#!/bin/sh

help() { echo "filter - filter input based on given expression to evaluate

USAGE:
	filter [OPTION]... <FILTER_EXPRESSION> [FILES]...

OPTIONS:
	-i  update files with filtered content
	-h  show this help message

FILTER_EXPRESSION is a contains the command string that will be executed to test
the input line. {} should be used as a placeholder that will be replaced to the
double-quoted string of the input line. If the command evaluates to true, the
line is printed to stdout.
"; }

err() { printf 'filter: %s\n' "$@" >&2; exit 1; }
while getopts 'ih' o; do case "$o" in
	i) iflag=1 ;;
	h) help; exit ;;
	*) err "invalid option -- '$OPTARG'" ;;
esac done
shift $((OPTIND - 1))

[ "$#" -lt 1 ] && help >&2 && exit 1
filter="$1"; shift

evaluate() {
	while IFS= read -r line; do
		escaped_line="$(printf "%s\n" "$line" | sed "s|\"|\\\\\\\\\"|")"
		cmd="$(printf "%s\n" "$filter" | sed "s|{}|\"$escaped_line\"|")"
		eval "$cmd" && printf "%s\n" "$line"
	done
}

if [ "$iflag" = 1 ]; then
	for file in "$@"; do
		[ -z "$1" ] && err "option -i requires file argument and none were passed"
		tmp="$(mktemp "/tmp/$file.tmp-filter.XXXXXX")"
		evaluate < "$file" > "$tmp"
		mv "$tmp" "$file"
	done
	exit
fi

if [ -n "$1" ]; then
	evaluate < "$@"
else
	evaluate
fi