewma_rtt + 0.2 * report.rtt

        # 2. 计算安全码率水位 (留 15% 余量给音频/重传)
        safe_bw = self.ewma_bw * 0.85

        # 3. 降级触发条件 (任一满足即触发)
        if self.loss_rate > 0.10 or safe_bw < self.layer_bitrate(self.current_layer) * 0.8:
            self._downgrade()
        # 4. 升级触发条件 (需连续 3s 满足)
        elif safe_bw > self.layer_bitrate(self._next_higher_layer()) * 1.3 and self.loss_rate < 0.02:
            self._upgrade()

    def _downgrade(self):
        # 优先降时间层 (保分辨率、降帧率，主观体验更平滑)
        if self.current_layer[1] > 0:
            self.current_layer = (self.current_layer[0], self.current_layer[1] - 1)
        # 时间层已到底，再降空间层
        elif self.current_layer[0] > 0:
            self.current_layer = (self.current_layer[0] - 1, 2)  # 降空间后恢复高帧率
        self._apply_layer()

    def _apply_layer(self):
        # 通过 RTCRtpSender.setParameters 热更新 active 层
        # 同时发送 PLI 请求新层关键帧，避免花屏
        pass

func (s *SFU) ForwardPacket(pkt *rtp.Packet, sub *Subscriber) {
    // 1. 解析 VP9/AV1 Payload Descriptor 获取 (S, T)
    sid, tid := parseScalabilityIDs(pkt)

    // 2. 订阅者能力裁剪
    if sid > sub.maxSpatialLayer || tid > sub.maxTemporalLayer {
        return // 静默丢弃，不回包，避免风暴
    }

    // 3. 关键帧同步保护
    if tid == 0 && !sub.keyFrameReceived {
        // 首包必须是关键帧，否则缓存等待下一个 IDR
        if !pkt.IsKeyFrame() {
            s.cachePendingKeyFrame(sub, pkt)
            return
        }
        sub.keyFrameReceived = true
        s.flushPendingFrames(sub) // 刷新缓存的后续帧
    }

    // 4. 层切换平滑过渡 (Layer Switch Smoothing)
    if sub.lastSID != sid || sub.lastTID != tid {
        // 插入合成关键帧或请求编码端生成 IDR
        if tid == 0 && sid != sub.lastSID {
            s.requestKeyFrame(sub.publisherID, sid)
        }
        sub.lastSID, sub.lastTID = sid, tid
    }

    // 5. 转发
    sub.transport.Write(pkt)
}

# FFmpeg 仅解码目标输出层 (如仅需 360p@15fps 输出)
ffmpeg -i input.webm 
  -c:v libvpx-vp9 
  -svc_params "layers=2:2:1"   # 仅解码 L1T1 (空间层1, 时间层1)
  -vf "fps=15,scale=640:360" 
  -b:v 800k 
  output.mp4

// 1. 使用 VideoDecoder 解码 SVC 流，手动控制输出层
const decoder = new VideoDecoder({
  output: frame => renderFrame(frame),
  error: e => handleDecodeError(e)
});
decoder.configure({
  codec: 'vp09.00.10.08', // VP9 Profile 0
  codedWidth: 1280,
  codedHeight: 720,
  // 关键：通过 scalabilityMode 告知解码器目标层
  scalabilityMode: 'L2T1' // 仅解码至 360p@15fps
});

// 2. 使用 EncodedVideoChunkMetadata 标记层级信息供编码器参考
encoder.encode(chunk, {
  temporalLayerId: 1,      // 显式指定时间层
  spatialLayerId: 0,
  dependencyId: 0,         // 依赖基础层
  duration: 33333          // 微秒
});