#!/bin/sh
set -eu

readonly DHCPD_CONF_FILE="${DHCPD_CONF_FILE:-/etc/dhcpd.conf}"
readonly DHCPD_LEASES_FILE="${DHCPD_LEASES_FILE:-/var/db/dhcpd.leases}"

DHCPD_CONF_FILE_AWK_SCRIPT=$(cat << 'EOF'
/^[ \t]*host[ \t]+[^ \t]+[ \t]+\{[ \t]*$/ {
	ip = ""
	hostname = $2
	next
}

/^[ \t]*fixed-address[ \t]+([0-9]{1,3}\.){3}[0-9]{1,3}[ \t]*;[ \t]*$/ {
	ip = substr($2, 1, length($2)-1)
	next
}

/\}/ {
	if (hostname && ip) {
		print hostname "\t" ip
	}

	hostname = ""
	ip = ""
	next
}
EOF
)

readonly DHCPD_CONF_FILE_AWK_SCRIPT

DHCPD_LEASES_FILE_AWK_SCRIPT=$(cat << 'EOF'
function parse_datetime(datetime) {
	command = "date -j -f \"%w %Y/%m/%d %H:%M:%S %Z;\" +%s \"" datetime "\""
	if (command | getline unix_time < 0) {
		exit 1
	}

	close(command)

	return unix_time
}

/^[ \t]*lease[ \t]+([0-9]{1,3}\.){3}[0-9]{1,3}[ \t]+\{[ \t]*$/ {
	ip = $2
	next
}

/^[ \t]*(starts|ends)[ \t]+[0-6][ \t]+[0-9]+\/[0-9]+\/[0-9]+[ \t]+[0-9]+:[0-9]+:[0-9]+([ \t]+[a-zA-Z0-9]+)?[ \t]*;[ \t]*$/ {
	if ($1 == "starts") {
		$1 = ""
		# remove the leading space caused by setting $1 to ""
		starts = parse_datetime(substr($0, 2))
	} else {
		$1 = ""
		# remove the leading space caused by setting $1 to ""
		ends = parse_datetime(substr($0, 2))
	}

	next
}

/^[ \t]*(client-)?hostname[ \t]+\"[^\"]+\"[ \t]*;[ \t]*$/ {
	hostname = gensub("\"|;", "", "g", $2)
	next
}

/^[ \t]*abandoned[ \t]*;[ \t]*$/ {
	abandoned = 1
	next
}

/\}/ {
	now = systime()

	if (!abandoned && hostname && ip && starts && ends && now >= starts && now < ends) {
		print hostname "\t" ip
	}

	hostname = ""
	ip = ""
	starts = 0
	ends = 0
	abandoned = 0
}
EOF
)

readonly DHCPD_LEASES_FILE_AWK_SCRIPT

(
	awk "${DHCPD_CONF_FILE_AWK_SCRIPT}" "${DHCPD_CONF_FILE}"
	awk "${DHCPD_LEASES_FILE_AWK_SCRIPT}" "${DHCPD_LEASES_FILE}"
) | sort -u | column -c 2 -t