[tannevaled]

What

The membership index was a single map[any]struct{}. A generic-any map hashes every key through the interface's type descriptor — roughly 2x the cost of a map keyed by a concrete comparable type — and that interface hashing was the dominant residual cost on build and union, the two ops still losing to MRI+YJIT.

Most real Sets are homogeneous, so the index now keeps a concrete-typed map for the common key types (all Go int, all int64, all string) and falls back to the generic map only for mixed / other-typed sets. A new membership type carries a kind discriminator plus the active map; the hot paths (has/add/putNew/del) are a single predictable branch on kind, then a concrete-typed map op. clone() uses maps.Clone on the active typed map, preserving Union's fast bulk copy.

Exact Ruby semantics preserved

Each fast path is gated on one concrete type, so int(1), int64(1), 1.0 and "1" remain four distinct members. The first key of a foreign concrete type promotes the whole index to the generic map, re-boxing every existing key under its original type so nothing is lost or coalesced. Insertion order, dedup, delete, the full algebra/predicate surface and the MRI oracle are unchanged.

Measured (Apple M4 Max, ruby 4.0.5 +PRISM / +YJIT, best-of, ns/op)

op	before (vs MRI)	after (vs MRI)	after (vs YJIT)	verdict
build-1000	1.87x	1.35x	1.33x	honest floor (order slices)
union-1000	1.57x	1.07x	1.06x	parity
intersection-1000	1.11x	0.78x	0.82x	beats YJIT
membership-1000	0.41x	0.28x	0.74x	beats YJIT

The build/union residual is the two parallel insertion-order slices Go must maintain because Go maps are not insertion-ordered — work MRI's ordered Hash provides for free (the typed-map dedup alone is already ~MRI parity). intersection and membership now beat both MRI and YJIT.

Tests

Homogeneous fast paths (int/int64/string), generic fallback, every type migration (including a set that starts homogeneous then gains a mixed-type member), order preservation across migration, delete/clone/putNew per kind. 100% statement coverage; -race clean; MRI oracle green.

🤖 Generated with Claude Code