#!/bin/sh
set -eu

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

help() {
	cat <<-'EOF'


		Options:
		  -v            notify when new
		  -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.  When -v is given, notify on STDERR
		that FILE has new content (useful for debugging purposes).


		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, and print
		  to STDERR when it does:

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


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

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

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


STDIN="$FILE.$(uuid).tmp"
trap 'rm -f "$STDIN"' EXIT

cat - > "$STDIN"
if ! cmp -s "$STDIN" "$FILE"; then
	MV_ARGS=''
	if [ "$VERBOSE" = true ]; then
		printf 'ifnew: "%s" will be updated.\n' "$FILE" >&2
		MV_ARGS='-v'
	fi
	mv $MV_ARGS "$STDIN" "$FILE"
else
	if [ "$VERBOSE" = true ]; then
		printf 'ifnew: "%s" left unchanged.\n' "$FILE" >&2
		rm -vf "$STDIN"
	fi
fi