dist_build() sorted the entire 151936-entry vocab by probability (qsort) on every
sampled token whenever 0 < g_nuc < 1 — the serve default — and again per draft
position under rejection sampling. Measured cost: 5.6-8.0 ms/call; the actual work
is finding the few-hundred-token head whose cumulative mass reaches g_nuc.
Replace the full qsort + linear scan with a max-heap partial select:
- Floyd heapify g_pidx over V by descending g_pbuf prob (O(V), cache-friendly)
- pop winners to the array's high end until cum >= g_nuc (k * O(log V))
- the remaining heap prefix IS the tail -> zero it, renormalize the head
Winners land in g_pidx[out..V-1] in descending order, so s2 accumulates in the
same order as before -> head is unchanged on tie-free shapes (ties were already
unspecified under the unstable qsort). All four dist_build/dist_sample contract
properties hold: g_pbuf stays id-indexed, g_pidx stays internal, the tail is
fully zeroed, the head renormalizes to 1.
No API change, no caller change, no new globals.
c/tests/test_topp.c (new): drives the REAL dist_build via the include-glm.c
pattern against an independent double-precision reimplementation of the OLD
algorithm. 123 cases: 6 sizes (1..1519) x 5 shapes (uniform/peaked/geometric/
plateau/sharptail) x 4 nuc values, plus the g_nuc>=1 guard-off paths and V=1.
Tie-free shapes compare head values within 1e-6 rel (float vs double renorm
noise); tie shapes compare value-multisets. No scratch files -> builds clean on
Windows MinGW without the unmerged mkdtemp shim (#352).
woolcoxm
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