Finite-difference (PDE) solvers for 1-D, 2-D and 3-D problems, one
scheme per file — numerical kernels only, independent of any payoff or
grid, so they are usable as a standalone FD toolkit.
Interpolation toolkit, one scheme per file — the single home for
every interpolation in the library (curves, smiles, surfaces) and a
standalone toolkit in its own right.
Multi-dimensional optimization for model calibration, one algorithm
per file. This is the fitting layer for every parametric model —
Heston today (equity::heston::calibrate),
SABR / Nelson-Siegel or any other least-squares fit tomorrow — so the
machinery lives in one place, like solvers
does for 1-D root finding.