remotely-save/docs/sync_algorithm.md

Sync Algorithm

Sources

We have three record sources:

1. Local files
2. Remote files
3. Local "delete-or-rename" history.

Assuming all sources are reliable.

Deal with them

We list all combinations mutually exclusive and collectively exhaustive.

ID	Remote Files	Local files	Local delete rename history	Extra	Decision
1	exist	exist	ignore	mtime_remote > mtime_local	download remote file, create local folder if not exists, clear local history if exists
2	exist	exist	ignore	mtime_remote < mtime_local	upload local file, create remote folder if not exists, clear local history if exists
3	exist	exist	ignore	mtime_remote === mtime_local && password === "" && size_remote === size_local	clear local history if exists (the file was synced and no changes after last sync)
4	exist	exist	ignore	mtime_remote === mtime_local && password === "" && size_remote !== size_local	upload local file, clear local history if exists (we always prefer local to remote)
5	exist	exist	ignore	mtime_remote === mtime_local && password !== ""	clear local history if exists (in encryption mode, file sizes are unequal. we can only rely on mtime(s))
6	exist	exist	ignore	If local is a folder. mtime_local === undefined	clear local history if exists. TODO: what if a folder and a previous file share the same name?
7	exist	not exist	exist	mtime_remote >= delete_time_local	download remote file, create folder if not exists
8	exist	not exist	exist	mtime_remote < delete_time_local	delete remote file, clear local history
9	exist	not exist	not exist		download remote file, create folder if not exists
10	not exist	exist	ignore	local may be folder or file	upload local files recursively, create remote folder if not exists, clear local history if exists
11	not exist	not exist	ignore		clear local history if exists