Price levels

Header: include/core/price_level.hpp

Fixed-capacity FIFO ring of OrderRef per tick, plus cached total_volume_.

Ring buffer

Field	Role
orders_	Slice of flat OrderRef array, bound at LOB construction
head_, tail_, count_	Ring indices
mask_	capacity - 1

Operation	Behavior
push	Tail insert; returns false when count == capacity
front	Head element
pop	Advance head, decrement count
empty	count == 0

Capacity must be power of two.

Volume

add_volume / remove_volume track aggregate level quantity. Matching and cancellation update order volume and level total. LOB calls clear() when total_volume() reaches zero after cancel or fill.

Tombstones

Cancel deallocates the pool slot and marks the locator inactive. The OrderRef remains in the ring until popped or compacted. Valid head requires matching generation and non-zero order volume.

Compaction

compact(Predicate) packs active refs to the front and discards inactive entries in O(n) for current queue depth.

Trigger	Location
Failed push during rest_order	Compacts, retries push
8 consecutive stale pops in clean_front	Compacts during match

Single stale head: O(1) pop. Batch threshold limits per-iteration tombstone walk at the touch.

Best price interaction

When level volume reaches zero, LOB clears the level and calls erase_best_ask or erase_best_bid. If the erased tick was best, next_ask / next_bid scans the side bit radar.

Level clear on solo cancel

When cancel removes the last volume at a tick, level.clear() resets head, tail, count, and volume. No tombstones remain. Tombstone chains require at least one other live order at the tick during cancel.