#!/bin/sh
#
# Shell command language preprocessor
#
# Copyright (C) 2015  Patrick "P. J." McDermott
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program.  If not, see <http://www.gnu.org/licenses/>.

set -u

VERSION='0.1.0'
FS="$(printf '\034')"

die()
{
	local fmt="${1}"
	shift 1

	printf "shpp: ${fmt}\n" "${@}" >&2
	exit 2
}

error()
{
	local fmt="${1}"
	shift 1

	printf "shpp: ${fmt}\n" "${@}" >&2
}

preprocess()
{
	local vars="${1}"
	local input="${2}"
	local output="${3}"
	local def=
	local var=
	local val=

	# Open output file.
	if [ "x${output}" = 'x-' ]; then
		if ! exec 3>&1; then
			die 'Cannot duplicate output file descriptor'
		fi
	else
		if ! exec 3>"${outfile}~"; then
			die 'Cannot open file "%s"' "${output}~"
		fi
	fi

	IFS="${FS}"
	for def in ${vars}; do
		case "${def}" in
			*=*)
				var="${def%%=*}"
				val="${def#*=}"
				;;
			*)
				var="${def%%=*}"
				val='true'
				;;
		esac
		case "${var}" in
			*[!A-Za-z0-9_]*)
				error 'Invalid variable name "%s"' "${var}"
				continue
				;;
			[0-9]*)
				error 'Invalid variable name "%s"' "${var}"
				continue
				;;
		esac
		printf "%s='" "${var}" >&3
		printf '%s' "${val}" | sed "s/'/'\\\\''/g" >&3
		printf "'\n" >&3
	done
	unset IFS

	# Read input file.
	if ! cat "${input}" >&3; then
		die 'Cannot read file "%s"' "${input}"
	fi

	# Close output file, make it executable, and set its name.
	if ! [ "x${output}" = 'x-' ]; then
		exec 3>&-
		if ! mv "${output}~" "${output}"; then
			die 'Cannot rename file to "%s"' "${output}"
		fi
	fi
}

usage()
{
	printf 'Usage: %s [option ...] <infile> [<outfile>]\n' "${0}"
}

help()
{
	usage
	cat <<EOF
Options:
  -h                Display this information
  -V                Display preprocessor version information
  -D <var>[=<val>]  Predefine <var> as a variable with value <val> (default
                    "true")
EOF
}

version()
{
	cat <<EOF
shpp ${VERSION}
Copyright (C) 2015  Patrick "P. J." McDermott
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
EOF
}

main()
{
	local opt=
	local vars=
	local infile=
	local outfile=

	while getopts 'hVD:' opt; do
		case "${opt}" in
			'h')
				help
				exit
				;;
			'V')
				version
				exit
				;;
			'D')
				vars="${vars}${OPTARG}${FS}"
				;;
		esac
	done
	shift $(($OPTIND - 1))

	case ${#} in
		1)
			infile="${1}"
			outfile='-'
			;;
		2)
			infile="${1}"
			outfile="${2}"
			;;
		*)
			usage >&2
			exit 1
	esac

	preprocess "${vars}" "${infile}" "${outfile}"
}

main "${@}"