flv文件解析及golang的流实现

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http.FileServer(http.Dir(base_dir)).ServeHTTP(w,r)

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//file
f,err := os.OpenFile(path,os.O_RDONLY,0644)
if err != nil {
    return err
}
defer f.Close()

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//get flv header(9byte) + FirstTagSize(4byte)
{
    buf := make([]byte, flv_header_size+previou_tag_size)
    if _,err := f.Read(buf);err != nil {return err}
    if bytes.Index(buf,flv_header_sign) != 0 {return errors.New(`no flv`)}
    streamChan <- buf
}

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for {
    t := getTag(f)
    if t.Tag == script_tag {
        streamChan <- *t.Buf
    } else if t.Tag == video_tag {
        if !first_video_tag {
            first_video_tag = true
            streamChan <- *t.Buf
        }

        if t.FirstByte & 0xf0 == 0x10 {
            if len(last_keyframe_video_offsets) > 2 {
                // last_timestamps = append(last_timestamps[1:], t.Timestamp)
                last_keyframe_video_offsets = append(last_keyframe_video_offsets[1:], t.Offset)
            } else {
                // last_timestamps = append(last_timestamps, t.Timestamp)
                last_keyframe_video_offsets = append(last_keyframe_video_offsets, t.Offset)
            }
        }
    } else if t.Tag == audio_tag {
        if !first_audio_tag {
            first_audio_tag = true
            streamChan <- *t.Buf
        }
    } else {//eof_tag 
        break;
    }
}

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type flv_tag struct {
    Tag byte
    Offset int64
    Timestamp int32
    PreSize int32
    FirstByte byte
    Buf *[]byte
}

//get tag func
var getTag = func(f *os.File)(t flv_tag){
    t.Offset,_ = f.Seek(0,1)//获取当前tag位置
    Buf := []byte{}
    t.Buf = &Buf//返回指针避免再次分配内存

    buf := make([]byte, tag_header_size)//读取tag头
    if size,err := f.Read(buf);err != nil || size == 0 {
        t.Tag = eof_tag//eof_tag为读取到结尾的标志，或许当前tag还没下载完成
        return
    }
    Buf = append(Buf, buf...)//将tag头储存到buf中，准备返回
    t.Tag = buf[0]//获取当前tag类型
    t.Timestamp = F.Btoi32([]byte{buf[7],buf[4],buf[5],buf[6]},0)//获取当前tag时间戳，Btoi32为4为字节转int32

    size := F.Btoi32(append([]byte{0x00},buf[1:4]...),0)//获取tag内容长度

    data := make([]byte, size)
    if size,err := f.Read(data);err != nil || size == 0 {
        t.Tag = eof_tag
        return
    }
    t.FirstByte = data[0]//获取当前内容的第一个字节，视频头/音频头

    pre_tag := make([]byte, previou_tag_size)//前一tag长度
    if size,err := f.Read(pre_tag);err != nil || size == 0 {
        t.Tag = eof_tag
        return
    } 
    t.PreSize = F.Btoi32(pre_tag,0)//前一tag长度
    
    Buf = append(Buf, append(data, pre_tag...)...)
    // if t.PreSize == 0{fmt.Println(t.Tag,size,data[size:])}

    return
}

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//seed to the second last tag
if len(last_keyframe_video_offsets) == 0 {flvlog.L(`W: `,`no keyframe`);return errors.New(`no keyframe`)}
f.Seek(last_keyframe_video_offsets[0],0)

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//copy
{
    last_available_offset := last_keyframe_video_offsets[0]//最后一个可用tag，万一发生错误，将回溯到此位置

    // last_Timestamp := last_timestamps[0]
    for {
        //退出
        select {
        case <-cancel:return nil;//客户退出了
        default:;
        }
        t := getTag(f)//逐个获取tag
        if t.Tag == eof_tag {//获取到了个残缺tag
            f.Seek(last_available_offset,0)
            time.Sleep(time.Second)
            continue
        } else if t.PreSize == 0 {//这是个无效的tag，或许读取发生了错误
            f.Seek(last_available_offset,0)
            f.Seek(seachtag(f),1)//seachtag返回可用tag位置
            continue
        } else if t.Tag == video_tag {
            // if t.FirstByte & 0xf0 == 0x10 {
            // 	video_keyframe_speed = t.Timestamp - last_video_keyframe_timestramp
            // 	fmt.Println(`video_keyframe_speed`,video_keyframe_speed)
            // 	last_video_keyframe_timestramp = t.Timestamp
            // }
            streamChan <- *t.Buf
        } else if t.Tag == audio_tag {
            streamChan <- *t.Buf
        } else if t.Tag != script_tag {//不复制Script脚本tag
            ;
        }
        
        last_available_offset = t.Offset//刷新最后可用tag

    }
}

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flusher, flushSupport := w.(http.Flusher);
if flushSupport {flusher.Flush()}//先进行这步，否则将可能会变成直接下载

var (
    err error
)
for err == nil {
    if b := <- byteC;len(b) != 0 {
        _,err = w.Write(b)
    } else {
        break
    }
    if flushSupport {flusher.Flush()}
}