#!/usr/bin/env bash

read -rd '' AWK_PREFILTER <<'EOF'
/(\/|^)[0-9]{4}-[0-9]{2}-[0-9]{2}-[0-9]{6}$/ {
	a[$0]
}

END {
	n = asorti(a)
	for (i = n; i >= 1; --i)
		print a[i]
}
EOF

read -rd '' AWK_FILTER <<'EOF'
function flr(n, s) {
	return int(n / s) * s
}

function mstart(t) {
	return mktime(strftime("%Y %m 01 00 00 00", t, 1), 1)
}

BEGIN {
	n = split("h d w M", arr)
	for (i = 1; i <= n; ++i) {
		b = arr[i]

		if (!match(opt_retain, "([*0-9]+)(\\^?)" b, md))
			continue

		ret[b] = md[1]
		newest_in[b] = (md[2] == "^")
		buckets[++n_buckets] = b
	}

	split("sun mon tue wed thu fri sat", arr)
	for (i in arr)
		arr2[arr[i]] = i - 1
	wshift = (11 - arr2[opt_week_start])*24*60*60
}

NR == 1 {
	++keep[$0]
}

{
	t = mktime(gensub(/(.*\/|^)(.+)-(..)-(..)-(..)(..)(..)$/,
		"\\2 \\3 \\4 \\5 \\6 \\7", 1), 1)
	if (t < 0) {
		++keep[$0]
		next
	}

	bt["h"] = flr(t, 60*60)
	bt["d"] = flr(t, 24*60*60)
	bt["w"] = flr(t + wshift, 7*24*60*60) - wshift
	bt["M"] = mstart(t)

	for (i = 1; i <= n_buckets; ++i) {
		b = buckets[i]

		if (b == "M" && "w" in ret) {
			if (newest_in["w"]) {
				if (mstart(bt["w"] + 6*24*60*60) > bt["M"])
					next
			} else if (bt["w"] < bt["M"]) {
				next
			}
		}

		if (newest_in[b] && bt[b] == last[b, b] && (i == 1 ||
				bt[buckets[i-1]] != last[b, buckets[i-1]]))
			next

		if ((b, bt[b]) in bkeep || ret[b] == "*" || bc[b]++ < ret[b])
			bkeep[b, bt[b]] = $0

		for (j in buckets)
			last[b, buckets[j]] = bt[buckets[j]]
	}
}

END {
	for (i in bkeep)
		++keep[bkeep[i]]

	n = asorti(keep)
	for (i = 1; i <= n; ++i)
		print keep[i]
}
EOF

err() {
	echo "${0##*/}: error: $*" >&2
}

run() {
	echo "* $*"
	if ! "$@"; then
		err "command failed: $*"
		return 1
	fi
}

current() {
	local str="${1}" interval="${2}"
	local s=0 val d

	while [[ "${interval}" =~ ([0-9]+)(.?) ]]; do
		val="${BASH_REMATCH[1]}"
		case "${BASH_REMATCH[2]}" in
			d) ((s += val * 24*60*60)) ;;
			h) ((s += val * 60*60)) ;;
			m) ((s += val * 60)) ;;
			*) ((s += val)) ;;
		esac

		interval="${interval#*${BASH_REMATCH[0]}}"
	done
	((s < 1)) && return 1

	d="${str:0:10} ${str:11:2}:${str:13:2}:${str:15:2}"
	(($(date -u +%s) / s == $(date -ud "${d}" +%s) / s))
}

filter() {
	local -n _f_src="${1}" _f_dest="${2}"
	local _f_dest_name="${2}" _f_label="${3}" _f_script="${4}"
	shift 4

	mapfile -td $'\0' "${_f_dest_name}" < <(
		printf '%s\0' "${_f_src[@]}" | awk -v 'RS=\0' -v 'ORS=\0' \
			"${@}" -- "${_f_script}")
	wait "$!"
	if [[ "$?" -ne 0 ]]; then
		err "${_f_label} failed"
		return 1
	fi
	if [[ ${#_f_src[@]} -gt 0 && ! -d "${_f_dest[0]}" ]]; then
		err "${_f_label} returned empty or invalid list"
		return 1
	fi
}

backup() {
	local src_path="${1}" dest_dir="${2}"
	local retain="${3}" interval="${4}" week_start="${5}"
	local dest_path snaps keep skip i ki

	if [[ ! -e "${src_path}" ]]; then
		err "source does not exist: ${src_path}"
		return 1
	fi

	mapfile -td $'\0' snaps < \
		<(shopt -s nullglob; printf '%s\0' "${dest_dir}/"*)
	filter snaps snaps 'pre-snapshot awk prefilter' "${AWK_PREFILTER}" \
		|| return 1
	[[ -n "${snaps}" ]] && current "${snaps[0]##*/}" "${interval}" \
		&& return

	dest_path="${dest_dir}/$(date -u '+%Y-%m-%d-%H%M%S')"
	if [[ -e "${dest_path}" || -e "${dest_path}.tmp" ]]; then
		err "destination already exists: ${dest_path}"
		return 1
	fi
	run rsync -ac --mkpath ${snaps[0]:+"--link-dest=${snaps[0]}"} \
		"${src_path}" "${dest_path}.tmp/" || return 1
	run mv -T "${dest_path}.tmp" "${dest_path}" || return 1

	if [[ ! "${retain}" =~ ^( *([0-9]+|\*)\^?[hdwM] *)+$ ]]; then
		err "invalid \$retain setting: ${retain}"
		return 1
	fi
	snaps+=("${dest_path}")
	filter snaps snaps 'post-snapshot awk prefilter' "${AWK_PREFILTER}" \
		|| return 1
	filter snaps keep 'awk filter' "${AWK_FILTER}" \
		-v "opt_retain=${retain}" -v "opt_week_start=${week_start}" \
		|| return 1

	for ((i = ${#snaps[@]} - 1, ki = 0; i >= 0; --i)); do
		if [[ "${snaps[i]}" == "${keep[ki]}" ]]; then
			((++ki))
		else
			run rm -rf "${snaps[i]}"
		fi
	done
}

declare -A def_opts=(
	['retain']='*h' # e.g. 24h7d4w12M or 24^h10d*M
	['interval']='0'
	['week_start']='mon'
)
declare -A opts

declare -A seen
while IFS='=' read -r f1 f2; do
	[[ -z "${f1}" ]] || [[ "${f1}" == '#'* ]] && continue

	if [[ "${f1}" =~ ^\[(.*)]$ ]]; then
		base_path="${BASH_REMATCH[1]}"
		declare -A opts="$(declare -p def_opts | sed '1s/[^=]*=//')"
	elif [[ "${f1}" == '$'* ]]; then
		opt="${f1:1}"
		[[ "${f2}" == '-' ]] && opts["${opt}"]= ||
			opts["${opt}"]="${f2:-"${def_opts["${opt}"]}"}"
	else
		dest="${base_path}/${f1}"
		printf '[%d] [%s] %s => %s%s\n' "$((++dcount))" \
			"${opts['retain']}" "${f2}" "${dest}"

		dest_rp="$(realpath -m "${dest}")"
		if [[ -v "seen["${dest_rp}"]" ]]; then
			err "destination already seen: ${dest}"
			((++errors))
			continue
		fi
		seen["${dest_rp}"]=

		backup "${f2}" "${dest}" \
			"${opts['retain']}" \
			"${opts['interval']}" \
			"${opts['week_start']}" \
			|| ((++errors))
	fi
done < "${1}"

((!errors))