#!/bin/sh
#
# Shell command language minifier
#
# 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'
HT='	'
LF='
'

c=
output=''

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

	printf "shmin: ${fmt}\n" "${@}"
	exit 2
}

fgetc()
{
	# Damn command substitutions eat trailing whitespace.  Even if
	# whitespace is the /only/ thing printed and exactly what we want from
	# the command.
	c="$(dd bs=1 count=1 <&3 2>/dev/null; printf '.')"
	c="${c%.}"
}

minify()
{
	local input="${1}"
	local buffer=''
	local escaped=false

	# Open input file.
	if ! exec 3<"${input}"; then
		die 'Cannot open file "%s"' "${input}"
	fi

	# Check for magic number or other first-line comment.
	fgetc
	if [ "x${c}" = 'x#' ]; then
		# Comment
		fgetc
		if [ "x${c}" = 'x!' ]; then
			# Magic number
			buffer="${buffer}#!"
			while :; do
				fgetc
				buffer="${buffer}${c}"
				if [ "x${c}" = "x${LF}" ]; then
					break
				fi
			done
		else
			while :; do
				fgetc
				if [ "x${c}" = "x${LF}" ]; then
					break
				fi
			done
		fi
	fi

	# Strip leading whitespace on the first line.
	while :; do
		case "${c}" in
			' ' | "${HT}" | "${LF}")
				;;
			*)
				break
				;;
		esac
		fgetc
	done

	# Parse the rest of the file.
	# Like any good parser, this is some very ugly and fragile code.
	# Whitespace code must come before comment code must come before newline
	# code must come before everything else.
	while :; do
		if [ "x${c}" = 'x' ]; then
			break
		fi
		if [ "x${c}" = 'x ' ] || [ "x${c}" = "x${HT}" ]; then
			# Whitespace
			while :; do
				fgetc
				case "${c}" in
					' ' | "${HT}")
						;;
					"${LF}" | '#')
						# Eat trailing whitespace.
						break
						;;
					*)
						# Condense whitespace.
						buffer="${buffer} "
						break
						;;
				esac
			done
		fi
		if [ "x${c}" = 'x#' ]; then
			# Comment
			while :; do
				fgetc
				if [ "x${c}" = "x${LF}" ]; then
					break
				fi
			done
		fi
		if [ "x${c}" = "x${LF}" ]; then
			# Strip empty lines and leading whitespace.
			while :; do
				fgetc
				case "${c}" in
					' ' | "${HT}" | "${LF}")
						;;
					*)
						buffer="${buffer}${LF}"
						break
						;;
				esac
			done
		fi
		if [ "x${c}" = 'x\' ]; then
			# Backslash
			buffer="${buffer}${c}"
			fgetc
			buffer="${buffer}${c}"
			continue
		fi
		if [ "x${c}" = "x'" ]; then
			# Single quotes
			buffer="${buffer}${c}"
			while :; do
				fgetc
				buffer="${buffer}${c}"
				if [ "x${c}" = "x'" ]; then
					break
				fi
			done
			fgetc
			continue
		fi
		if [ "x${c}" = 'x"' ]; then
			# Double quotes
			buffer="${buffer}${c}"
			while :; do
				fgetc
				buffer="${buffer}${c}"
				if ${escaped}; then
					escaped=false
					continue
				fi
				if [ "x${c}" = 'x\' ]; then
					escaped=true
					continue
				fi
				if [ "x${c}" = 'x"' ]; then
					break
				fi
			done
			fgetc
			continue
		fi
		buffer="${buffer}${c}"
		fgetc
	done

	# We made it.  Now close input file.
	if ! exec 3<&-; then
		die 'Cannot close file "%s"' "${input}"
	fi

	# Write the output.
	if ! printf '%s' "${buffer}" >"${output}~"; then
		die 'Cannot write file "%s"' "${output}~"
	fi

	# Set output file name.
	if ! cat "${output}~" >"${output}"; then
		die 'Cannot rename file to "%s"' "${output}"
	fi
	if ! rm "${output}~"; then
		die 'Cannot remove file "%s"' "${output}~"
	fi
}

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

help()
{
	usage
	cat <<EOF
Options:
  -h           Display this information
  -V           Display minifier version information
  -o <output>  Write the minified output to <output>, instead of replacing the
               existing file
EOF
}

version()
{
	cat <<EOF
shmin ${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 input=

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

	if [ ${#} -ne 1 ]; then
		usage >&2
		exit 1
	fi
	input="${1}"

	if [ "x${output}" = 'x' ]; then
		output="${input}"
	fi

	minify "${input}"
}

main "${@}"