feat: Add Smart Playlist Sync to download only new/updated tracks

[khushipy]

Requested Feature

Feature Request: Smart Playlist Sync

Description

Add functionality to download only new or modified tracks from a playlist since the last download, rather than re-downloading the entire playlist.

Problem

Currently, when updating a local copy of a playlist, users must either:

1. Re-download the entire playlist (wasting time and bandwidth)
2. Manually track which tracks are new (time-consuming and error-prone)

Possible implementation

Proposed Solution

1. Track Playlist State:

   • Store a checksum or timestamp of the playlist's current state
   • Save this information in a local database or metadata file

2. Incremental Downloads:

   • When downloading a playlist, first check its current state against the stored state
   • Only download tracks that are new or have been modified since the last download

3. CLI Options:

   # Basic usage (automatically detects changes)
   spotdl sync [https://open.spotify.com/playlist/](https://open.spotify.com/playlist/)...
   
   # Force full re-download
   spotdl sync --force [https://open.spotify.com/playlist/](https://open.spotify.com/playlist/)...
   

The Smart Playlist Sync feature tracks changes in Spotify playlists by maintaining a local state file with playlist hashes. When syncing, it compares the current playlist state with the stored hash, downloading only new or modified tracks to save bandwidth and time. The implementation includes a new 'spotdl sync' command with options for custom output formats and force updates, while ensuring efficient state management and error handling. This feature is particularly useful for frequently updated or collaborative playlists, especially for users with limited bandwidth.

[Shajal-Kumar]

Hi! I'll be working on this issue. Will post any updates here.

[vhmaadhav]

I'm happy to take this!

My plan is to:

Create a state file (spotdl/utils/sync_state.py using platformdirs) to store track IDs and metadata for each playlist.

Implement a new sync command (spotdl/console/sync.py) that will compare the current playlist against the saved state, diff the tracks, and only download what's new or changed. It will include a --force flag to re-download everything.

Add unit tests and update the documentation.

Let me know if this works. I'll start on the PR!

[Shajal-Kumar]

@vhmaadhav Go ahead.