(bash experts) computationally efficient circular buffer?

Lynx · March 20, 2022, 2:37pm

What is most efficient way in bash to maintain an array of historical values? Oldest value needs to be read from array and newest value appended to the end.

At the moment I have:

(( ${delays[0]} )) && ((sum_delays--))
unset 'delays[0]'
delay=0
(($rtt_delta > $delay_thr)) && delay=1 && ((sum_delays++))
delays+=($delay)
delays=(${delays[*]})

Is there a more efficient way?

Context:

github.com

lynxthecat/CAKE-autorate/blob/59a193095d0092790117318a1ac3b4b97461f332/CAKE-autorate.sh#L232

      
        
            while true
            do
            	while read -r timestamp reflector seq rtt_baseline rtt rtt_delta
            	do 
            		t_start=${EPOCHREALTIME/./}
            		if ((($t_start - "${timestamp//[[\[\].]}")>500000)); then
            			(($debug)) && echo "WARNING: encountered response from [" $reflector "] that is > 500ms old. Skipping." 
            			continue
            		fi
            
            
		(( ${delays[0]} )) && ((sum_delays--))
            		unset 'delays[0]'
            		delay=0
            		(($rtt_delta > $delay_thr)) && delay=1 && ((sum_delays++))
            		delays+=($delay)
            	       	delays=(${delays[*]})
            	
            		update_loads
            
            
		dl_load_condition="low_load"
            		(($rx_load > $high_load_thr)) && dl_load_condition="high_load"

Lynx · March 20, 2022, 5:46pm

Thanks to some suggestions from @colo and @Alfr on IRC I tried:

(( ${delays[$delays_idx]} )) && ((sum_delays--))
delay=0
(($rtt_delta > $delay_thr)) && delay=1 && ((sum_delays++))
delays[$delays_idx]=$delay
(( delays_idx=(delays_idx+1)%$bufferbloat_detection_window ))

This avoids having to re-index the array, albeit performance seems roughly similar - perhaps because of the overwrite?