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Re: EMMS MPD consume patch

From: Ian Eure
Subject: Re: EMMS MPD consume patch
Date: Wed, 15 Mar 2023 15:03:34 -0700
User-agent: mu4e 1.4.15; emacs 28.2

Yoni Rabkin <yoni@rabkins.net> writes:

Ian Eure <ian@retrospec.tv> writes:

Yoni Rabkin <yoni@rabkins.net> writes:

Ian Eure <ian@retrospec.tv> writes:

Hi again,

Seeing as I’m currently unemployed, therefore no employer can make
IP claims on my work, I started hacking on an enhanced cleanroom
on my previous work. I’m sending over a WIP patch, since I’ve run into a situation I’m not sure how to deal with, and figured you
have some feedback.

We'd be happy for that work; thank you.

I don’t have a great solution for this. While leaning on track
certainly isn’t robust, it is very efficient. The only solution
coming up with is shoving the songid into the EMMS track structure
emms-player-mpd-get-tracks-1), and doing a linear scans to locate
correct song(s). I dislike this, since my own EMMS/MPD usage has
playlists with thousands of songs in them.

Why not maintain a hash table to avoid the linear scan? Even a
100,000-item hash table remains small in terms of memory footprint
very fast.

I don’t think a hash table buys anything here. For it to be useful,
the key would be the song ID, and the value would be either the
playlist or buffer position of that song in the playlist (both amount to the same thing). But in consume mode, songs are deleted after
they’re played, which means the hash table has to be updated to
reflect the new positions, which is an O(n) operation as well. So I think it’s more complexity for no gain over scanning the buffer.

I'm probably not tracking the real problem since most hash table
operations are O(1) on average and only O(n) at their worst.

Do we:

Load in the remote playlist to emms. That's N operations where N is the
number of entries. The hash table is populated at that time.

MPD consumes a track X. It takes O(1) to find track X in the hash, which
means we can go right to it in the playlist and delete it from a
playlist no matter where it is.

Track X is then deleted from the hash as well (since we know which track
was played, we know what track X is.)

But I don't actually know what I'm talking about. This is how I imagine
it to work. What am I missing?

The problem to solve is: given two songids, how do we move from songid A to songid B in a playlist?

To solve this with a hash table, the keys need to be a songid, and the value needs to be the playlist position, which is a 0-based index into the playlist. The first song has position 0, second 1, on up to N-1, where N is the number of songs in that playlist.

Now you can do things like look up a songid, get its playlist index, go to the beginning of the playlist buffer, and advance some number of lines equal to the value from the hash; or look up two values, subtract them, and move that number of lines.

This works great as long as the playlist never changes. If you delete a track from the playlist (which consume mode does for you automatically, after it’s been played), the index of every following track is decremented. The values in the hash table are now off by one, and the error grows the more songs are removed.

Fixing that means updating the values in the hash table. Updating a single value is O(1), but O(n) operations have to be performed to update everything which changed. So you end up with the worst outcome, more complex code for the same O(n) performance characteristic of a linear scan of the buffer.

I think I have an approach that will work, I’ll try to hack on that a bit more this week. It’s just not terribly elegant, which I dislike.

Taking a step back, the hash table would not be an mpd-player specific tool, but an extension of emms-playlist-mode, which would provide a different way of accessing tracks. This manner of access, which would need to be kept syncronized with the playlist, can help accelerate access to a specific track in cases where people create a playlist with
many thousands of tracks.

A lot to think about...

I don’t have a strong opinion on this, as it’s mostly outside the scope of what I’m trying to accomplish.

FWIW, consume mode issues aside, EMMS handles large playlists much, much better than other Emacs MPD clients I’ve tried. Emmet, for example, refreshes the entire playlist on any change, which is slow enough to make my whole Emacs session pause on every track change. My typical MPD use is to load nearly my entire music library, shuffle it, then run that in consume mode until it’s empty, which means I regularly have 8000+ songs in the playlist; so this is immediately disqualifying.

Also, can we please move this to the emms-help mailing list? I didn't CC this to there because I don't want to do so without your permission.

Definitely, I’ve moved you to bcc and redirected to the ML.


 — Ian

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