Struct Sum 

pub struct Sum;

Additive commutative monoid: old ⊕ op = old + op. Associative and commutative, not idempotent (re-adding double-counts). The unordered-exactly-once tier (analysis §2): a running counter / sum.

Trait Implementations

impl<T> MergePolicy<T> for Sum

where T: Add<Output = T> + Clone,

const COMMUTATIVE: bool = true

true iff ⊕ is commutative — reordering ops converges to the same state (merge(merge(a, b), c) == merge(merge(a, c), b)).

const IDEMPOTENT: bool = false

true iff ⊕ is idempotent — re-applying an op is a no-op (merge(merge(a, b), b) == merge(a, b)).

fn merge(old: &T, op: T) -> T

Fold op into old. MUST be associative.

const CONFLATES: bool = true

true iff coalescing (merging accumulated ops into one) actually bounds the state — the precondition for the Conflate overflow action. All conflating policies (band / monoid / semilattice) bound; only RawFifo (concat: order + multiplicity are meaning) grows without bound, so it cannot conflate and a relay MUST reject Conflate for it (analysis §4.3).