#!/bin/sh
set -eu


usage() {
	cat <<-'EOF'
		Usage:
		  htmlesc [-e|d] [CONTENT...]
		  htmlesc -h
	EOF
}

help() {
	cat <<-'EOF'


		Options:
		  -e            escape the string (the default action)
		  -d            unescape (de-escape?) the string
		  -h, --help    show this message

		  CONTENT       a literal string to be escaped


		Convert data to/from HTML escaping.  If CONTENT is not given,
		get data from STDIN.


		Examples:

		  Escape HTML control characters in a string:

		    $ htmlesc 'a > 5 && !b'
		    a &gt; 5 &amp;&amp; !b


		  Unescape the content from a file:

		    $ htmlesc -d < file.html
	EOF
}


for flag in "$@"; do
	case "$flag" in
		(--)
			break
			;;
		(--help)
			usage
			help
			exit
			;;
		(*)
			;;
	esac
done

ENCODE=false
DECODE=false
while getopts 'edh' flag; do
	case "$flag" in
		(e)
			ENCODE=true
			;;
		(d)
			DECODE=true
			;;
		(h)
			usage
			help
			exit
			;;
		(*)
			usage >&2
			exit 2
			;;
	esac
done
shift $((OPTIND - 1))

if [ "$ENCODE" = true ] && [ "$DECODE" = true ]; then
	printf 'Both -e and -d given.  Pick one.\n' >&2
	usage >&2
	exit 2
fi

decode() {
	sed \
		-e 's|&amp;|\&|g'  \
		-e 's|&lt;|<|g'   \
		-e 's|&gt;|>|g'   \
		-e 's|&quot;|"|g' \
		-e "s|&apos;|'|g"
}

encode() {
	sed \
		-e 's|&|\&amp;|g'  \
		-e 's|<|\&lt;|g'   \
		-e 's|>|\&gt;|g'   \
		-e 's|"|\&quot;|g' \
		-e "s|'|\&apos;|g"
}

if [ "$DECODE" = true ]; then
	FN=decode
else
	FN=encode
fi

if [ $# = 0 ]; then
	"$FN"
else
	for s in "$@"; do
		printf '%s\n' "$s" | "$FN"
	done
fi