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Re: EMMS MPD consume patch

From: Yoni Rabkin
Subject: Re: EMMS MPD consume patch
Date: Wed, 15 Mar 2023 18:35:09 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux)

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
>>>>> make
>>>>> any
>>>>> IP claims on my work, I started hacking on an enhanced cleanroom
>>>>> take
>>>>> 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
>>>>> might
>>>>> 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
>>>>> order
>>>>> certainly isn’t robust, it is very efficient.  The only solution
>>>>> I’m
>>>>> coming up with is shoving the songid into the EMMS track
>>>>> structure
>>>>> (in
>>>>> emms-player-mpd-get-tracks-1), and doing a linear scans to locate
>>>>> the
>>>>> 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
>>>> and
>>>> 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.

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
>> 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.

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.

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.

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

>> 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.
> Thatnks,
>  — Ian

thank you

   "Cut your own wood and it will warm you twice"

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