pairwise_offsets now computes each signal's rFFT once at a shared padded
length (2*max_len) and reuses it for every pair: O(K) forward FFTs +
cheap per-pair spectrum products instead of O(K^2) full FFTs. Each
pair's lag search stays capped at +/-(la+lb)/2 — the same window the
per-pair transform used — and gcc_phat() itself is unchanged for
callers (drift windows, tests). Measured 1.8x on 6 cams x 10 min;
the advantage grows with camera count.
New test locks the cached path against direct gcc_phat, including
unequal-length signals where the padded sizes genuinely differ.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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