#!/bin/sh
# backup garbage collector
# keep:
#   1 backup per year the previous years
hd=3600		# hour duration:  60s * 60
h12=43200	# 12h
dd=86400	# day duration:   hd * 24
wd=604800	# week duration:  dd * 7
md=2592000	# month duration: dd * 30
yd=31557600	# year duration:  dd * 365.25

now=$(date +'%Y%m%d_%H%M%S')

#date2ts() { d=${1%_*} t=${1#*_}; date -d "$d${t%??}" +%s; }
date2ts() {
	t=$1; r=${t#????}; Y=${t%$r}
	t=$r; r=${t#??}; m=${t%$r}
	t=$r; r=${t#??}; d=${t%$r}
	t=${r#_}; r=${t#??}; H=${t%$r}
	t=$r; r=${t#??}; M=${t%$r}
	S=${t#??}
	date -d "$Y-$m-$d $H:$M:$S" +%s
}

ts2date() { date -d "@$1" +'%Y%m%d_%H%M%S'; }

tsn=$(date2ts "$now")

# minimal retention delay, according to backup age
retention_delay() {
	d=$((tsn - $1))
	if [ $((d < h12)) = 1 ]; then
		r=0	# keep all backups in the last 12 hours
	elif [ $((d < dd)) = 1 ]; then
		r=$hd	# keep 1 backup per hour in the last day
	elif [ $((d < wd)) = 1 ]; then
		r=$dd	# keep 1 backup per day in the last week
	elif [ $((d < md)) = 1 ]; then
		r=$wd	# keep 1 backup per week in the last month
	elif [ $((d < yd)) = 1 ]; then
		r=$md	# keep 1 backup per month in the last year
	else
		r=$yd	# keep 1 backup per year in the previous years
	fi
	echo $r
}

while getopts :nv opt; do
	case $opt in
	(n) optn=1 ;;
	(v) optv=1 ;;
	(*) echo "Usage: $0 [-nv] [dir]"; exit 1;
	esac
done
shift $((OPTIND - 1))

# sorted list of backups, most recent first
dest=${1:-/.history}
lbu=$(ls -rv "$dest")
for d in $lbu; do
	tsc=$(date2ts "$d")
	if ! [ "$tsp" ]; then
		[ "$optv" ] && echo "keep $dest/$d"
		tsp=$tsc
		continue
	fi
	mrd=$(retention_delay "$tsp")
	dp=$((tsp - tsc))
	if [ $((dp < mrd)) = 1 ]; then
		[ "$optv" ] && echo "delete $dest/$d"
		[ "$optn" ] || rm -rf "${dest:?}/$d"
	else
		[ "$optv" ] && echo "keep $dest/$d"
		tsp=$tsc
	fi
done