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ADR 0039: PCA epochs and archive rebuild on refit

Date: 2026-02-12

Status: Accepted

Decision

  • Treat the PCA projection as an epoch-scoped coordinate system for MAP-Elites behaviour measures.
  • When PCA refits (epoch increments), rebuild the island archive by reprojecting existing elites into the new coordinates and replacing persisted map_elites_archive_cells rows.
  • Persist samples_since_fit in map_elites_states.snapshot so MAPELITES_DIMENSION_REDUCTION_REFIT_INTERVAL works across one-shot reductions.
  • Align successive projections with orthogonal Procrustes alignment on elite anchors (when available) to reduce coordinate jitter between epochs. The resulting orthogonal matrix may include a reflection when PCA axes flip.