Struct TagHints 

pub struct TagHints { /* private fields */ }

The per-keyspace append hints for one handle.

D9 widened the append SPLIT from “rightmost leaf of the tree” to “rightmost leaf of the key’s own tag”. This widens the append HINT the same way, which is the other half: the split policy was already right for a tag that is not the tree’s last, but every one of those inserts still paid a full root-to-leaf descent to reach the leaf the policy then acted on.

WHY A CACHED FENCE IS SAFE. fast_path_leaf’s next_leaf() == 0 test is self-validating – it reads the truth off the page every time – and a cached separator is not. It is exact only while the tree’s shape is the one the arming descent walked, so every way that shape can move is answered:

• a split of the hinted leaf subdivides its interval – the cached next_leaf changes, so the page-side check catches it, and the split path forgets the slot as well;
• a neighbour redistribution moves separators BETWEEN siblings without touching the chain, so it forgets the slots of every page in its window itself;
• a delete, a graft, a bulk pack, a rollback and a checkpoint can merge leaves, collapse the root or hand a page back to the allocator, and each of those clears the whole cache.

SACRIFICE (Law 4): about 1.5 KB of fixed state per handle, and a linear scan of sixteen slots per insert. Bought: an ascending run that is not the tree’s rightmost – which is every run but one, in a store where D4 makes every feature a tag in the same tree – inserts with one page access instead of the tree’s full height.

Implementations

impl TagHints

pub fn clear(&self)

Forget everything. The answer to every structural change this cache does not track page by page.

pub fn clear_tree(&self, tree: u16)

Forget one tree’s slots, for a tree whose root is being freed or whose handle slot is being recycled.

pub fn attempts(&self) -> u64

Diagnostics: inserts that found a slot, and inserts that then landed on its leaf. Counted, not reasoned about – the budget tests read these.

pub fn hits(&self) -> u64

pub fn misses(&self) -> u64

Inserts no slot claimed.

pub fn disabled() -> Self

A cache that is off. Every insert descends, as it did before K1.

pub fn set_enabled(&self, on: bool)

Turn the cache on or off for this handle. Turning it off forgets everything it held, so a handle cannot come back to a stale fence.

pub fn enabled(&self) -> bool

Trait Implementations

impl Default for TagHints

fn default() -> Self

Returns the “default value” for a type.