[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: EMMS MPD consume patch

From: Ian Eure
Subject: Re: EMMS MPD consume patch
Date: Wed, 15 Mar 2023 16:13:20 -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:

Yoni Rabkin <yoni@rabkins.net> writes:

Ian Eure <ian@retrospec.tv> writes:

Hi again,

Seeing as I’m currently unemployed, therefore no employer can
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
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
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
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
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
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,
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
was played, we know what track X is.)

But I don't actually know what I'm talking about. This is how I
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.

This only happens the result of the track being consumed by MPD is that it is also killed from the playlist buffer. If you visually mark the track as "consumed by mpd" in some way, then all of the tracks remain in

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
tool, but an extension of emms-playlist-mode, which would provide a different way of accessing tracks. This manner of access, which
need to be kept syncronized with the playlist, can help accelerate access to a specific track in cases where people create a playlist
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

Please bear with me here as I try to understand the problem and the use
case. I don't use MPD. I apologize for my slow uptake here.

No problem, we’re pretty deep in the weeds at this point.

If you start by loading 8,000 tracks and shuffling them in MPD, then that will be the order that they appear in Emms. Why do you need to do
anything to find the next song? Isn't it simply the next track?

What you are describing seems to be that the MPD playlist is shuffled, but the Emms playlist has a different order. Is that correct? Why can't we have the Emms playlist mirror the shuffled MPD playlist? Looking for that next track would always take exactly one step. The track doesn't even need to be killed from the playlist buffer as long as there is an indication for the user that the remote end is in "consume" mode.

Shuffle is a command that performs a one-time randomization of the playlist, so yes, if you shuffle, then connect up EMMS, things will be fine. However, MPD also has four mode bits which change its behavior when it reaches the end of a song:

- "repeat" loops the song.
- "random" chooses a random songe in the playlist instead of advancing to the next one.
- "consume" deletes the just-played song.
- "single" stops playback after the current song.

Any combination of these can be enabled at the same time, so you can both play random songs and delete when they finish. When random is enabled, you can’t assume the next song in playlist order will be the next to get played, whether you shuffled the playlist or not. MPD clients need to handle all these modes, singly, and in combination.

(Note that I’m using MPD terms here. Many players have a "shuffle" option which is the equivalent of MPD’s "random.")

In any case, I'll let you move forward with your idea. It is indeed a unique use case. Assuming modern music, which has about 3-minute tracks, you are describing over 16 days of non-stop music playing. My machines will typically not be on for that long, let alone play music for that

I have a low-power single-board computer dedicated to music playback which I never turn off; but MPD persists its state anyhow, so my playlist would survive a reboot. It typically takes me a few months to get through the whole playlist. Then I load it up and do it over again.

The problem of correctly handling MPD mode bits in EMMS isn’t necessarily unique to how I use it, though it certainly exposes those kind of bugs quickly.

 — Ian

reply via email to

[Prev in Thread] Current Thread [Next in Thread]