Function canonical_level_bits 

pub fn canonical_level_bits(v: f64) -> u64

Canonical bit key for a floating-point categorical / grouping level.

Factor dummies, random-effect groups, by= gates and factor-smooth blocks all identify a level by the raw bits of its numeric code — they intern the observed codes with f64::to_bits() and, at fit/predict time, gate each row by data_bits == level_bits. Raw to_bits() is a bit identity, not the numeric equality IEEE-754 defines, and the two disagree in exactly two places:

• Signed zero. +0.0 is 0x0000_0000_0000_0000 and -0.0 is 0x8000_0000_0000_0000, yet IEEE-754 guarantees +0.0 == -0.0. Keying on raw bits splits one physical group into two: a row whose code is -0.0 matches no +0.0 dummy, so its factor / random effect silently drops and the prediction collapses onto the intercept / population mean. Signed zero arises routinely from ordinary float arithmetic on a computed group column (-1.0 * 0.0, a centred/differenced column landing on -0.0, np.round emitting -0.0), and the miss is silent — no schema error, check() still reports ok=True. See #2145 (random effect) and #2146 (factor dummy).
• NaN. Every quiet/signalling NaN payload and sign bit denotes “not a number”, so 2^53-ish distinct bit patterns would otherwise intern as distinct levels. (NaN group codes are rejected upstream on most paths, but canonicalising here keeps the key numerically honest regardless.)

This maps both encodings to a single canonical key while leaving every ordinary finite value bit-stable, so that canonical_level_bits(a) == canonical_level_bits(b) iff a and b name the same real-valued level. Interning and lookup must both route through this function; because it is idempotent, applying it to an already-canonical frozen level set is a no-op.