89. Circular Buffer-I

Question.5

A developer replaces the modulo in a circular buffer with a bitmask for performance:

#define BUF_SIZE 16  // Power of 2

cb->head = (cb->head + 1) & (BUF_SIZE - 1);
// Instead of: (cb->head + 1) % BUF_SIZE

Is this optimization valid? What is the constraint?

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Always valid — bitmask and modulo are always equivalent
Valid only when BUF_SIZE is a power of two — the bitmask (N-1) produces a pattern of all 1s that wraps correctly
Never valid — bitmask gives different results than modulo
Valid only for 8-bit buffer sizes