#!/bin/bash

# This algorithm implements a variation of the Towers of Hanoi rotation method
# (see http://en.wikipedia.org/wiki/Backup_rotation_scheme#Towers_of_Hanoi).
#
# Unlike traditional ToH rotation, which uses a finite set of physical tapes,
# we operate on a set of snapshots whose size doesn't necessarily have to be
# bounded. Note that the number of snapshots only grows logarithmically with
# time, which makes it very hard to fill your hard disk (even when running
# unbounded).
#
# The result is that once we've run this for long enough, we'll find that for
# recent dates (e.g. last few days) almost all snapshots are available, and the
# older the date we're searching the more spread available snapshots will be.

set -e

. /etc/default/zfs

# Does $1 belong to class $2 ?
isclass ()
{
	if [ "$2" == "$AUTOSNAP_MAX_SNAPSHOTS" ] ; then
		# Special-case. Treat all snapshots as if they belong to
		# this class (in addition to their real class).
		return 0
	fi
	local remainder=$((2 ** ($2 - 1)))
	local divisor=$((${remainder} * 2))
	[ $(($1 % ${divisor})) == ${remainder} ]
}

classify ()
{
	local creation="$1"
	local creation_in_days="$(($(LANG=C date +%s -d "@${creation}") / 86400))"
	local class="1"
	# Find the class ${creation_in_days} belongs to.
	while ! isclass "${creation_in_days}" "${class}" ; do
		class=$((${class}+1))
	done
	echo "${class}"
}

for fs in $AUTOSNAP_FILESYSTEMS ; do
	# Create today's snapshot.
	echo zfs snapshot ${fs}@autosnap-$(date +%F)
	zfs snapshot ${fs}@autosnap-$(date +%F)

	# Remove any snapshots in the same class as today's addition.
	LANG=C zfs list -r ${fs} -t snapshot -H -o name,creation \
	| grep @autosnap- \
	| while read name creation_human ; do
		creation="$(LANG=C date +%s -d "${creation_human}")"
		echo "$(classify "${creation}") ${creation} ${name}"
		done \
	| sort -nr \
	| while read class creation name ; do
		echo ${class} ${name}
		if [ ${class} == "${previous_class}" ] ; then
			echo zfs destroy "${name}"
			zfs destroy "${name}"
		fi
		previous_class="${class}"
		done
done

exit 0
