Compiled Faust devices

MAGDA has two kinds of Faust device:

• Compiled (this skill): the .dsp is compiled to C++ at build time and statically linked. Fast, ships in the binary. Lives in magda/daw/audio/faust_dsp/*.dsp + magda/daw/audio/plugins/compiled/Magda*CompiledPlugin.{hpp,cpp}.
• Interpreted/JIT (FaustPlugin / FaustInstrumentPlugin): loads .dsp at runtime. Not this skill.

The pipeline (how a .dsp becomes a device)

faust_dsp/magda_x.dsp
  --(CMake: magda_compile_faust_dsp, runs `faust -lang cpp`)-->
cmake-build-debug/compiled_dsps/magda_x.generated.cpp   (NOT checked in)
  --(#include'd by)-->
plugins/compiled/MagdaXCompiledPlugin.cpp   (the host wrapper)
  --(custom UI)-->
ui/components/chain/custom_ui/XUI.cpp

• The generated .cpp is produced by the build — there is no checked-in generated file to hand-edit. Just edit the .dsp and rebuild (make debug); CMake reruns faust. Confirm the result in cmake-build-debug/compiled_dsps/<name>.generated.cpp.
• Registration lives in magda/daw/audio/CMakeLists.txt via magda_compile_faust_dsp(<dsp> <ClassName> <OUT_VAR>); the generated path is added to target_sources and the wrapper #includes <name>.generated.cpp.
• Validate DSP before building with the faust-mcp-magda MCP tools (compile_faust, search_faust_libraries, get_faust_library).

Host-slot contract (THE thing to get right)

Each control is a Faust hslider/nentry tagged [idx:N]. N is the host slot index — the stable contract between dsp, wrapper, and UI. The wrapper harvests zones by reading the [idx:N] metadata.

Three places must agree on slot indices:

1. .dsp — [idx:N] on each param.
2. MagdaXCompiledPlugin.hpp — kFooSlot = N constants + kHostSlotCount.
3. MagdaXCompiledPlugin.cpp — hostSlotInfo_[kFooSlot] = {.name, .scale, .minValue, .maxValue, .defaultValue, .choices}.
4. XUI.{hpp,cpp} — its own kFooSlot constants + kNumParams, label, layout.

Adding a parameter (common case)

Append at the end (idx = old kHostSlotCount) so existing slot indices stay stable — never insert in the middle and renumber.

1. .dsp: add the hslider/nentry with the next free [idx:N]; wire it into the DSP graph. Reuse-by-copy from a sibling .dsp when matching behaviour (e.g. the drive lerp (1-d)*x + d*(tanh(4x)/tanh(4)) from magda_filter_svf.dsp).
2. Wrapper .hpp: add kFooSlot = N, bump kHostSlotCount. The std::array<…, kHostSlotCount> members grow automatically.
3. Wrapper .cpp: add hostSlotInfo_[kFooSlot] = {…}. Use ParameterScale::Discrete + .choices = {...} for menus, Linear/Logarithmic/FaderDB otherwise.
4. UI: add kFooSlot, bump kNumParams, add a label, place it in resized().
5. make debug (regenerates the dsp), confirm the param appears in the generated .cpp.

[style:menu{'A':0;'B':1}] in the dsp = a discrete choice param; mirror with .choices in slot info.

Custom UI + linking

• The UI exposes std::vector<LinkableTextSlider*> getLinkableSliders(); each slider carries its slot via setParamIndex(). DeviceSlotComponent::setupCustomUILinking() wires mod/macro/automation/MIDI-Learn off that list. Any control that must be linkable has to be a LinkableTextSlider in that list.
• To present a slot as something else (e.g. segmented buttons for a menu): keep the hidden LinkableTextSlider in controls_/getLinkableSliders() (carries value + linking) and drive it from the custom widget. Set selection explicitly for segmented buttons (setToggleState(i==sel)), not via JUCE radio groups (radio exclusivity left two segments lit).
• Theme-font buttons: setLookAndFeel(&FlatTabButtonLookAndFeel::getInstance()) (flat tab) or SmallButtonLookAndFeel (rounded), both in themes/SmallButtonLookAndFeel.hpp; clear with setLookAndFeel(nullptr) in the destructor.
• Labels/fonts: FontManager::getInstance().getUIFont(...), colours via DarkTheme::getColour(...).

Reusing a curve view across devices

Curve/response views (e.g. CompiledFilterCurveView) are normally bound to one plugin's slots via a DeviceInfo snapshot. To reuse one in a different device, add a raw-value entry point (setRawState(...)) that sets the target values directly and bypasses the device-snapshot path, plus optional per-instance theming (e.g. setCurveColour). Drive it from the host UI on every relevant param change.

Gotchas

• Generated .cpp is build output — don't look for it in git; rebuild to refresh.
• Never reorder/insert [idx:N]; append. (Unreleased features: no migration needed, but stable indices keep diffs sane.)
• ba.selectn(n, sel, ...) is strict — every branch runs, so cascading/parallel filter modes is cheap and safe.
• Polyphonic synths use Faust's mydsp_poly (group=false) — per-voice zones are harvested into voiceZonesBySlot_.
• Build with the building skill (make debug), never raw faust/cmake.