remotely-save/docs/algorithm.md

# 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                                |