#!/bin/sh

set -eu

prev_hash=
prev_bk_file=

# Back up a database and output a SHA-256 hash of its SQL dump.
backup()
{
	local db_file="${1}"
	local bk_file="${2}"
	shift 2

	sqlite3 "${db_file}" ".backup ${bk_file}~"
	sqlite3 "${bk_file}~" '.dump' | sha256sum | cut -d ' ' -f 1
	return 0
}

# Read previous backup's hash and file name.
# Returns 0 if the new backup's hash is equal to the previous backup's hash, or
# 1 otherwise.
compare_hashes()
{
	local db_file="${1}"
	local bk_hash="${2}"
	shift 2

	{ read -r prev_hash prev_bk_file 0<"${db_file}.bakhash"; } 2>/dev/null \
		|| :

	case "${prev_hash}" in "${bk_hash}") return 0;; esac
	return 1
}

# Make a symbolic link with the new backup file name to the previous backup
# file.
# No path resolution is performed, so either absolute paths should be used or
# all backup files should be in the same directory.
# The linking of the final backup file and the unlinking of the temporary file
# are not atomic, but as long as they're done in this order, the only possible
# consequence is that the linking is done but an extraneous temporary file
# remains.
link_to_prev()
{
	local bk_file="${1}"
	shift 1

	ln -s "${prev_bk_file}.xz" "${bk_file}.xz"
	rm "${bk_file}~"

	return 0
}

# Compress and save a backup file.
save_backup()
{
	local db_file="${1}"
	shift 1

	xz "${bk_file}~"
	mv "${bk_file}~.xz" "${bk_file}.xz"

	return 0
}

# Save a backup file's name and hash for future comparison and linking.
save_hash()
{
	local db_file="${1}"
	local bk_file="${2}"
	local bk_hash="${3}"
	shift 3

	printf '%s %s\n' "${bk_hash}" "${bk_file}" 1>"${db_file}.bakhash~"
	mv "${db_file}.bakhash~" "${db_file}.bakhash"

	return 0
}

usage()
{
	printf 'Usage: %s <database-file> <backup-file>\n' "${0}"

	return 0
}

main()
{
	local db_file=
	local bk_file=
	local bk_hash=

	if [ ${#} -ne 2 ]; then
		usage 1>&2
		return 1
	fi
	db_file="${1}"
	bk_file="${2}"
	shift 2

	bk_hash="$(backup "${db_file}" "${bk_file}")"
	if compare_hashes "${db_file}" "${bk_hash}"; then
		link_to_prev "${bk_file}"
	else
		save_backup "${bk_file}"
		save_hash "${db_file}" "${bk_file}" "${bk_hash}"
	fi

	return 0
}

main "${@}"