#!/bin/sh
set -eu

usage() {
	cat <<-'EOF'
		Usage:
		  ifnew FILE
		  ifnew -h
	EOF
}

help() {
	cat <<-'EOF'


		Options:
		  -h, --help    show this message

		  FILE          the target to compared against


		Compare STDIN to FILE, and only write to FILE if STDIN content
		differs from FILE content.


		Examples:

		  Create a new file, don't update it afterwards, and update it
		  as the content changes:

		    $ echo aaa | ifnew  new-file
		    $ stat --print='%y' new-file
		    # timestamp 1

		    $ echo aaa | ifnew  new-file
		    $ stat --print='%y' new-file
		    # timestamp 1

		    $ echo bbb | ifnew  new-file
		    $ stat --print='%y' new-file
		    # timestamp 2


		  Only change file when content changes in a Makefile:

		    i18.cfg: ALWAYS
			gen-cfg | ifnew $@
			po4a $@
	EOF
}


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

while getopts 'h' flag; do
	case "$flag" in
		h)
			usage
			help
			exit
			;;
		*)
			usage >&2
			exit 2
			;;
	esac
done
shift $((OPTIND - 1))

FILE="${1:-}"
eval "$(assert-arg -- "$FILE" 'FILE')"


STDIN="$(mkstemp)"
trap 'rm -f "$STDIN"' EXIT

cat - > "$STDIN"
if ! cmp -s "$STDIN" "$FILE"; then
	mv "$STDIN" "$FILE"
fi