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docs/sync_algorithm_v3.md

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+# Sync Algorithm V3
+
+Drafted on 20240117.
+
+An absolutely better sync algorithm. Better for tracking deletions and better for subbranching.
+
+## Huge Thanks
+
+Basically a combination of algorithm v2 + [synclone](https://github.com/Jwink3101/syncrclone) + [rsinc](https://github.com/ConorWilliams/rsinc) + (some of rclone [bisync](https://rclone.org/bisync/)). All of the later three are released under MIT License so no worries about the licenses.
+
+## Features
+
+Must have
+
+1. true deletion detection
+2. deletion protection (blocking) with a setting
+3. transaction from the old algorithm
+4. user warning show up, **new algorithm needs all clients to be updated!** (deliberately corrput the metadata file??)
+5. filters
+6. conflict warning
+7. partial sync
+
+Nice to have
+
+1. true time and hash
+2. conflict rename
+
+## Description
+
+We have _five_ input sources: local all files, remote all files, _local previous succeeded sync history_, local deletions, remote deletions.
+
+Init run, consuming local deletions and remote deletions :
+
+TBD
+
+Later runs, use the first, second, third sources **only**.
+
+TBD