# C5 — Freshness-gated recompute and sample-and-hold

**Guarantee.** The `cycle_id` advances everywhere (a cheap counter), but a node
re-evaluates only when ≥1 of its own inputs is fresh this cycle (detected by
run-count); otherwise it holds its last output. Stale inputs contribute their
last (held) value.

**Forbids.** Recomputing every node every cycle ("push all" is true for the
*clock*, not for *recompute*); treating a held value as missing.

**Why.** Total recompute does not scale to many sparse high-frequency sources;
freshness-gating is the performance discipline that keeps the synchronous model
fast.

## Current state

Freshness is a run-count epoch. Each input slot carries `SlotState { fresh_at,
last_ts }` (`crates/aura-engine/src/harness.rs`); a slot is fresh this cycle iff
`fresh_at == cycle_id`. The firing predicate `fires` re-evaluates a node when any
`Firing::Any` input is fresh this cycle, or when a barrier group completes (≥1
member fresh this cycle and every member carrying `last_ts == ts`) — the barrier
/ co-freshness machinery itself is C6.

A node that does not fire pushes nothing, so downstream consumers keep reading the
last value via `window[0]`: sample-and-hold falls out of the push model rather
than being a separate mechanism. A held value is a real past value, never treated
as missing.

## See also

- [C6](c06-firing-policy.md) — the firing predicate and the barrier /
  co-freshness rule this gating builds on.
- [C1](c01-determinism.md) — freshness is a deterministic function of `cycle_id`,
  so gating never perturbs the bit-identical run.
- [C2](c02-causality.md) — a held value is past data, not look-ahead.