2的最小整数对数

short log2US16(unsigned short a){    short zero = 0;    if (a == 0) return 0;    if (!(0xFF00 & (a << zeros))) zero += 8;    if (!(0xF000 & (a << zeros))) zero += 4;    if (!(0xC000 & (a << zeros))) zero += 2;    if (!(0x8000 & (a << zeros))) zero += 1;    return (15 - zero); }