Cross-compiles the nx_eigen C++ NIF (Eigen-backed Nx backend) for one Android or iOS target ABI and archives the result as libnx_eigen.a. The archive gets static-linked into the user app's main native binary alongside crypto.a, libemlx.a, and any other static NIFs.

Why static-link this NIF?

Same constraints as every other NIF mob ships on phones:

• Android. dlopen'd children inherit RTLD_LOCAL, hiding the parent's enif_* symbols from a separately-loaded libnx_eigen.so. on_load then fails with "cannot locate symbol". Static linking sidesteps that — BEAM finds nx_eigen_nif_init via dlsym(RTLD_DEFAULT) against the main app binary.
• iOS. App Store forbids loading unsigned dylibs / dlopen; every NIF must already be present in the signed binary.

Per-target deltas

All four targets compile the same two source files (@sources) with the shared base CXXFLAGS list (@base_cxxflags). The deltas are:

STATIC_ERLANG_NIF_LIBNAME

We pass -DSTATIC_ERLANG_NIF_LIBNAME=nx_eigen rather than plain -DSTATIC_ERLANG_NIF. The reason: nx_eigen_nif.cpp uses Fine's FINE_INIT(...) macro, which expands to ERL_NIF_INIT_DECL(NAME) — passing the literal token NAME as MODNAME. With STATIC_ERLANG_NIF alone the emitted symbol would be the unhelpful NAME_nif_init. Setting LIBNAME overrides MODNAME entirely and forces the symbol to nx_eigen_nif_init, which is what mob's driver_tab references.

FFT — Eigen's built-in kissfft backend

We don't use NxEigen's bundled FFT variants. Instead, mob_dev ships its own priv/cpp_nif/nx_eigen_fft_eigen.cpp bridge that calls Eigen's unsupported/Eigen/FFT module (kissfft underneath — header only, embedded in the Eigen tarball). This gives Nx.fft/3 and Nx.ifft/3 working on-device with no additional cross-compile.

Kissfft is roughly 2x slower than FFTW for large transforms but microseconds for audio-sized buffers; switch to a FFTW variant later if a real workload measures a bottleneck.

Phases

Each build/2 call:

1. Precheck — nx_eigen source dir + Fine + Eigen headers + erts include all present; Android/iOS toolchain reachable.
2. Compile — for each of @sources, run <cxx> <cxxflags> -c -o obj src.
3. Archive — <ar> rcs libnx_eigen.a obj1 obj2 then <ranlib> ....
4. Verify — <nm> libnx_eigen.a, scan for the expected symbol. Symbol missing is a :precondition_failed (means our compile didn't actually produce nx_eigen_nif_init — usually because Fine or NxEigen's source moved out from under us, or LIBNAME wasn't picked up).