interval.cpp

著名的 helix realplayer 基于手机 symbian 系统的 播放器全套源代码

/* ***** BEGIN LICENSE BLOCK ***** 
 * Version: RCSL 1.0/RPSL 1.0 
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 * the file under the RealNetworks Community Source License Version 1.0 
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 * in which case the RCSL will apply. You may also obtain the license terms 
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 * applicable to this file, the RCSL.  Please see the applicable RPSL or 
 * RCSL for the rights, obligations and limitations governing use of the 
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 * This file is part of the Helix DNA Technology. RealNetworks is the 
 * developer of the Original Code and owns the copyrights in the portions 
 * it created. 
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#include "hlxclib/stdlib.h"
#include "interval.h"

/*
* Minimum average time between RTCP packets from this site (in
* seconds).  This time prevents the reports from `clumping' when
* sessions are small and the law of large numbers isn't helping
* to smooth out the traffic.  It also keeps the report interval
* from becoming ridiculously small during transient outages like
* a network partition.
*/
double const RTCP_MIN_TIME = 5.0;
/*
* Fraction of the RTCP bandwidth to be shared among active
* senders.  (This fraction was chosen so that in a typical
* session with one or two active senders, the computed report
* time would be roughly equal to the minimum report time so that
* we don't unnecessarily slow down receiver reports.) The
* receiver fraction must be 1 - the sender fraction.
*/
double const RTCP_SENDER_BW_FRACTION = 0.25;
double const RTCP_RCVR_BW_FRACTION = (1-RTCP_SENDER_BW_FRACTION);
/* To compensate for "unconditional reconsideration" converging to a
* value below the intended average.
*/
double const COMPENSATION = 2.71828 - 1.5;


double rtcp_interval(INT32 members,
                     INT32 senders,
                     double rs_byterate,
		     double rr_byterate,
                     BOOL we_sent,
                     double avg_rtcp_size,
                     BOOL initial,
		     double rtcp_min_time)
{
    double rtcp_byterate = rr_byterate + rs_byterate;
    double t;                   /* interval */
    INT32 n = members;          /* no. of members for computation */

    /*
    * Very first call at application start-up uses half the min
    * delay for quicker notification while still allowing some time
    * before reporting for randomization and to learn about other
    * sources so the report interval will converge to the correct
    * interval more quickly.
    */
    if (initial) 
    {
	rtcp_min_time /= 2;
    }

    /*
    * If there were active senders, give them at least a minimum
    * share of the RTCP bandwidth.  Otherwise all participants share
    * the RTCP bandwidth equally.
    */
    if (senders > 0 && senders < members * RTCP_SENDER_BW_FRACTION) 
    {
	if (we_sent) 
	{
	    rtcp_byterate = rs_byterate;
	    n = senders;
	} 
	else 
	{
	    rtcp_byterate = rr_byterate;
	    n -= senders;
	}
    }

    /*
    * The effective number of sites times the average packet size is
    * the total number of octets sent when each site sends a report.
    * Dividing this by the effective bandwidth gives the time
    * interval over which those packets must be sent in order to
    * meet the bandwidth target, with a minimum enforced.  In that
    * time interval we send one report so this time is also our
    * average time between reports.
    */
    t = avg_rtcp_size * n / rtcp_byterate;
    if (t < rtcp_min_time) 
    {
	t = rtcp_min_time;
    }	

    /*
    * To avoid traffic bursts from unintended synchronization with
    * other sites, we then pick our actual next report interval as a
    * random number uniformly distributed between 0.5*t and 1.5*t.
    */
    t = t * (((double)rand() / (double)RAND_MAX) + 0.5);
    t = t / COMPENSATION;

    return t;
}