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Collectors

Aggregation functions for group_by operations in constraint streams.

Collectors aggregate values within groups created by group_by(...). They transform a stream of individual matches into grouped summaries. For simple fairness rules, prefer balance(...); use collectors when you need explicit counts, totals, or custom imbalance data.

Using Collectors

Pass a collector as the second argument to group_by:

factory.shifts()
    .group_by(
        |shift: &Shift| shift.employee_idx,   // grouping key
        count(),                              // collector
    )
    // Result: grouped stream of (key, usize)

Available Collectors

count()

Counts the number of matches in each group. Returns usize.

.group_by(|s: &Shift| s.employee_idx, count())
// → (key, usize)

sum(mapper)

Sums numeric values in each group. The mapper extracts the value to sum.

.group_by(|s: &Shift| s.employee_idx, sum(|s: &Shift| s.hours))
// → (key, i64)

load_balance(key_fn, metric_fn)

Measures load imbalance across a grouping key. Returns a LoadBalance<K> with unfairness metric.

.group_by(
    |s: &Shift| s.department_idx,
    load_balance(|s: &Shift| s.employee_idx, |s: &Shift| 1i64),
)

Balance Stream Operation

For simple load balancing without group_by, use the balance stream operation directly:

factory.shifts()
    .balance(|shift: &Shift| shift.employee_idx)
    .penalize_soft()
    .named("Fair distribution")

The key function returns Option<K> — None values (unassigned entities) are skipped.

See Also