`Sma` and `BollingerBands` both maintained their running `sum` (and
`sum_sq` for Bollinger) with a single-subtract incremental update. That
is correct in exact arithmetic, but in f64 the sequence `sum -= old;
sum += new` on long streams with alternating large/small magnitudes
can accumulate catastrophic-cancellation error. Bollinger's existing
`.max(0.0)` clamp on the computed variance was a band-aid for the same
root cause — the drift had already driven the running variance below
zero.
The fix: every `16 · period` finite updates, reseed `sum` (and `sum_sq`
for Bollinger) from the live window. Amortised cost stays at O(1) —
`O(period)` work amortised over `O(period)` updates — and the reseed
strategy is named after the constant `RECOMPUTE_EVERY` so the
intention is clear at the call site.
Behaviour is unchanged on inputs that did not drift to begin with
(every existing test still passes, including `batch_equals_streaming`
and the SMA proptest). Two new stress tests
(`long_stream_drift_stays_bounded` in each module) feed a
magnitude-alternating stream for `5 · RECOMPUTE_EVERY · period`
updates and assert the reported value tracks a fresh from-scratch
computation over the live window to within tight tolerance — these
would have failed without the reseed on Bollinger's `sum_sq`.
The misleading `sma.rs` comment that claimed drift was already
bounded by recomputing the sum after each pop is rewritten to
describe the actual reseed strategy (audit finding L2-Rust).
kingchenc
510013fc5a