1) There was a logic error in FileBuffer where when it was trying to
add a new 64kb chunk to hold output, it was adding an empty chunk,
but making sure we have space capacity to hold 64 thousand chunks.
So that was a bit of pointless juggling to get nothing good.
2) In UV_vertex_key, instead of trying to combine three members into
a hash value, badly, by doing some ad-hoc shifts and xors, use
get_default_hash_3 instead, which combines them way more properly.
Also avoid copying the whole hash map object.
On my windows box (Ryzen 5950X, VS2022), exporting Stanford Lucy 3D
scan:
- Binary: 13.4 -> 5.4 sec
- ASCII: 29.3 -> 14.6 sec
So basically 2x faster for two tiny changes.
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