PDDL3 preferences

ferroplan compiles soft-goal preferences away (Keyder & Geffner, JAIR 2009) and minimizes the :metric.

• Goal preferences, including (forall (?x) (preference p phi)), are expanded into one instance per binding; (is-violated p) counts violated instances.
• Precondition preferences become satisfied/violated action variants, so a violation is charged exactly once per application.
• The metric must be linear in (is-violated …) and (total-cost) (the IPC-5 simple-preferences shape) — plus any monotone numeric term (e.g. rovers' sum-traverse-cost), which is folded into total-cost; maximize / negative / scaled metrics fall back to a satisficing plan with a clear note.

The optimizer (0.4.0)

The default is an exact-closure metric optimizer: it searches real states with metric-bounded acceptance and closes the compiled preference bookkeeping with a provably-optimal collect/forgo phase tail. Three pieces make it scale:

• Static preference simplification at compile — statically-satisfied preference instances are dropped before grounding (storage's 62k-instance quadratic forall collapses ~97%).
• Barrier-free full-DNF guidance — the search sees a preference's forgone cost directly instead of behind a compilation barrier.
• A budget-escalating branch-and-bound — a tightening probe that hits its per-iteration eval cap without improvement retries the same bound with the remaining budget rather than giving up. The deterministic, thread-count-independent budget is FF_PREF_EVAL_BUDGET (default 2M evals) — a real quality dial.
• Anytime sweeps + a diversified restart ladder — each sweep tightens its bound in place on every acceptance (a restart happens once per cap, not once per improvement), and a capped sweep that fails to improve rotates the open-list weights through a fixed profile ladder before the final full-budget escalation — a stuck h-ordering is a direction problem, not a budget problem. This is what broke the storage/tpp large-instance plateau (storage now beats SGPlan5 on p01–p07).

For resource-coupled domains an opt-in ESPC penalty loop (FF_ESPC, after Hsu–Wah's extended-saddle-point method) prices a shared resource as a global constraint across a partitioned search — the lever that puts openstacks ahead of SGPlan5 on its larger instances. Every knob has a restore hatch (FF_PREF_COMPILED, FF_PREF_NO_STATIC, FF_PREF_BARRIER, FF_PREF_NO_ESCALATE, FF_ESPC_MONO); see the tuning reference.

On the largest instances exact optimization may return a best-found plan (flagged not proven optimal) within the budget. Full per-instance results vs SGPlan5: benchmarks/ipc5-scoreboard.md.

Trajectory constraints ((:constraints ...)) — enforced since 0.7

The six untimed modal operators — always, sometime, at-most-once, sometime-after, sometime-before, at end — are enforced on the classical path by compiling each ground constraint instance into a small monitor automaton: fresh 0-ary monitor facts transitioned by conditional effects on every action, with acceptance checked at the goal. forall outside a (preference ...) multiplies instances (so (is-violated name) counts violated instances); and/forall inside a preference body stay ONE instance, violated at most once — the PDDL3 instance boundary.

• Hard constraints become goal conjuncts: a plan that violates one is simply not a plan.
• Soft (preference name ...) constraints lower to ordinary goal preferences priced by the metric machinery above — the whole optimizer stack applies unchanged, and (is-violated name) works across goal and constraint preferences in one namespace.
• The independent verifier (ferroplan::verify) replays the ORIGINAL constraint semantics over the trajectory — never the compiled monitors — so reported metrics are cross-checked by construction, and validate_plan rejects constraint-violating plans.
• Statically decidable instances are simplified away before grounding (quadratic forall constraints over static relations stay tractable); FF_PREF_NO_STATIC=1 restores the blind expansion.

The timed operators (within, always-within, hold-during, hold-after) and constraints on durative-action domains are rejected by name — never silently dropped. FF_CONSTRAINTS_REJECT=1 restores the pre-0.7 blanket rejection. Measured results on the IPC-5 qualitative-preferences track: benchmarks/ipc5-qualitative-scoreboard.md.