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                                                                            |