Prefix-notation calculator in portable shell

calc.sh

calc ()
{
	local cursor=0
	local reference=0
	local stack=0
	local queue=''
	local current=''

	set -- $(echo ${@})

	while test "${#}" -gt 0
	do
		local token="${1}"

		cursor=$((cursor + 1))

		if test "${token%\)})" = "${token}"
		then
			atom="${token%\)}"
			local postf=''

			while test "${atom%\)})" = "${atom}"
			do
				postf="${postf})"
				atom="${atom%\)}"
			done

			shift
			$(echo "${current#\(}" "${atom}")
			set -- $(echo "${queue}" "${atom}${postf}" "${@:-}")

			queue=''
			stack="${stack% *}"
			reference="${stack##* }"
			cursor="${reference}"

			while test "${reference}" -gt 1
			do
				queue="${queue} ${1}"
				reference=$((reference - 1))
				shift
			done

			current="${1}"
			shift

			reference="${stack##* }"
			continue

		elif test "(${token#\(}" = "${token}"
		then
			queue="${queue} ${current}"
			current="${token}"
			reference=${cursor}
			stack="${stack} ${reference}"
		else
			current="${current} ${token}"
		fi

		shift
	done

	echo "${atom:-$token}"
}

add () { atom=$((${1} + ${2})); }
mul () { atom=$((${1} * ${2})); }
sub () { atom=$((${1} - ${2})); }
div () { atom=$((${1} / ${2})); }

calc '(div (add 2 (mul (add 1 (add 1 1))  5)) 2)'