The app shuffled by flattening every album's meta.json in scope into one
array and choosing from it. On a hundred-thousand-track library that
meant a six-figure object array living in the phone's JS heap — marked
by every garbage collection — and copied again on each draw. Measured on
device: the JS thread never went idle during playback and the phone
reached 'serious' thermal state.
Picking twenty songs never required any of that. The manifest already
holds per-album track counts, so the choice is arithmetic over numbers
and only the albums actually picked are read.
Uniform over TRACKS, not albums: choosing an album first would make a
two-track EP as likely as a sixty-track box set, so albums are weighted
by count via a running total and a binary search. Same distribution as
shuffling everything and taking the first n, which is what a shuffle
means. Results are re-shuffled before returning, because grouping the
reads by album would otherwise hand back something sorted by album.
