into {}, frequencies, group-by, set, into #{} and persistent! all built
their result by folding an immutable assoc/conj per element — each call
rebuilt the O(log32 n) trie path and allocated a fresh wrapper. Add a
one-pass bottom-up HAMT builder (phm-from-pairs) and route the builders
through it, the map/set analog of the pvec bulk build in #153.
phm-from-pairs partitions entries by hash and constructs the bin/array/
collision nodes directly, with the same bin<=16 / array-node>=17 promotion
the incremental path uses — so the trie is byte-identical to one built by
phm-assoc (validated across the size and branching boundaries, including
hash collisions, duplicate keys and the nil key). persistent! map/set and
the set constructor bulk-build; into {} keeps the small-scalar-map-stays-a
-struct rule via bulk-map-from-pairs; frequencies/group-by switch to the
canonical transient form and ride the fast persistent!.
50k A/B: into {} 704->270ms, frequencies 582->160, set 615->241,
into #{} 702->240, group-by 1358->919 (bound on persistent vector conj).
Gate: conformance x3, full suite (4718 >= baseline), new maps/sets bulk
boundary specs.
Co-authored-by: Yogthos <yogthos@gmail.com>
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