The map build already used a transient map, but each bucket was rebuilt with
a persistent (conj (get ret k []) x) per element — an O(log n) trie path
rebuild + alloc each. A coarse grouping (few large buckets) was bound on that
conj, not the map build. Buckets are now native arrays (transient vectors,
O(1) push) frozen once; distinct keys are tracked in a side vector so the
buckets freeze in place with no second map rebuild. A bucket's first element
stays a cheap persistent [x] and only promotes to a transient on the second,
so an all-singletons grouping pays no transient alloc.
coarse (10/100 buckets, 50k): ~313ms -> ~125ms (~2.5x)
2 buckets (50k): ~322ms -> ~129ms (~2.5x)
all-unique (50k): ~949ms -> ~892ms (no regression)
Surfaced a latent bug: canon-key returns nil for a nil key and Janet tables
drop a nil key, so the canon-keyed transient map silently lost a nil-key
entry — group-by/frequencies/assoc!/into{} dropped the whole nil bucket
((group-by identity [nil nil 1]) gave {1 [1]}, not {nil [nil nil], 1 [1]}).
Route nil through a sentinel (tbl-key) at the transient-map keying sites;
persistent!/count/dissoc! work unchanged since the real [nil v] pair is kept
as the stored value, and phm already has its own has-nil slot. The transient
set has the analogous bug (needs phs nil support) — filed separately.
Co-authored-by: Yogthos <yogthos@gmail.com>
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