docker-mailserver/target/helper_functions.sh

#!/bin/bash

# expects mask prefix length and the digit
function _mask_ip_digit() {
	if [[ $1 -ge 8 ]]; then
		MASK=255
	else 
		if [[ $1 -le 0 ]]; then
			MASK=0
		else
			VALUES=('0' '128' '192' '224' '240' '248' '252' '254' '255')
			MASK=${VALUES[$1]}
		fi
	fi
	echo $(( $2 & $MASK ))
}

# transforms a specific ip with CIDR suffix like 1.2.3.4/16
# to subnet with cidr suffix like 1.2.0.0/16
function _sanitize_ipv4_to_subnet_cidr() {
	IP=${1%%/*}
	PREFIX_LENGTH=${1#*/}

	# split IP by . into digits
	DIGITS=(${IP//./ })

	# mask digits according to prefix length
	MASKED_DIGITS=()
	DIGIT_PREFIX_LENGTH="$PREFIX_LENGTH"
	for DIGIT in "${DIGITS[@]}" ; do
		MASKED_DIGITS+=( $(_mask_ip_digit $DIGIT_PREFIX_LENGTH $DIGIT) )
		DIGIT_PREFIX_LENGTH=$(( $DIGIT_PREFIX_LENGTH - 8 ))
	done

	# output masked ip plus prefix length
	echo ${MASKED_DIGITS[0]}.${MASKED_DIGITS[1]}.${MASKED_DIGITS[2]}.${MASKED_DIGITS[3]}/$PREFIX_LENGTH
}