smart beaconing: fix

[dl9sau]

Someone said, smart-beaconing implementation is not so smooth.
I looked in the code and found situations where the result is negative. The negative result is assigned to unsigned long, which gives us a very high variable -> the next beacon is either in a few weeks, or user drops speed, or drives a curve.
Concrete: sb_max_speed-sb_min_speed is negative i.e. in 100max-120current

Current code

   nextTX = (sb_max_interval-sb_min_interval)/(sb_max_speed-sb_min_speed)*(sb_max_speed-average_speed_final)+sb_min_interval;
   if (nextTX < sb_min_interval) {nextTX=sb_min_interval;}
   if (nextTX > sb_max_interval) {nextTX=sb_max_interval;}

Solution:
either work with signed long variable, do the checks, and afterwards assign the result to nextTX.
Or test it inline.

-> My suggestion:

nextTX = ((sb_max_speed > average_speed_final) ? ((sb_max_interval-sb_min_interval)/(sb_max_speed-sb_min_speed)*(sb_max_speed-average_speed_final)+sb_min_interval) : sb_min_interval);
//if (nextTX < sb_min_interval) {nextTX=sb_min_interval;}   // already assured (  (sb_max_speed <= average_speed_final) -> sb_min_interval)
if (nextTX > sb_max_interval) {nextTX=sb_max_interval;}

Also to consider:
If user configures sb_max_speed equal to sb_min_speed, the division
(sb_max_interval-sb_min_interval)/(sb_max_speed-sb_min_speed)
may lead to division by zero.
boot code and / webinterface should check that sb_max_speed-sb_min_speed is >= 0, and sb_max_interval-sb_max_interval is > sb_max_speed-sb_min_speed.

[dl9sau]

Feature is stable in my development version