Struct SnapshotRing 

pub struct SnapshotRing { /* private fields */ }

A bounded ring of ArenaSnapshots: “current” (the most recently pushed entry, ring index 0) plus up to depth older entries. Never retains more than depth + 1 entries, no matter how many times Self::push is called – the “a design with no $past retention need pays no retention cost beyond entry 0” guard (see snapshot_ring_depth_0_design_allocates_no_ring below). Whether a ring is allocated at all for a given instance is a decision this type does not make – sched::control::Engine only ever constructs one when the design has at least one assertion/clocking-block/$past site to begin with (§1.4’s “not every instance, to avoid taxing designs with no SVA at all”); a design with none never calls Self::new and pays nothing.

Implementations

impl SnapshotRing

pub fn new(depth: usize) -> Self

depth = N (16.9.3/16.9.4’s largest literal number_of_ticks argument across the whole design that needs this ring; 0 if $past/$past_gclk is never called with a nonzero tick count for it). Allocates nothing up front (VecDeque::new(), never with_capacity) – the first Self::push call is this ring’s first real allocation.

pub fn depth(&self) -> usize

N as passed to Self::new – how many entries beyond “current” this ring retains.

pub fn len(&self) -> usize

How many entries are currently retained (<= depth + 1) – a test/ diagnostic accessor; Self::push/Self::get already enforce the bound internally without consulting this.

pub fn is_empty(&self) -> bool

true before the first Self::push (clippy’s len_without_is_empty).

pub fn push(&mut self, snap: ArenaSnapshot)

Appends snap as the new “current” entry, evicting the oldest entry once the ring already holds depth + 1 entries – never grows past that bound no matter how many times this is called.

pub fn get(&self, ticks_back: usize) -> Option<&ArenaSnapshot>

The entry ticks_back pushes before the most recent one (0 = “current”, the value Self::push most recently stored). None when ticks_back exceeds how many entries are actually retained – either because the ring hasn’t been pushed to that many times yet (simulation start; 16.9.3’s own “before the Nth previous clock tick, treat the value as X” case, left to the $past lowering task to turn into the right X-valued result) or because ticks_back > depth (a caller bug this primitive reports rather than panics on – unlike RegionQueues’s “this region can never be serviced at all” structural impossibility, this is a plain out-of-range read).