A pvec is a 32-way trie, but na-chunk-first built each block by calling
pvec-v on the full backing vector — materializing all n elements to a
flat Scheme vector — then copying the 32-wide window out of it. That made
chunk-first O(n), so walking a vector chunk-by-chunk (Clojure's real
chunked map/filter fast path) was O(n^2): a ported chunked map over 500K
elements took 39s, superlinear to ~700s at 2M.
na-chunk-size equals pv-width and blocks are 32-aligned, so a block is
exactly one trie leaf — pv-chunk-for hands it back in O(log n). Copy that
leaf directly; fall back to per-index reads for the rare window that
crosses a leaf boundary. Chunked map is now linear, ~133x faster at 500K
(293ms) and within ~2.3x of the native seq loop, which makes a
clojure-in-clojure seq tier viable.
Corpus rows pin chunk-first window contents + chunk-rest boundaries
against JVM; fixed a stale 'always false' chunked-seq? label.
6d441e2d00