Beats per minute calculator for the command line.

BPM.sh

#!/bin/sh
# Calculates Beats per Minute and confidence.

# Trigger function on ctrl+c
trap ctrl_c_handler INT
function ctrl_c_handler() {
	echo -e "\n -> $index beats in $(echo "scale=2; $delta_sum /1"| bc) seconds. "
	exit 0
}

if [[ $1 == '-q' ]]; then
	q_to_quit='true'
fi

index=0
delta_sum="0.0"
delta_sum_sq="0.0"
confidence="%100 "

if [[ -n $q_to_quit ]]; then
	msg="Start tapping any key to calculate BPM, press \"Q\" to stop."
else
	msg="Start tapping any key to calculate BPM, press Ctrl+C to stop."
fi

cols=$(tput cols) # terminal width
echo -ne ${msg:0:$cols} # truncate msg to fit terminal.
read -n 1 -s # get character
read -t 0.00001 -s # flush
msg=" BPM: ...                                                    "
echo -ne "\r${msg:0:$cols}"

while true; do
	start_time=$(date +%s.%N)
	read -n 1 -s "char" # get character
	end_time=$(date +%s.%N)
	read -t 0.00001 -s # flush

	if [[ -n $q_to_quit ]] && [[ $char == "q" || $char == "Q" ]]; then
		ctrl_c_handler
	fi

	((index++))
	delta_time=$(echo "$end_time - $start_time"| bc)
	delta_sum=$(echo "$delta_sum + $delta_time"| bc)
	delta_sum_sq=$(echo "$delta_sum_sq + ($delta_time * $delta_time)"| bc)

	if [ $index -gt 1 ]; then
		# Use naïve algorithm for calculating variance.
		# from https://en.wikipedia.org/wiki/Algorithms_for_calculating_variance#Na.C3.AFve_algorithm
		variance=$(echo "scale=10; ($delta_sum_sq - ($delta_sum * $delta_sum) / $index) / ($index - 1)"| bc)

		# Calculate confidence: abs(x - v) / x
		diff=$(echo "$delta_time - $variance"| bc)
		if [ ${diff:0:1} == '-' ]; then diff=${diff:1}; fi # abs( diff )
		confidence=$(echo "scale=5; $diff / $delta_time * 100"| bc)
		if [ `echo "$confidence < 100"| bc` -eq 1 ]; then
			confidence="%${confidence:0:4}"
		else # when confidence was less than -100
			confidence="%00.0"
		fi
	fi

	BPM=$(echo "scale=2; ($index / $delta_sum) * 60"| bc)
	echo -ne "\r BPM: $BPM   $confidence   "
done