Struct UniversalHeader 

pub struct UniversalHeader {
    pub magic: u32,
    pub capacity: u32,
    pub state: AtomicU64,
    pub insert_count: AtomicU64,
    pub contains_count: AtomicU64,
    /* private fields */
}

Fields

magic: u32

capacity: u32

state: AtomicU64

Packed state, single AtomicU64 for atomic update / atomic reader-load:

• bits 63..32 = version: u32 (bumps per migration within the current generation; wraps to 0 at u32::MAX)
• bits 31..16 = generation: u16 (bumps when version wraps; ensures a reused (generation, version) pair NEVER appears in the same lifetime, so readers comparing the full u64 state always observe wrap-around and re-open)
• bits 15..0 = strategy: u16 (low byte is the Strategy discriminant; high byte reserved for strategy variants)

True exhaustion: generation u16 AND version u32 both at MAX (= 2^48 = 281 trillion migrations). Returns VersionExhausted.

insert_count: AtomicU64

Bumped by every insert; consumed by the writer’s local policy to decide when to migrate.

contains_count: AtomicU64

Bumped by every contains; same role as insert_count.