Profiling on the Lyra found vector-tile decode dominating everything else
in the app by about two orders of magnitude: a median cached tile took
1236 ms of pure CPU, the largest 3.4 s, against 16 ms of SD read for a
whole viewport. Decoded tiles are memoised for the session, so this is a
first-pan cost -- but it runs on a GIL-bound worker thread, so it starves
the render loop for those seconds rather than running beside it.
Roughly 40% of that work was thrown away. A planet tile ships building,
housenumber, poi and mountain_peak; _draw_tile looks up ten layer names
and none of them is those. So decode_tile now takes the set of layers the
caller actually wants, reads each layer's identity with a cheap header
walk, and decodes the features only for the ones asked for. Unwanted
layers are still returned, named and empty -- which matters, because the
tile source decides whether a cache entry is real by whether the bytes
parsed into layers, and a tile whose every layer we skip must not read as
blank and get its perfectly good cache entry pruned and re-downloaded
every session. That test moves off "has features" and onto "parsed into
layers", which is sound because the other two ways a tile isn't real --
empty and corrupt -- decode to nothing and raise respectively.
Only the decode narrows; whole tiles are still cached, so drawing more
later costs a re-decode and never a re-download.
Measured on the device across the 46 real cached tiles: 183 s -> 51 s,
3.6x, with the drawn layers byte-identical on every one. The median tile
is 1259 -> 353 ms, the largest 3545 -> 759 ms.
_Reader.skip stopped slicing out the bytes it steps over while it was
here, which is most of what makes the header walk cheap.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Jean-Pierre MartineauandClaude Opus 5
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