2nd/10_Wiki/Topics/Programming & Language/Index Masking.md

id, title, category, status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, verification_status, tags, raw_sources, last_reinforced, github_commit, tech_stack

id	title	category	status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	verification_status	tags	raw_sources	last_reinforced	github_commit	tech_stack
wiki-2026-0508-index-masking	Index Masking	10_Wiki/Topics	verified	self	Bitmask Index Power-of-Two Masking	none	A	0.9	applied	optimization low-level ring-buffer hash		2026-05-10	pending	language framework C/C++/Rust/JavaScript General

Index Masking

매 한 줄

"매 capacity 가 power-of-two 일 때 % N 의 & (N-1) replacement". 매 modulo 의 expensive (CPU cycle 10-30) 의 single-cycle AND 로 대체. Ring buffer, hash table bucket, fixed-size LRU, lock-free queue 의 hot path 의 universal trick. CPU branch predictor 와 cache line 의 friendly.

매 핵심

매 정확성 조건

• N 이 power-of-two: 2^k.
• index % N == index & (N - 1) (unsigned 또는 non-negative).
• N - 1 이 모든 lower bit 의 1 의 mask.

매 대표 사용 처

• Ring buffer / circular queue.
• Open-addressing hash table bucket index.
• Game loop frame index modulo history size.
• Hardware register address calc.
• Lock-free SPSC/MPMC queue (Disruptor pattern).

매 응용

1. LMAX Disruptor (Java) — sequence & ringMask.
2. Linux kernel ringbuffer.
3. Redis hash table — sizemask.
4. V8 hidden class transitions table.

💻 패턴

Basic ring buffer (C)

#define RB_CAP 1024  // must be power of two
typedef struct {
  int data[RB_CAP];
  uint32_t head, tail;  // free-running counters
} RingBuf;

static inline void rb_push(RingBuf *r, int v) {
  r->data[r->head & (RB_CAP - 1)] = v;
  r->head++;
}
static inline int rb_pop(RingBuf *r) {
  int v = r->data[r->tail & (RB_CAP - 1)];
  r->tail++;
  return v;
}

TypeScript fixed-size circular log

class CircularLog<T> {
  private buf: (T | undefined)[]
  private mask: number
  private head = 0
  constructor(capacityPow2: number) {
    if ((capacityPow2 & (capacityPow2 - 1)) !== 0)
      throw new Error('capacity must be power of two')
    this.buf = new Array(capacityPow2)
    this.mask = capacityPow2 - 1
  }
  push(v: T) {
    this.buf[this.head & this.mask] = v
    this.head++
  }
  recent(n: number): T[] {
    const out: T[] = []
    for (let i = 0; i < n; i++) {
      const v = this.buf[(this.head - 1 - i) & this.mask]
      if (v !== undefined) out.push(v)
    }
    return out
  }
}

Round-up to next power of two

uint32_t next_pow2(uint32_t v) {
  v--;
  v |= v >> 1; v |= v >> 2; v |= v >> 4; v |= v >> 8; v |= v >> 16;
  return v + 1;
}

Hash table bucket

struct Map<K, V> {
    buckets: Vec<Option<(K, V)>>,
    mask: usize,
}
impl<K: Hash + Eq, V> Map<K, V> {
    fn bucket(&self, k: &K) -> usize {
        let mut h = DefaultHasher::new();
        k.hash(&mut h);
        (h.finish() as usize) & self.mask  // mask = capacity - 1
    }
}

SPSC lock-free (Disruptor-style)

// producer
void produce(RingBuf *r, int v) {
  uint32_t h = atomic_load(&r->head);
  while (h - atomic_load(&r->tail) >= RB_CAP) cpu_relax();  // full
  r->data[h & (RB_CAP - 1)] = v;
  atomic_store(&r->head, h + 1);
}

Branch-free wrap (negative-safe)

// for signed wrap, use bit-and after cast
int idx = ((int)(counter) & ((int)CAP - 1));  // counter must be non-negative

매 결정 기준

상황	Approach
Hot path, capacity 가 control 가능	Power-of-2 + masking
Capacity 가 user-input arbitrary	% (or round up to pow2)
Capacity 가 dynamic resize	resize 시 mask recompute
Signed counter	unsigned cast 또는 modulo
Hardware-aligned (cache line)	pow2 + alignment 조합

기본값: 매 ring/hash table 는 power-of-two + bitmask. 매 capacity 의 dynamic 이면 round-up.

🔗 Graph

• 응용: Object Pooling (오브젝트 풀링)
• Adjacent: SharedArrayBuffer로 스레드 간 메모리 공유 효율 높이기 · Pointer Compression

🤖 LLM 활용

언제: 매 high-throughput data structure, lock-free queue, HFT, game engine frame loop. 언제 X: capacity 의 control 안 되거나 readability priority 인 일반 application code.

❌ 안티패턴

• Non-pow2 size 에 mask 적용: 매 silent index out-of-range, data corruption.
• Signed negative index 의 masking: -1 & (N-1) 의 N-1 이 됨 → 매 unsigned cast 또는 ((i % N) + N) % N pattern.
• Capacity 1 의 mask 0: 매 always index 0, degenerate.
• Premature application: 매 측정 없이 micro-optimization. 매 hot path 만 의 적용.

🧪 검증 / 중복

• Verified (Hacker's Delight Ch.3, LMAX Disruptor paper, Linux kfifo source, Redis dict.c).
• 신뢰도 A.

🕓 Changelog

날짜	변경
2026-05-08	Phase 1
2026-05-10	Manual cleanup — power-of-2 masking patterns + ring/hash applications