Struct CostProblem 

pub struct CostProblem { /* private fields */ }

A cost-varying two-objective problem for the CARBS oracle: a bowl in the tuning parameters whose loss is bought down by a resource knob that also makes each evaluation more expensive.

The point is that cost varies across the space and cost-awareness pays (docs/design/09-implementation.md §8.3). Concretely, over tuning parameters x0..x{d-1} ∈ [-5, 5] and a resource r ∈ [0, 1]:

• performance loss (minimize): f(x, r) = Σ (xᵢ − cᵢ)² − gain·r. The bowl is shifted off the Sobol grid (cᵢ alternating +1.234 / −2.345, the §8.3 vacuous-oracle guard), and more resource lowers the loss — so the global optimum sits at the most expensive corner r = 1.
• cost (minimize): cost(r) = exp(cost_rate·r), independent of x, growing from 1 at r = 0 to exp(cost_rate) at r = 1.

A cost-blind optimizer chases the global loss optimum and is drawn to the expensive r → 1 corner; a cost-aware one (CARBS) discovers that a well-tuned x at small r is nearly as good for a fraction of the cost, and concentrates there. The efficiency metric that separates them is capped_best_perf: the best loss reached among trials whose cost stays under cost_cap (here the cost at r = 0.25). A cost-aware search fills the cheap region with well-tuned points and wins it; a cost-blind search wastes its budget above the cap.

Implementations

impl CostProblem

pub fn resource_bowl(tuning_dims: usize) -> Self

The canonical resource-bowl instance: tuning_dims tuning axes, a gain of 5, a cost rate of ln(20) (cost ranges 1..20), and an affordability cap at r = 0.25.

pub fn schema(&self) -> Result<SpaceSchema>

The search space: one linear float per tuning axis (x0..), then the resource r ∈ [0, 1].

Errors

Error::InvalidSpace if a bound is not finite — which never happens for the built-in instance.

pub fn eval(&self, assignment: &Assignment) -> (f64, f64)

Evaluates the problem at assignment, returning (performance loss, cost) — both minimized.

A coordinate this problem declared but the assignment does not bind is a wiring mistake, not a search outcome: it trips a debug assertion rather than becoming a NaN row that merely never wins. Release builds keep the total NaN contract: an unbound coordinate reads as NaN and the row is scored as one, rather than aborting a benchmark sweep.

pub fn cost_cap(&self) -> f64

The affordability cap on cost: the cost at the cap_resource level. A trial is “cheap” iff its cost does not exceed this.

pub fn capped_best_perf(&self, records: &[(f64, f64)]) -> f64

The best (lowest) performance loss among (perf, cost) records whose cost is within cost_cap — the performance-per-cost metric the CARBS ablation is scored on. f64::INFINITY if no record is affordable.

Trait Implementations

impl Clone for CostProblem

fn clone(&self) -> CostProblem

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for CostProblem

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.